Lakhasly

3.2 Sensors and actuators
This section describes a range of sensors and actuators used in common A/C systems. It describes the underpinning theory associated with the sensors and actuators and provides practical examples of their application, and also includes wiring diagrams and measured data in various forms representing the output of a sensor or control signal used for an actuator. The sensors and actuators included are as follows:
Sensors:
● Temperature – NTC and PTC.
● Sun load – photovoltaic and solar cells.
● Pressure–capacitive,straingauge,piezoelectric.
● Position – linear and rotary potentiometer.
● Speed – Hall effect and inductive.
● Humidity – capacitive.
● Pollution – metal oxide semiconductor (MOS).
Actuators:
● Solenoids–relays,coolantvalves.
● Motors – permanent magnet.
● Steppermotors–DCpermanentmagnet,variablereluctance,hybrid.
Temperature sensor
Theory of operation
A temperature sensor uses direct measurement. It is a temperature dependent, non-linear semi- conductor which is termed ‘thermistor’. It comes in various packages and is fitted in a range of environments. Temperature sensors are generally NTC or PTC type semiconductor material.
NTC temperature sensor
These are generally made from different oxides of metals such as iron, cobalt, nickel copper and zinc. The sensor is extensively used in temperature measurement. The NTC (Negative
Temperature Coefficient) thermistor, decreases in resistance as it increases in temperature. The sensors are generally arranged in a potential divider circuit with a fixed resistor. The fixed resistor will be inside the control module to allow the module to measure a variation in volt- age or current, see Figure 3.2.
Sensor monitoring locations:

1. Engine coolant temperature sensor – direct measurement of coolant temperature.


2. Ambient temperature sensor – fitted near the condenser/front bumper.


3. Evaporator temperature sensor – fitted to the evaporator to measure its surface tempera-
   ture informing a module to control the compressor in case icing occurs.


4. Cabintemperaturesensor–integraltotheA/Ccontrolspaneland/orfittedtotheairducting.
   Sensor failure
   If a temperature sensor like the exterior temperature sensor fails, then often the A/C module will apply a fixed temperature value, e.g. 10°C. If the interior temperature sensor fails then a temperature of approximately 24°C is fixed. This allows the system to operate with a fault.
   An example application is an air vent outlet and interior temperature sensor with integrated fan (Figure 3.18)
   Sensor monitoring location:


5. Built into the heater controls or 2. Fitted to the air ducts.
   The sensor unit has a small electric fan (Fig. 3.18) to prevent distortion of the temperature due to heat build-up. The outlet and cabin temperature sensors are the most important values for the A/C control unit. A comparison is made between the cabin temperature and the desired temperature in order to decide if the mixed-air temperature should be increased or decreased. The cabin temperature is adjusted with respect to the outside temperature so that the temper- ature experienced matches the set temperature. As the difference between selected tempera- ture and adjusted temperature increases, the interior fan speed will increase. When the ignition is turned off, the suction fan on some systems may continue to operate for up to 10 minutes. This decreases the risk of incorrect temperature value when restarting after a short stop.
   Sensor wiring diagram
   The motor is powered by the A/C control module (Fig. 3.19). Some systems will have the unit built inside the module while other systems have a separate unit near the centre vent area to accurately measure interior temperature. The motor will be powered by a 12 V feed and the sensor by a 5 V feed. The sensor will be a part of a potential divider circuit enabling the meas- urement of a volt drop across a fixed resistor situated inside the module.
   The technician is able to measure the volt drop across the sensor by placing a voltmeter or oscilloscope (trend measurement) across pins 4 and 6.
   Waveform
   The plot in Fig. 3.20 shows the volt drop across an NTC sensor. As the temperature applied to the sensor by the A/C system increases the resistance reduces with a corresponding reduction in volt drop. A DC analogue signal can also be measured using the Min and Max selection of an oscilloscope. This enables the measurement of the total variation in voltage to be known which can be compared to a known value for analysis.
   Signal checks:


6. Measuring the temperature of the air flowing past the sensor to carry out a comparison should be carried out.


7. The waveform should correspond to an NTC or PTC graph (Figs 3.16 and 3.22). 3. The peak voltage should be referenced to the specification of the sensor.


8. Voltage transitions should be steady and reflect a change in temperature
   The measured temperature of the face vent outlet temperature at the start of the waveform is 2.58 V at 4°C and 0.55 V at 60°C.
   PTC temperature sensor/element
   These are generally made from barium titanate. A characteristic of a positive temperature coefficient sensor/element is that there is an increase in resistance due to an increase in tem- perature. The increase in temperature can also be caused by the current flowing through the sensor. A PTC element can be used as a protection device, overload protection due to a large heating effect from the current flow. When the current flows and the element increases in tem- perature its resistance also increases, this has a counter effect and reduces the current flow. This characteristic prevents the PTC element from overheating.
   Example recirculation flap controlled by PTC sensors
   The air recirculation flap on some A/C systems is operated by a DC motor. The flap has only two positions (100% open or 0% closed). When the motor has turned the flap to one of the end positions, the current passing through the motor winding is limited by two PTC resistors built into the motor. Once the motor reaches its maximum position the current will increase and heat the PTC sensors. The sensors increase in temperature and reduce the current flow due to a corresponding increase in resistance.
   Sensor monitoring location:


