Page 73 - Mechatronics with Experiments

CLOSED LOOP CONTROL 59

Clock
On clock interrupt
T

Foreground program
Background program
main() Control algorithm
1. Determine desired response
2. A/D conversion of sensor signal
check operator I/O 3. Calculate control : u
check error conditions 4. D/A conversion of control: u
check machine I/O
5. Return to background program

Mesured y(kT) Mesured y(kT)

t t

Control u(kT) Control u(kT)
T

t t
kT t 1 t 1 - for step(1)
t - for step(2)
t 2 2
t - for step(3)
t 3 3
t 4 t - for step(4)
4
FIGURE 2.10: Effective time delay due to signal conversion, processing, and update methods
in a digital control system.

as soon as possible. In the foreground program, it handles the closed loop control update
for the current sampling period. This involves the following tasks,

1. determine the desired response (sample from A/D if necessary),
2. A/D sample of the sensor signal,
3. calculate the control action,
4. send the control action to D/A,
5. return to the background program.

This implementation and timing of the sequence of operations are shown in Figure 2.10 on
the left-hand side. Note that A/D conversion is an iterative process and the conversion times
may vary from one cycle to the next. As a result, the effective update period for control