On whom does the idle speed of the DC motor with separate excitation depend? How can this engine speed be practically obtained?
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Answer:
The idle speed control mode is used to prevent engine stall during idle. The goal is to allow the engine to idle at as low an RPM as possible yet keep the engine from running rough and stalling when power-consuming accessories, such as air-conditioning compressors and alternators, turn on.
The control mode selection logic switches to idle speed control when the throttle angle reaches its zero (completely closed) position as detected by a switch on the throttle that is closed and engine RPM falls below a minimum value. This condition often occurs when the vehicle is stationary. Idle speed is controlled by using an electronically controlled throttle bypass valve, as seen in Fig. 6.6, which allows air to flow around the throttle plate and produces the same effect as if the throttle had been slightly opened such that sufficient flows to maintain engine operation.
There are various schemes for operating a valve to introduce bypass air for idle control. One relatively common method for controlling the idle speed bypass air uses a special type of motor called a stepper motor. One stepper motor configuration consists of a rotor with permanent magnets and two sets of windings in the stator that is powered by separate driver circuits. The configuration of a stepper motor is similar to that of a brushless DC motor as explained in Chapter 5 (see Fig. 5.36). Such a motor can be operated in either direction by supplying pulses in the proper phase to the windings as explained in Chapter 5. This is advantageous for idle speed control since the controller can very precisely position the idle bypass valve by sending the proper number of pulses of the correct phasing.
A digital engine control computer can precisely determine the position of the valve in a number of ways. In one way, the computer can send sufficient pulses to close completely the valve when the ignition is first switched on. Then, it can open pulses (phased to open the valve) to a specified (known) position. The physical configuration for the idle speed control is depicted in Fig. 6.7A. A block diagram for an exemplary idle speed control is depicting Fig. 6.7B. The variables have the same notation as given in Chapter 4.
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