9. Fitted inside the recirculation motor which is attached to the recirculation flap.
   Sensor failure
   If a temperature sensor like the exterior temperature sensor fails, then often the A/C module works with a fixed temperature value, e.g. 10°C. If the interior temperature sensor fails then a temperature of approximately 24°C is fixed. This allows the system to operate.
   Sun load (photovoltaic diode and solar cells)
   Theory of operation
   The sun load sensor (Fig. 3.26) can be a photoelectric diode which is designed to exploit the pho- tovoltaic effect and measures sun intensity. If a voltage in reverse bias is placed across the diode and light is directed on it, a reverse current will flow (photovoltaic current). Exposing the diode to
   light energy produces more electron hole pairs (free charge carriers) which pass the junction of the diode and increase current flow. The amperage of this current is proportional to light intensity. Sensor monitoring location:


10. The sensor is located above the instrument panel near the windshield.
    Explanation of P and N type material (Fig. 3.25)
    N type
    Once impurities are added to a base material their conductive properties are radically affected. For example, if we have a crystal formed primarily of silicon (which has four valence electrons), but with arsenic impurities (having five valence electrons) added, we end up with ‘free electrons’ which do not fit into the crystalline structure. These electrons are loosely bound. When a volt- age is applied, they can be easily set in motion to allow electrical current to pass. The loosely bound electrons are considered the charge carriers in this ‘negatively doped’ material, which is referred to as N type material. The electron flow in an N type material is from negative to posi- tive. This is due to the electrons being repelled by the negative pole and attracted by the posi- tive pole of the power supply.
    P type
    It is also possible to form a more conductive crystal by adding impurities which have one less valence electron. For example, if indium impurities (which have three valence electrons) are used in combination with silicon, this creates a crystalline structure which has ‘holes’ in it, that
    is, places within the crystal where an electron would normally be found if the material was pure. These so-called ‘holes’ make it easier to allow electrons to flow through the material with the application of a voltage. In this case, ‘holes’ are considered to be the charge carriers in this ‘positively doped’ conductor, which is referred to as P material. Positive charge carriers are repelled by the positive pole of the DC supply and attracted to the negative pole; thus ‘hole’ current flows in a direction opposite to that of electron flow.
    Sensor failure
    If the sun load sensor fails, the A/C module works with a value corresponding to darkness. Photo diode can be tested using a powerful 60 W light. This will alter the light intensity and vary the sensor’s output. If more than one sensor is housed within the sun load sensor, for example dual zone A/C with left and right photo diodes, the light is placed towards one side to test the sensor and more towards the opposite side to test the other sensor. This can be moni- tored via a scope, diagnostic tester or volt/current meter. An amp clamp (low current sensing
    clamp) could be fitted to measure the current flow, access permitting.
    Wiring diagram
    The sensor (Fig. 3.28) receives a 5 V signal from the A/C module pins 1 and 2. The A/C module will also have resistors in series with both the left and right sensor. This will provide a poten- tial divider circuit. The division will be based on the amount of light falling on the diode caus- ing it to conduct in a reverse bias direction. Pin 3 is the sensor ground (earth).
    A typical sensor that works on the photo diode principle has the following output:
    Dark signal voltage: 4.6 volts measured at pin 22/23 Light signal voltage: 0.4 volts measured at pin 22/23
    Sun load sensor using solar cells
    A solar cells converts light energy into electrical energy. The sensing element contains a PN junction, so the charge is carried separately in its electric field before proceeding to the metal contacts on the semiconductor’s surface (see page 118 for an explanation of P and N type material. A DC electric (photoelectric voltage) is produced across the terminals; the electrical potential is between 0.5 and 1.2V depending on the semiconductor material being used. Silicon is the most common material for solar cells.
    Sun load sensor (infrared)
    Theory of operation
    Infrared radiation lies in the optical waveband and forms part of the electromagnetic spec- trum. The infrared range is adjoined by visible light of long wavelengths. Every warm body releases infrared radiation. The higher the body’s temperature the more energy is released in the form of infrared radiation. This makes the measurement of heat source and intensity pos- sible with infrared sensors. The infrared sun load sensor is situated on the top of the dashboard and contains five infrared-sensitive elements: left, right, front, rear and top. It is supplied with power in the form of battery pulses from the climate control module. Voltage from the five sensor elements is sent consecutively to the climate control module. For the control module to be able to detect which sensor element is reporting, the transmission is preceded by the pulse pattern 5 V–0 V–5 V. The control module synchronises the solar sensor pulse transmis- sion by sending short ground pulses at 25 Hz from pin 16. With the voltage from the five sensor elements, the control module calculates the solar intensity, azimuth and height. The val- ues are used to calculate the current temperature at head height for the front seat passenger and driver.
    Pressure sensor – capacitive type
    Theory of operation
    The pressure sensor contains two metal plated ceramic discs mounted in close proximity. The disc located closest to the pressure connection is thinner and bends when subjected to pressure. By this means, capacitance between the metal plating of the discs is changed based on the pres- sure. A circuit integrated in the sensor converts the capacitance to an analogue voltage. Capacitive measurements are based on the principle of a capacitor with the physical property of storing electrical charge. Its ability to store a charge is based on the distance between the two plates, size of the plates and the material it is made from. The distance is the main variable which determines the plates’ charge difference. If the plates are far apart then this creates a low charge difference (Figure 3.31). If the plates are close together then the charge difference will increase proportionally (Figure 3.32). Linear pressure sensors allow for greater fan control.
    Sensor monitoring locations:


11. A/C high pressure sensor is fitted to the A/C pipe work or receiver-drier.
    Waveform
    The trend plot (Fig. 3.33) shows the A/C system off, producing a voltage reading of 1.3 volts and a pressure of approximately 5 bar. The A/C was then switched on under light load; the voltage increased proportionally with an increase in pressure. The A/C stabilised at a voltage of approxi- mately 2 volts which is approximately 12 bar. The load on the system was increased by setting the interior temperature to the lowest value on the climate control system (low 16°C) and increasing the blower speed to maximum. This caused the sensor voltage to peak at 2.2 volts, approximately 13.5 bar.
    Signal checks:
    Figure 3.34 Results from a pressure sensor


