Idle speed partial rotation motor (rotary idle valve)
In this design (Figure 1.47), the motor armature is restricted by mechanical stops from rotating through more than approximately 60°. Connected to the end of the armature is an air flap or air valve assembly which, when opened and closed, will regulate the air passing into the engine, thus enabling idle speed to be controlled.
In the simplest type, a spring keeps the motor armature rotated against one of the mechanical stops.
However, when an electric current is applied to the motor (creating electromagnets), this will cause the armature to rotate against the spring. The ECU controls the average current flowing in the circuit by altering the duty cycle of the control signal. The greater the average current, the more the armature will rotate against the spring force. By continuously altering the duty cycle it is then possible to alter the angular position of the armature.
1.9.1 ECU functioning as a switch in a circuit
The control signal provided by the ECU to an actuator is most commonly a digital signal, which effectively switches the actuator on or off; this is achieved in most cases by making the ECU a part of the actuator electrical circuit. The ECU is therefore acting as a sophisticated switch that makes or breaks (switches on or off) the actuator circuit (Figures 1.48a and 1.48b).
As previously described (see the text about amplifiers in section 1.3.3), the ECU usually contains a final stage power transistor, which is effectively the actuator circuit switch. The low voltage signal from the ECU’s microprocessor simply controls the power transistor, which then replicates or copies the control signal. But because the power transistor is the switch within the actuator circuit, when the microprocessor control signal is on or off, it causes the power transistor to switch on or off, thus making or breaking the actuator circuit.
transistor to switch on and off the actuator circuit.
However, it is not just simple on or off control that is provided by the ECU: most control signals will cause the actuator to switch on or off for different lengths of time and at different speeds or frequencies (measured in hertz (Hz)).
When examining the control signal, the duration of the on or off period can be referred to as pulse width. It is however general practice that the pulse width refers to the on time only.
Figure 1.49a shows an on/off control signal where the on time or pulse width is 1⁄2second, and the off time is also 1⁄2second. In this case the frequency is 1 hertz, which means that the actuator is switched on and off once every second.
Figure 1.49b shows a similar signal, but the on time is 1⁄4second, with the off time being 3⁄4second. The frequency is therefore still 1 hertz but the on and off times are different.
Figure 1.49c shows a control signal with equal on and off times but the frequency is 10 hertz.
The completion of the on and off process is one complete cycle of operation or 1 cycle. Therefore, when the signal completes one on and one off pulse, this is also referred to as 1 cycle. If there are 10 cycles within one second, this is a frequency of 10 cycles per second, which is referred to as 10 hertz (10 Hz).
If the durations of the on and off times are the same, this is referred to as a duty cycle of 50%, i.e. the on time is 50% of one cycle. If however the on time is 1⁄4of the total cycle time then this is referred to as a duty cycle of 25%.
Figure 1.50a shows two control signals, each with a 50% duty cycle. Although the durations and frequencies of the two signals are different, the duty cycles are 50%
in both cases. Figure 1.50b shows two control signals, ECU/actuator control signals 33
Figure 1.48 Switching an actuator circuit a Normal switch controlling an actuator circuit
b ECU acting as the switch and controlling the actuator circuit.
Note that the power transistor in the ECU directly switches the circuit in response to the signal from the ECU microprocessor
Note that for most ECU controlled actuator circuits, the ECU (power transistor) forms part of the earth or return circuit (negative path). The positive path from the power supply (the battery) can be directly connected to the actuator or it may contain a switch such as an ignition switch. Fuses and relays are also generally connected into the positive side of the circuit. The ECU, which provides the controlling function, is therefore making and breaking (switching on and off) the earth or negative side of the circuit.
Most control signals provided by the ECU are therefore simple on/off pulses that cause the power
Figure 1.49 ECU control signal duration and frequency a ECU control signal with equal on and off duration of 1⁄2second and a frequency of 1 hertz
b ECU control signal with on duration of 1⁄4second and off duration of 3⁄4second but with a frequency that is still 1 hertz c ECU control signal with equal on and off duration, but with a frequency of 10 hertz
Figure 1.50 Duty cycles
a Control signals with 50% duty cycle b Control signals with 25% duty cycle
each with a 25% duty cycle, and again, although the durations and frequencies are different, the duty cycles are the same.
Important note: In the control signal examples illustrated, the off time is shown as the higher portion of the pulse, i.e. as a voltage level. The on time is therefore shown as zero volts. When a switch (in this case the ECU) is connected into the earth or negative path of an actuator circuit, the earth circuit will in fact be at zero volts when the circuit is switched on and at battery level voltage when the circuit is switched off. It is important to note when using test equipment, such as multimeters or oscilloscopes, that the measurements displayed may be the reverse of the expected readings, e.g. the duty cycle could be shown as 25% instead of 75%.
