0195136047.pdf

812 BASIC CONTROL SYSTEMS

in which the chip itself is programmed to perform specific functions. Digital watches and auto

fuel-injection systems are examples. With microprocessor control, not only the same hardware

can be used to perform a variety of tasks by reprogramming, but alsodistributed computation

(which results in less expensive and more reliable operation than centralized control) is possible

with interconnection and intercommunication of several microprocessors.

Figure 16.3.9 shows a block diagram of a microprocessor-controlled dc motor system, in

which the controlled process consists of a dc motor, the load, and the power amplifier.

The block diagram of Figure 16.3.10 illustrates a typical microprocessor system used to

implement the digital PI controller. It would be simple to include the derivative operation to

implement the PID controller. While an analog timer is shown in Figure 16.3.10 to determine

the start of the next sampling period, a software timing loop can be used to keep track of when

Tseconds have elapsed. The output pulse, once everyTseconds, is applied to the interrupt

line of the microprocessor. This will cause the processor to execute the interrupt routine to

output the next value of the control,u[(k+ 1 )T], which is sent to the D/A converter, whose

output in turn controls the power amplifier. The timing pulse from the timer is also sent to the

“sample” command line, thereby triggering the sample-and-hold circuitry; the motor velocity

ω(t) is sampled and held constant for one sampling period. The value ofω(kT) is then converted

to anN-bit binary number by the A/D circuitry. The microprocessor is signaled via “data ready”

line (which may be attached to the interrupt line of the microprocessor) that the sampled data

have been converted. The second interrupt will cause the processor to read in the value of

D/A

converter

Dc motor and

power amplifier

Feedback signal

from motor

A/D

converter

Microprocessor-

based

controller

Output, updated

every T seconds

Desired

system

motion Sampler with

sampling period T

Figure 16.3.9Block diagram of a microprocessor-controlled dc motor system.

D/A

converter

Power

amplifier

Input

port

Sample-

and-hold

A/D

converter

ω(t)

Accumulator (ac)

Data lines

Control

signal

"Data ready" line

"Sample"

command

line

Motor

Timer

Tachometer

Microprocessor

Output

port

Figure 16.3.10Block diagram of discretized proportional and integral (PI) controller.