12. Monitor a steady change in voltage level directly proportional to a change in pressure.


13. Check for glitches in the signal (drops to zero or rise to reference voltage).


14. Volt drop on the reference voltage and ground signal should not be greater than 400 mV
    (see Power-to-power test and Earth-to-earth test under section 3.3).
    Pressure sensor using strain gauge
    Theory of operation
    A micro-machined membrane sensor with a strain gauge is used to measure pressure. A strain gauge is a group of resistors printed onto a membrane in the form of a bridge circuit (Wheatstone bridge). The bridge circuit Figure 3.37 shows four resistors (two series resistors in parallel with each other). The potentials at points UM are equal when the bridge is balanced (both sets of parallel resistors have the same potential difference across them). When pressure is applied to the sensor’s membrane the resulting mechanical force changes the electrical resistance of some of the resistors. The resistors are arranged so that two resistors increase in length and two resistors decrease in length due to deformation. This causes the bridge to become unbalanced giving a potential difference output across point UM. The output of the potential difference is proportional to the deformation of the membrane. It is important to remember that resistance is proportional to length and inversely proportional to cross-sectional area. By changing the shape of the resistors the bridge becomes unbalanced. The sensor and the hybrid circuitry for signal processing are located together in a single housing.
    Sensor monitoring locations:


15. A/C high pressure sensor is fitted to the A/C pipe work or receiver-drier.
    Pressure sensor using piezoelectricity
    Theory of operation
    The piezoelectric effect can best be illustrated by means of a quartz crystal on which pressure is exerted. The quartz crystal is electrically neutral in its rest state, that is, the positively and negatively charged atoms (ions) are in balance. External pressure exerted on a quartz crystal causes the crystal’s lattice to deform. This results in ion displacement. An electric voltage is generated as a consequence. The direct piezo effect is primarily utilised in sensors. Today’s technologies use high performance piezoceramic materials instead of quartz crystals. As sen- sors, piezoceramics convert a force acting upon them into an electrical signal when the ceramic material is compressed against its high rigidity. Owing to dielectric displacement (dielectric  electrically non-conductive), surface charges are generated and an electric field builds up. This
    field can be registered as a (measurable) electrical voltage via electrodes. In the case of sen- sors, mechanical energy is converted into electrical energy by means of a force acting on a piezoelectric body.
    Sensor monitoring locations:


16. A/C high pressure sensor is fitted to the A/C pipe work or receiver-drier.
    Angle sensors (potentiometer)
    Theory of operation
    An angle sensor simply measures the angular rotation of a component (e.g. air distributor door or throttle valve). Figure 3.39 shows a sliding contact and a control track. A reference voltage for the sliding contact is supplied via a contact track. This contact track has a constant, low ohmic resistance from start to end. The sliding-contact position sensor is actuated by the move- ment of the distribution door (which is actuated by a motor), the resistance of the variable resist- ance track changes along its length. The sensor track operates on the principle that resistance is proportional to length and inversely proportional to cross-sectional area. This means if you double the length of a conductor the resistance will double and if you halve the cross-sectional area of a conductor the resistance will double. The slider moves across the track measuring the voltage at different points.
    The rotary distribution door has a DC permanent magnet motor (not shown) with a poten- tiometer which provides closed loop feedback to verify its position.
    A reference voltage and ground is applied to the resistive track. This creates a 5 V volt drop across the resistor. The signal output sliding contact makes contact with the surface of the resistive track and divides (potential divider circuit) the circuit creating two volt drops across its length (Fig. 3.41).
    If the sliding contact position sensor is in the middle of the resistive track, 50% of travel has occurred on the rotary door. Then the volt drop theoretically should be equal at 2.5 volts. This is due to the resistor track being divided into two halves of equal proportion so equal volt drops exist.
    If the rotary distribution door is closed, which represents a 0% movement ratio, then the output of the sensor would be very low at around 0.1 V due to a low resistance between the ref- erence voltage and the output. If the rotary door was in the fully open position, 100% move- ment ratio, then the volt drop would be high due to the high resistance.
    A formula to calculate voltage output (Vout) of a rotary sensor:
    VØ
    V 150/ 300 S
    Vs – voltage supply
    Ø – angle of rotation
    300 – total available angle of rotation
    e.g. V  150°
    5 V  2.5 V
    out
    Pressure sensor sliding contact potentiometer (linear)
    A linear potentiometer (Fig. 3.43) operates on the same principle as a rotary potentiometer except the movement is linear and not rotary, hence its name. Pressure from the refrigerant is applied to a membrane that deforms and transfers this movement to the potentiometer slider (1). This varies the linear movement and the sensor’s output.
    Sensor monitoring locations:


17. A/Cpressuresensor(linearmeasurement)isfittedtotheA/CpipeworkSchradervalveor receiver drier housing (high pressure side of A/C system).


18. Throttle position (angle measurement) is found in the throttle housing, accelerator pedal position.


19. Heater door position feedback is attached to heater door motor or mechanism.
    Waveform
    The waveform represents an increase in voltage output with a corresponding increase in angu- lar movement.
    Example specification:
    Voltage output approximately 0.8 V at rest position, throttle valve closed Voltage output approximately 4.8 V at Wide Open Throttle (WOT) position
    Resistance of the sensor in the rest position – 1.1 k
    Resistance of the sensor in fully open position – 4.4 k
    
    Signal checks:


20. Monitor a steady change in voltage level directly proportional to a change in rotary or lin- ear motion.


21. Check for glitches in the signal (drops to zero or rise to reference voltage).


22. Volt drop on the reference voltage and ground signal should not be greater than 400 mV
    (see Power-to-power test and Earth-to-earth test under section 3.3).