1.9.2 Using the signal to control the actuator
By understanding that the duration (pulse width), duty cycle and frequency of the control signal can be altered, it is possible to understand how an actuator can be controlled so that the task it performs can be varied. An
example is a fuel injector, which can be provided with a control signal where the duty cycle or pulse width varies. This means that the injector can be opened for longer or shorter time periods, thus allowing different quantities of fuel to be delivered to the engine.
The control signals affect how the actuator operates in different ways because the actuator is altering the current flow in the circuit. It was explained previously that the ECU is effectively an on/off switch, but this is only part of the whole story.
Altering the control signal duty cycle
Altering the duty cycle or pulse width has the effect of altering the average current flow and applied voltage in a circuit.
As an example, a simple 12 volt light circuit is switched on and off by a simple switch (Figure 1.51a).
When the circuit is switched on, the voltage on the power supply to the bulb will be 12 volts. Because the light bulb has a 2 ohm resistance, the current will therefore be 6 amps and the bulb will produce its maximum light output. However, when the switch is off, the voltage and current will both be zero and the bulb will produce no light.
If the light switch could be switched on and off very rapidly, for example at 100 times a second
(Figure 1.51b), and the duty cycle was 50%, i.e. the on and off pulses were both 50% of 1 cycle (of equal duration), the result would be that the light would be on for only half of the time. This means that the average voltage, the average current and the average amount of light produced by the bulb would also be 50% of the maximum value had the bulb been switched on all the time.
ECU/actuator control signals 35
Figure 1.51 Altering the duty cycles to affect the average voltage and current
a Simple 12 volt light circuit and switch
b Average voltage and current in a light circuit with a 50% duty cycle
c Average voltage and current in a light circuit with a 25% duty cycle
In this example where the bulb is rapidly switched on and off, the average voltage on the power supply circuit is 6 volts because this is 50% of the maximum supply voltage (50% of 12 volts). The average current is therefore 3 amps (50% of 6 amps). The total amount of light produced by the light bulb should in theory be 50% of the light that would have been produced if the bulb had been illuminated for all of the time.
If the duty cycle was changed so that the on time was only 25% of the total cycle (Figure 1.51c), then 12 volts would be available to power the bulb for only 25% of the time, but zero volts would be supplied for 75% of the time. The average voltage would therefore be 25% of 12 volts, i.e. 3 volts. The average current would therefore also be 25% of the maximum 6 amps, i.e. 1.5 amps. The average amount of light produced would therefore in theory also be 25% of the maximum.
If this same process of altering the duty cycle is applied to a control signal that is being used on an actuator such as an electric motor, it is then possible to alter the power produced by the motor. The same applies to any actuator control signal, where altering the duty cycle will influence the way in which the actuator functions.
Altering the control signal timing and frequency If the control signal consists of simple on and off pulses, an actuator will also be switched on and off. It is therefore possible to provide the on and off pulses at a specified time. A common example is when a fuel injector used on a modern fuel injection/engine management system is required to open at a certain time in the engine operating cycle. The injectors on some modern systems will open just before, or at the start of, the intake stroke (possibly just before or just as the intake valve opens). A sensor (usually the camshaft position sensor) is used by the ECU as a timing reference to calculate when the intake stroke is about to start, allowing it to provide the on pulse in the control signal at the right time.
The frequency of the control signal also affects how an actuator behaves. For instance, a simple solenoid could be used to open and close a small valve (which could be allowing fuel to pass through a pipe). If the control signal had a 50% duty cycle, and provided on and off pulses that occurred very slowly, e.g. every 10 seconds, the solenoid would open the valve for 10 seconds and close the valve for 10 seconds. Although this would regulate the flow of fuel in the pipe, it is not a very effective means of control. If, however, the control signal pulses occurred 100 times every second (100 hertz), this would mean that the solenoid would be trying to open and close 100 times a second. The solenoid would in fact adopt a half open position i.e. it would never reach the fully open or fully closed positions. Therefore, altering the duty cycle will affect the average opening time of a solenoid controlled valve, but it is more effective if the frequency is high (such as 100 hertz) than it is if the frequency is low.
Web links
Engine systems information www.bosch.com
www.sae.org www.imeche.org.uk www.picotech.com www.autotap.com www.visteon.com www.infineon.com
www.kvaser.com (follow CAN Education links)
Teaching/learning resources
Online learning material relating to powertrain systems:
www.auto-training.co.uk