Aftermarket April 2022

34 AFTERMARKET APRIL 2022 TECHNICAL/SNAP-ON www.aftermarketonline.net Fig.1 Fig.2 Fig.3 Fig.4 rotating in a clockwise rotation. With switches S2 and S3 closed, the current flow through the motor results in the motor rotating in an anti-clockwise rotation. N.B. if switches S1 and S2 or S3 and S4 are closed simultaneously then the motor will be shorted circuited and the circuit may be damaged. Feedback and control As previously outlined the engine control module uses the feedback from the accelerator pedal position sensor to determine driver demand. Vehicle demands may also exist in the form of a traction control system command or from the engine speed limiter. However, for correct operation of the electronic throttle control motor a closed loop control system must be implemented using throttle position sensors. In the simplified example as seen in Fig.8, an operational amplifier voltage comparator uses inputs from both the desired throttle position and actual throttle position. The output (Vo) from the comparator is the voltage difference between both signal voltages applied. This output voltage is used to control a proportional, integral, derivative (PID) driver which then outputs a pulse width modulated (PWM) control to the H drive circuit. Current flow and voltage waveform analysis Fig.9 to Fig.14 show oscilloscope waveforms captured from a vehicle. The yellow trace is the current flow through the motor and both the green and blue traces are the motor voltage control wires with a hard ground as the reference for the oscilloscope. As can be observed from the Fig.9 waveform, when the voltage on both wires is equal there is no current flow through the motor. Then one wire is given a ground (green trace) and the other with is controlled with a PWM command. The more positive the PWM, the greater the current flow will be. This particular trace illustrates the point. Fig.10 shows a waveform similar to the one seen in Fig.9. However, at the beginning of the trace, the wire being tested with the green trace is given a ‘hard’ live and the other wire is controlled with a PWM, this time on the ground side of the control. This has the effect of reversing the polarity of the current flow through the motor. After a period of time the polarity of the control voltage changes and the current flow becomes positive. Again, note the increase in current flow as the PWM supply increases. Fig.11 shows the current flow switching from negative to off when the ignition is switched off. The voltage is held at approximately 5 volts. This voltage can be used to monitor the system for faults and open/short circuits. Note: Some electronic throttles will only be actuated when the engine is running or cranking as opposed to key on, engine off. The Fig.12 waveform is similar to the previous trace with the exception that the current flow is positive before the system is shut-off. Fig.13 shows a low PWM which results in a reduction in current flow through the throttle motor. Note in the middle of the trace both control wires are given a ‘hard’ ground which causes current flow to cease. This is to allow constant control of the throttle valve and to allow for smooth operation and controlled idle. The PWM is approximately 10-15% and the average voltage is 1 amp. The final waveform, Fig.14, shows a comparatively large positive PWM control which increases current flow through the motor, resulting in a greater throttle valve angle. The PWM is approximately 80-85% and the average voltage is 4 amps.