23. Min and max values indicate maximum angular or linear motion.


24. Signal noise may indicate worn or faulty wiper contact.
    Speed sensor (Hall effect)
    Theory of operation
    If a current flows through an electrical conductor positioned at right angles (90°) to a magnetic field, the charge carriers (electrons) are deflected (Lorentz force). The Hall effect (Figs 3.45 and 3.46) is generated by means of a semiconductor plate (Hall plate) which receives a defined volt- age (U). Application of the supply voltage U results in an evenly distributed electron flow over the entire surface of the Hall plate. As a result, a magnetic field builds up around the Hall plate. The evenly distributed electron flow leads to charge equalisation (UH  0 V) on both sides of the Hall plate.
    Note – changes in the magnetic field lead to corresponding changes in electron flow. If the north pole of a permanent magnet meets the north pole of a Hall plate magnetic field, the field moves away from the permanent magnet.
    As a result, the electrons (negatively charged particles) driven by the longitudinal potential are suddenly deflected vertically with respect to the current’s direction of flow, away from the permanent magnet (repulsion of electron flow). The resulting charge difference between the two sides of the Hall plate gives rise to a Hall voltage(UH ). If the south pole of a permanent magnet
    meets the north pole of a Hall plate magnetic field, the field moves toward the permanent mag- net. As a result, the electrons (negatively charged particles) driven by the longitudinal potential are suddenly deflected vertically with respect to the current’s direction of flow, toward the per- manent magnet (attraction of electron flow). The sudden changes in electron flow correspond- ingly change the polarity of the Hall voltage (from positive to negative or vice versa). Hall voltages are generally very low. Lying in the millivolt range, these voltages must be processed appropriately. Sensor technology usually makes use of Integrated Circuits (ICs) to process Hall voltages and output them as square-wave signals to the terminal device (e.g. PCM). The square- wave signals can be made visible with the aid of an oscilloscope or tested with an LED.
    Note – The Hall plate magnetic field can also be deflected by moving an iron element (e.g. a ferrous pulse wheel) toward it. In this case, there is no alternation of electron flow between the sides of the Hall plate. The magnetic field and electron flow are always dis- placed in just one direction: from charge equalisation to charge difference (0 signal edge/ high signal edge).
    Sensor Monitoring locations:


25. Interior blower speed feedback is fitted on blower inside heater assembly. 2. Vehicle speed sensor is fitted on the transmission housing output shaft.
    Example vehicle speed sensor
    Vehicle speed is an important factor used to assist in calculating the cooling rate of the interior due to increased natural flow rates (ram air). Often an increase in vehicle speed will cause the control module in automatic mode to reduce the blower speed (automatic climate control sys- tem). The accuracy of the exterior temperature sensor is also adversely affected by the ram air. For this reason, the A/C module calculates the exterior temperature from the measurement of the exterior temperature sensor and vehicle speed. The vehicle speed is detected by the Vehicle Speed Sensor (VSS), which also supports other systems (speedometer, PCM (Powertrain Control Module), suspension, braking). It is designed as a Hall sensor and sends a digital signal to the A/C module in the form of a square-wave signal.
    Signal checks:
    (approx. 130 km/h)
    Figure 3.48 A frequency modulated signal from a Hall effect vehicle speed sensor
    1. Monitor a steady change in frequency which is proportional to a change in vehicle speed.


26. Check for glitches in the signal (drops to zero or rises to reference voltage).


27. Volt drop on the reference voltage and ground signal should not be greater than 400 mV.


28. Peak–peak voltages should be the same and equal reference voltage, allow 400 mV differ-
    ence (see Power-to-power test and Earth-to-earth test under section 3.3).


29. The lower horizontal lines should almost reach zero, allow for 400 mV difference.


30. Signal transitions should be straight and vertical.


31. Check for any background interference.
    Inductive type speed sensor
    Theory of operation
    Figure 3.49 shows a permanent magnet with north and south poles. An electrical conductor is positioned between the north and south poles. If the conductor is moved in the direction of the arrow, it intersects with the permanent magnet’s field lines. Charges inside the conductor are displaced in this process. Free electrons move to one end of the conductor. Correspondingly, a shortage of electrons occurs at the other end. If you can arrange for the conductor to keep moving in the magnetic field then you will have a continuous force of electrons available to do work. The resulting potential between the conductor’s ends is termed induction voltage.
    Sensor monitoring locations:


