Closed-loop Control of DC Drivers直流驱动的闭环控制A basic scheme of the closed-loop speed control system employing current limit control, also known as parallel current control, is shown in Fig. 1-5B-1. ω* sets the speed reference. A signal proportional to the motor speed is obtained from the speed sensor. The speed sensor output is filtered to remove the AC ripple and compared with the speed reference.The speed error is processed through a speed controller. The output of the speed controller u c adjusts the rectifier firing angleαto make the actual speed close to the reference speed. The speed controller is usually a PI (proportional and integral) controller and serves three purposes—stabilizes the drive and adjusts the damping ratio at the desired value, makes the steady-state speed-error close to zero by integral action, and filters out noise again due to the integral action. The drive employs current limit control, the purpose of which is to prevent the current from exceeding safe values. As long as IA<Ix, where, Ix is the maximum permission value of IA, the current control loop does not affect the drive operation. If IA exceeds Ix, even by a small amount, a large output signal is produced by the threshold circuit, the current control, and the speed error iscorrected essential at a constant current equal to the maximum permission value.When the speed reaches close to the desired value, IA falls below IX, the current control goes out of action and speed controlled takes over. Thus in this scheme, at any given time the operation of the drive is mainly controlled either by the speed control loop or the current control loop, and hence it is also called parallel current control.Another scheme of closed-loop speed control is shown in Fig.1-5B-2. It employs an inner current control loop within an outer speed loop. The output of the current controller uc adjusts the converter firing angle such that the actual speed is brought to a value set by the speed commendωm*.Any positive speed error, cause by either an increase in the load torque, produces a higher current reference I a*. The motor accelerates due to an increase in I a, to correct the speed error and finally settles at a new I a* which makes the motor torque equal to the load torque and the speed error closed to zero.For any large positive speed error, the current limiter saturates and the current reference I a* is limited to a value I am*, and the drive current is not allowed to exceed the maximum permission value. The speed error is corrected at the maximum permissible armaturecurrent until the speed error becomes small and the current limiter comes out of saturation. Now the speed error is corrected with IA less than the permissible value.A negative speed error will set the current reference IA at a negative value. Since the motor current cannot reverse, a negative IA is of no use. It will however “charge” the PI controller. When the speed error becomes positive the “charged” PI controller will take a longer time to respond, causing unnecessary delay in the control action. The current limiter is therefore arranged to set a zero-current reference for negative speed errors.Since the speed control loop and the current control loop are in cascade, the inner current control is also known as cascade control. It is also called current guided control. It is more commonly used than the current-limit control because of the following advantages: 1.It provides faster response to any supply voltage disturbance.This can be explain by considering the response of two drives toa decrease in the supply voltage. A decrease in the supplyvoltage reduces the motor current and torque. In the current-limit control, the speed falls because the motor torque is less than the load torque that has not changed. The resulting speed error is brought to the original value by setting the rectifier firing angle at a lower value. In the case of inner currentcontrol, the decrease in motor current, due to the decrease in the supply voltage, produces a current error which changes the rectifier firing angle to bring the armature current back to the original value. The transient response is now governed by the electrical time constant of the motor. Since the electrical time constant of a drive is much smaller compared to the mechanical time constant, the inner current control provides a faster response to the supply voltage disturbance.2.For certain firing schemes, the rectifier and the control circuittogether have a constant gain under continuous of 5 percent.Under discontinuous conduction, the gain reduces. The higher the reduction is in the conduction angel, the greater the reduction is in the gain. The drive response becomes sluggish in discontinuous conduction and progressively deteriorates as the conduction angle reduces. If an attempt is made to design the drive for discontinuous conduction current control loop providesa closed loop around the rectifier and the control circuit, andtransient response of the drive with the inner current loop is superior to that with the current-limit control.3.In the current-limit control, the current must first exceed thepermissible value before the current-limit action can be initiated.Since the firing angle can be changed only at discrete intervals,substantial current overshoot can occur before the current limiting becomes effective.Small motors are more tolerant to high transient current. Therefore, to obtain a fast transient response is then needed only for abnormal values of current. In such cases because of the simplicity, current-limit control is employed.Both the schemes have different responses for the increase and decrease in the speed command. A decrease in speed command at the most can make the motor torque zero; it cannot be reversed as braking is not possible. The drive decelerates mainly due to the load torque. When load torque is low, the response to a decrease in the speed command will be slow. These drives are therefore suitable for application with large load torque, such as paper and printing machines, pumps, and blowers.。