32. Compressor speed sensor – measures the rotational speed of the pulley or main compres- sor shaft.


33. Blowermotorspeedsensor–measurestherotationalspeedoftheblowermotorforfeedback control.
    The inductive speed sensor contains a permanent magnet and soft permeable pole pin sur- rounded by a coil. The speed sensor is mounted so that its front face is a defined distance from the sensor ring. The rotation of the sensor ring induces a voltage proportional to the periodic variation in magnetic flux. The variation in magnetic flux is caused by the movement of the ferro- magnetic sensor ring. When the magnetic flux increases or decreases an emf is induced into the coil windings. As a tooth of the ring approaches the sensor a positive emf is generated in the coil due to the lines of flux being cut in the magnetic field. When the tooth is aligned with the sensor there is no change in magnetic flux so the emf is zero. As the tooth rotates away from the sensor it again breaks the lines of magnetic flux which generates a negative emf. The changes in the magnetic flux induce an alternating voltage in the inductive sensor coil. A uniform tooth pattern will create a near sinusoidal voltage curve.
    Waveform
    Voltages generated by induction constantly alternate in amplitude and polarity (Fig. 3.51). Accordingly, they are also termed alternating voltage. Alternating voltage rises from 0 V to its positive peak value (amplitude), then drops back via the 0 V level to its negative peak value,
    rises again to its positive peak value etc. The number of complete alternations (periods) per second is termed the voltage frequency.
    The air gap of this particular type of sensor is crucial to its operation. Because air is not very permeable if the gap is too large the amplitude of the output is very low or zero.
    Humidity sensor
    Theory of operation
    Humidity sensors determine relative air humidity using capacitive measurement technology. For this principle, the sensor element is built out of a film capacitor on different substrates (glass, ceramic etc.). The dielectric is a polymer which absorbs or releases water proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor, which is measured by an onboard electronic circuit (see Pressure sensor – capacitive type (above) for additional information).
    A temperature sensor is often used with a humidity sensor. The temperature and humidity sensors together form a single unit, which enables a precise determination of the dewpoint without incurring errors due to temperature gradients between the two sensor elements. The sensors are coupled to an amplification, analogue-to-digital (ND) conversion and serial inter- face circuit all on the same chip. The integration provides improved signal quality and insensi- tivity to external disturbances (EMC).
    Air quality sensor
    Theory of operation
    If the vehicle is in an exhaust gas cloud, the air intake process will always be stopped and the system will switch to recirculation mode. This is to prevent the air quality in the passenger com- partment from becoming contaminated by the air outside. Recirculation will also be disengaged if the mode was permanently on and the air in the passenger compartment was not being exchanged at all; in this case the system works dynamically. In exceptional cases such as these, the mode of operation ensures that an adequate supply of fresh air is fed into the system.
    An Air Quality Sensor (AQS) (Fig. 3.52) is located in the main air inlet duct of the HVAC system. When the threshold for carbon monoxide or nitrogen dioxide is reached, the AQS rap- idly communicates to the HVAC system to initiate the air recirculation mode. The Metal Oxide Semiconductor (MOS) sensor consists of a sensing material and a transducer (substrate). Surface reactions at the sensitive layer change its resistivity. The transducer keeps the sensing material at an elevated temperature, and its resistance is measured. Changes in the composi- tion of the ambient atmosphere create a corresponding change in the resistance of the sensing
    layer, allowing the sensor to detect a wide range of toxic gases even at very low concentrations. The sensing layer is a porous thick film of polycrystalline tin oxide (SnO2). In normal ambient air, oxygen and water vapour-related gases are absorbed at the surface of the SnO2 grains. For reducing gases such as CO, a reaction takes place with the preabsorbed oxygen and water vapour-related gases which decreases sensor resistance.


34. Sensor monitoring location: Main air inlet duct of the HVAC system.
    The sensor uses integrated signal conditioning electronics mounted with the sensing element on a circuit board. An integral microcontroller monitors the pollution level and creates a Pulse Width Modulated (PWM) or serial output signal in relation to pollution levels.
    Benefits:
    ● Improved comfort.
    ● Fast,automaticprotectionagainstpotentiallyharmfulexternalpollutants.
    ● Cabin pollutant concentrations reduced by 20%.
    ● Occupant discomfort due to odours reduced by 40%.

Actuators

1. Relay.


2. Solenoids.


3. DC permanent magnet motor.


4. Stepper motors – variable reluctance, permanent magnet and hybrid.
   Relay
   Theory of operation
   Current flowing through a conductor like a straight copper wire creates a magnetic field around itself. If the copper wire is twisted (Fig. 3.55) and shaped like a coil and current passed through it then a magnetic flux is created. The relationship between the direction of current flowing through a conductor and the direction of magnetic flux is expressed by ‘Ampere’s rule of the right-hand screw effect’. If the current is reversed then the magnetic poles will reverse.
   If a permeable material (material easily magnetised) is placed under the coil it will become magnetised and attracted by the magnet field generated in the coil. The action of the coil and current is the principle of electromagnetic force. The force created is not strong enough in Fig. 3.55
   to move the metal piece. If an iron core is inserted (Fig. 3.56), the number of magnetic fluxes is intensified and the metal is under a greater force. This is because the coil and the bar create lines of magnetic force. This principle explains the operational characteristics of a relay.
   Actuator example relay
   In Figure 3.57 a current flows through the relay coil (6) and creates a magnetic field, which is magnified by the coil’s core (7). This Magnetic Motive Force (MMF) is applied to the armature (2) and it is attracted to the centre of the coil. The force is great enough for the armature to overcome spring force and the armature makes contact with the coil. Contact points are attached to the armature. Once the armature has made contact with the coil the relay contacts are closed from (4) to (5). This creates a closed circuit on the high current side of the relay.
   Relays are electrically controlled switches. The function of a relay is to use a low current to operate a high current. The switch inside the relay will be in one of two positions, depending on whether the electromagnetic relay coil is energised or de-energised. In basic relays, there is one input and either one or two outputs. Relays are either Normally Open (NO) or Normally Closed (NC). In either case, the relay switch input (Fig. 3.59a) is always connected to pin 30. Pin 30 not only designates the input to the relay switch, but in accordance with DIN standards, we also know that it’s connected to battery positive. The relay outputs on the other side of the relay switch are designated either 87, 87a or 87b. The two remaining relay terminals are connected
   to the relay coil. Applying current to the coil is what makes the relay close or open. According to DIN standards, pin 85 should be connected to ground (usually controlled by another switch) and pin 86 should be connected to battery positive (usually protected by a fuse). This uses a low current (switched by the A/C module) to operate the relay thus switching on a consumer which uses a large current (condenser fan). This means that the relay must have at least two circuits, a low current and a high current circuit.
   The relay in Fig. 3.58 shows two circuits running vertically. The left which obtains its power from fuse F30 (15A) is the low current side (switching side). The right circuit is the high cur- rent side (switched side). Generally the switching side is operated by a switch or a module and the switched side operates a high current consumer like the compressor or condenser fan. The diode is used to protect the switching device (generally a control module) from back-emf.
   Relay pin coding
   DIN standard 72 552 pin codes and ISO pin designation
   Testing relays
   Table 3.1 offers some appropriate tests for a relay. Some common relay applications:
   ● A/C relay.
   ● WOT(WideOpenThrottle)relayforthecompressorclutch.
   ● Blower motor relay.
   ● Condensor fan relay.
   Actuator locations:


5. Battery junction box, central junction box.
   Solenoid
   Theory of operation
   A solenoid (Fig. 3.60) operates in a similar manner to a relay except when a permeable mate- rial like an iron bar is inserted into a coil shaped like a cylinder and current flows through the coil, the bar is pulled to the centre of the coil. The applied current can be DC, AC, or a pulse width modulated control signal.
   A water control solenoid can be used as an example (Figure 3.61). The key to lifting the plunger inside the valve body is the generation of magnetism ‘lines of force’. These lines of force need to be strong enough to move the mass of the plunger (the so-called iron bar). They quite often need to be strong enough to overcome a spring force (used to hold the plunger closed) and the force of gravity (plunger is in the vertical position). The more lines of force the stronger the magnetic field or ‘flux ( )’. The unit for flux is the weber (Wb). The flux density (B) of a magnetic field is the amount of flux (Wb) per unit area perpendicular to the magnetic field. The unit for magnetic flux density is either the weber-per-metre-squared or tesla which has the designated letter (B). Another factor, which determines the strength of a magnetic field, is reluctance. Reluctance is the ratio of magnetic motive force (mmf) to flux in a mag- netic conductor. It is the equivalent to electrical resistance and so is proportional to length and inversely proportional to cross-sectional area. The unit of reluctance is the ampere-per-weber and is designated the letter (R).
   Electric motor permanent magnet
   Theory of operation
   All conventional electric motors depend for their operation on a conductor such as a wire or a coil creating or operating within a magnetic field. Current flowing through a conductor like
   a straight copper wire creates a magnetic field around itself. The relationship between the direction of current flowing through a conductor and the direction of magnetic flux is expressed by ‘Ampere’s rule of the right hand screw effect’. If this conductor is placed between the poles of a magnet and current is passed through it. There will be a reaction between the two magnetic fluxes produced.
   The strength of the electromotive force varies in proportion to the density of the magnetic flux, the current flowing through the conductor and the length of the conductor within the magnetic field.
   The magnitude of the force varies directly with the strength of the magnetic field and the amount of current flowing in the conductor:
   FI
   L
   B
   F – force (newtons)
   I – current (amperes)
   L – length (metres)
   B – magnetic flux (webers/m2)
   If a conductor formed in the shape of a square coil (Figs 3.63 and 3.64) is placed between the north and south poles of a magnet and a commutator segment is fitted to the end of the coil with brushes to enable an electrical contact, an interesting relationship occurs.
   When no current flows through the conductor only one magnetic flux is present, which is created by the permanent magnet (Fig. 3.64). When current flows from the battery to the con- ductor via the brushes and commutator a magnetic flux is produced. The composition of the mag- netic fluxes from the magnets and the conductor creates reactive forces (F1 and F2). The reactive forces are in different directions due to current flowing forwards on the left side of the square coil and in the opposite direction on the right side of the square coil; this is due to the layout of the conductor within the magnetic field. The direction of the two forces F1 and F2 adheres to ‘Fleming’s left hand rule’ and causes the conductor to rotate around its axis P. Problems occur when the conductor rotates vertically and counter forces are created that try to reverse the direction of the conductor. As a solution (Fig. 3.65) to this problem individual commuta- tors are fitted to each end of the coil to periodically reverse the current flow whenever rotated 180°; this allows the magnetic forces to be applied in a fixed direction and allows the conduc- tor to fully rotate.
   Construction of a permanent magnet motor
   Electric motors basically consist of a rotor (moving part) and a stator (stationary part). Generally, the stator comprises a housing with magnets. The brushes and electrical connections are located in the housing cap.
   The rotor (Fig. 3.67) consists of the armature and an axle, which are bearing mounted in the housing cap. In electrical engineering, the term ‘armature’ refers to a moving component; it can rotate or it can move back and forth like the armature in a solenoid.
   The standard brush type DC motor found in a car has a wound rotor and a permanent mag- net stator (starter motors are an exception as their stators are wound as well). Commutation, switching from one phase to another, is accomplished by incorporating commutator bars on the rotating rotor and stationary brushes in the housing. As the rotor turns, the brushes contact the next phase, allowing the motor to continue to rotate. Commutation is simple and done automatically. Regardless of motor speed, the commutation happens at the right time and no electronic control is required.
   So-called brushes (Fig. 3.68) (usually made from graphite) are used to transfer the power via the connections (commutator) of the moving armature.
   The brushes are pressed against the commutator (Fig. 3.69) by means of a spring. In the event of excessive power consumption, e.g. due to blocking, bimetal switches (thermoswitches) are used for overload protection. These interrupt the circuit to the electric motor and the con- tact is only closed again once the motor has cooled down.
   Because a permanent magnet motor uses the magnet as a stator the direction of rotation of the motor is determined by the electrical polarity of the supply voltage to the armature. If thebattery supply is reversed then the motor will run backwards. Permanent magnets also provide constant field strength.
   Note – not all motors use permanent magnets as a stator. Some motors operate in a simi- lar manner to previously described but use a wire wound (called electromagnetic) stator. The stator can be series wound which means it uses the same current as supplied to the armature winding. The back-emf produced by a series wound motor when started is negligible which allows a large current to flow producing high torque This is typical of a starter motor. Most motors used on the A/C system are low torque and require fine control. This results in most motors being either DC permanent magnet or stepper motor type.
   Stepper motors
   Theory of operation
   In essence, step motors are electrical motors that are driven by digital pulses rather than a con- tinuously applied voltage. They are also referred to as Electronic Commutation (EC) motors due to the absence of a commutator and brushes which are generally present in conventional motors. Inherent in this concept is open loop control, wherein a train of pulses translates into a number of shaft revolutions, with each revolution requiring a given number of pulses. Each pulse equals one rotary increment, or step (hence, step motors), which is only a portion of one com- plete rotation. Therefore, counting pulses can be applied to achieve a desired amount of shaft rotation. The count automatically represents how much movement has been achieved, without the need for feedback information. The precision of step motor controlled motion is determined primarily by the number of steps per revolution; the more steps, the greater the precision. For even higher precision, some step motor drivers divide normal steps into half-steps or micro- steps. With the appropriate logic, step motors can be bi-directional, synchronous, provide rapid acceleration, stopping, and reversal, and will interface easily with other digital mechanisms.
   The main advantages of a stepper motor are as follows:


6. Nobrushesmeansamaintenancereductionandnobrushresiduecontaminationtobearings or the environment.
   2. Becausethereisnobrusharcingorbrushcommutation,brushlessmotorsaremuchquieter both electrically and audibly.


7. Feedbackpositionisnotrequiredbecausethecontrolsystemcancountstepsfromaknown starting point and calculate position.
   Disadvantage:


8. Complex control electronics for commutation. Range of stepper motors:


9. Variable reluctance. 2. Permanent magnet. 3. Hybrid.
   Full-step
   In full-step operation, the motor steps through the normal step angle, e.g. 200 steps/revolution motors take 1.8 steps while in half-step operation, 0.9 steps are taken. There are two kinds of full-step modes. Single phase full-step excitation is where the motor is operated with only one phase energised at a time. This mode should only be used where torque and speed perform- ance are not important, e.g. where the motor is operated at a fixed speed and load conditions are well defined. This mode requires the least amount of power from the drive power supply of any of the excitation modes. Dual phase full-step excitation is where the motor is operated with two phases energised at a time. This mode provides good torque and speed performance with a minimum of resonance problems. Dual excitation, provides about 30 to 40% more torque than single excitation, but does require twice the power from the drive power supply.
   Half-Step
   Half-step excitation is an alternate single and dual phase operation resulting in steps one half the normal step size.
   Micro-step
   In the micro-step mode, a motor’s natural step angle can be divided into much smaller angles. For example, a standard 1.8 degree motor has 200 steps/revolution. If the motor is micro- stepped with a ‘divide-by-10’, then each micro-step would move the motor 0.18 degrees and there would be 2000 steps/revolution.
   Permanent magnet stepper motor
   This motor is constructed in almost the opposite manner to a DC permanent magnet motor. The armature becomes a two pole permanent magnet and the stator is wound. Commutation is achieved by electronically controlling the current flowing through the stator. Permanent magnet stepper motors have a permanent magnet rotor with no teeth, and are magnetised per- pendicular to the axis. In energising a number of phases in sequence, the rotor rotates as it is attracted to the magnetic poles. Direction of rotation depends on the polarity of the stator when current is applied. Altering the frequency of the pulses to the stator varies the motor’s speed of rotation. Increasing the number of stator and rotor poles reduces the step angle. Step angles are generally 90°, 45°, 18°, 15°, 7.5°. Permanent magnet stepper motors have a holding torque (detent torque) when not energised due to using a permanent magnet as a rotor. Rotor direction can be in the opposite direction by changing the sequence of pulses.
   The motor shown in Figure 3.71 will take 90 degree steps as the windings are energised in sequence ABCD. Half-steps can be achieved by these motors. Generally they have step angles of 45 or 90 degrees and step at relatively low rates, but they exhibit high torque and good damping characteristics.
   Advantages:


10. High torque compared to variable reluctance.


11. Permanent magnet stepper motors have a holding torque (detent torque) when not ener-
    gised due to using a permanent magnet as a rotor.
    Disadvantages:


12. Complex control electronics for commutation.


13. Fall of performance due to changes in magnetic strength.
    Variable reluctance
    Permeable material rotor – low torque
    A variable reluctance stepper motor has a soft iron rotor with radial poles and a stator which is wound. The stator has more poles than the rotor.
    The rotor in the centre is made of a permeable material and has eight poles. The stator has 12 poles. The stator is wound with copper cable. Only one phase of the motor is wound on Figure 3.80, normally there would be three-phase windings. To operate the stepper motor the windings would be pulsed in a specific sequence. This example shows four poles of the stator wound in series. When current flows through the four poles of the stator they create a magnetic field. The rotor aligns due to mutual induction to give the shortest magnetic path.
    This is the path of minimum reluctance. From this point all that is required is the correct sequence of pole sets to be energised to give the motor its clockwise or anticlockwise motion. To move clockwise, poles 1 and 2 are the closest so these set of four poles should be energised.
    These stepper motors operate at high frequency and small step angles. Current in windings does not change direction. To change direction the order of the sequence of steps is changed. Step angles of 15, 7.5, 1.8, 0.45 can be achieved.
    Disadvantages of reluctance stepper motors:


14. Complex control electronics for commutation. 2. No holding torque (when not energised).
    Hybrid stepper motor
    Stepper motors are used for precise mechanical angular positioning. These motors (Fig. 3.82) feature a rotor made from a magnetic material (e.g. steel) with non-magnetised poles.
    The stator consists of a large number of pole pairs and energised windings. The stator is designed in a claw pole configuration with two or four ring coils. Each coil assembly is sur- rounded by a stator core, which is divided into two parts – the lower and upper stator core. Eachstator core features numerous teeth. These teeth are all offset to one another and are arranged so that they project in the direction of the rotor. The controller cycles the current from one sta- tor pole to the other, deflecting the rotor poles. A torque is generated. If, for instance, four sta- tor cores are installed each with 12 teeth, this means that a total of 48 teeth are available as opposite magnetic poles. As a result, 48 steps per revolution (step angle of 7.5°) are achieved.
    Waveform
    Figure 3.87 shows two coils being pulsed in a sequence one after the other to move the motor in one direction. A signal was sent from the A/C control panel (in this example via touch screen display) to change the temperature from 27°C to low (below 16°C) so a sequence of pulses is sent to the coils to move the blend door ensuring less air from the evaporator travels through the heat exchanger and flows directly to the air distribution doors.
    Signal check:


15. Check consistent sawtooth pattern (due to inductance reactance), all the signals should be in line.


16. Check for glitches in the signal (drops to zero or rises to reference voltage).


17. Volt drop on the reference voltage and ground signal should not be greater than 400 mV.


18. Maximum peak voltages should be equal to each other.


19. The lower horizontal lines should almost reach zero, allow for 400 mv difference.


20. Check for any background interference.
    A/C modules and displays
    Air-conditioning control modules vary depending on the system they are controlling. The module generally incorporates the A/C controls unless the vehicle is of a high specification, which includes satellite navigation, DVD player, telephone system, which requires a graphical interface. High specification vehicles will often have a multi-zone A/C system which requires a great deal of control compared to a manual system. The following examples are provided to allow the reader an appreciation of the differences between a simple and more complex A/C module.
    Module used for manual control (Fig. 3.88)
    The manual control system illustrated has a manual blower control selection, manual air dis- tribution, manual temperature control, and switch operated A/C and recirculation. The only information the module must process is based on A/C switch input, recirculation input, and temperature variation for the solenoid operated water control valve. The blend door and air distribution are all Bowden cable operated.
    The electronic circuitry required for the operation of such a system is very simple. The module has no memory functions (EPROM), cannot be programmed and is not a part of a multiplex network. This means that the circuitry is designed as an ASIC (Application Specific Integrated Circuit). The output from the unit can be monitored by more electronically advanced modules like the Powertrain Control Module (PCM). These modules (PCM) often make the final deci- sion on whether the A/C compressor clutch will be energised or not.
    Semi-automatic systems (Fig. 3.89)
    The semi-automatic control system as illustrated in Figure 3.89 has a manual and automatic blower control selection, manual air distribution, manual and automatic temperature control, often a display (LCD), and switch operated A/C and recirculation.
    With semi-automatic temperature control, the air distribution (except in ‘DEFROST’ mode) must be set by the user. In addition, the user can switch on/off automatic operation and set the temperature for a single zone (whole interior space).
    The system is often more advanced and can be programmed electronically on and off the vehicle (has EPROM). Semi-automatic modules often incorporate self-test diagnostics (dis- playing fault codes via the LCD) as well as being able to communicate via multiplex commu- nication networks.
    Fully automatic control system (Fig. 3.90)
    The fully automatic control system has a manual and automatic blower control selection, air distribution and temperature control. Switch operated A/C and recirculation is built into the module to activate the system although recirculation will be automatically controlled in some A/C modes. The blend door and air distribution and recirculation are motorised.
    The module incorporates an interior temperature sensor and fan which can be seen in Figure 3.90 at the bottom of the module on the left hand side behind the grille.
    Using advanced graphical displays often requires the A/C module to be a separate unit. Multiplex communication is used to communicate user selection information between the two modules (Fig. 3.91).
    Wiring diagram
    Figure 3.92 shows a mid-speed CAN network. On the MS CAN bus, the transmission rate is 125 kbit/s. Cabling between two nodes, the touch screen A363 module pins 15 and 5 and the A/C module A205 (EATC) pins 18 and 19. CAN high signal is between pins 5 and 18 and CAN low signal between pins 15 and 19.
    This bus is used to transfer all the A/C information the occupants select from the touch screen menu. For example, if auto A/C control is selected by the occupants with a control temperature of 16°C then a signal via the bus is sent to the A/C module allowing it to process the command and compare data against programmed values within its memory. The A/C module will take readings on internal and external air temperature, vehicle speed for natural air flow calculations, blower speed and compare these to the desired temperature. If required the A/C system will then control blower speed, blend door position and air distribution to meet the desired temperature as quickly as possible. All this information will be in the form of a data stream.
    Figure 3.93 shows CAN high and low signal (anti-phase). See section 3.6 for additional information.

Testing A/C modules
Table 3.2 provides the reader with a number of possible tests which can be used to diagnose system faults on A/C modules.