THE PID CONTROLLER — Part 1 •••••
simple and easy to construct.
The schematic has the same
physical layout as the block diagram.
Op-amp U1 is used as the summing
junction for the set-point and measured
motor position. The individual P, I,
and D functions are implemented by
U2, U3, and U4, respectively. Finally,
op-amp U5 sums the individual PID
terms. The P and I terms are inverted,
while the D term is not. Darlington
transistors have been added to U5 to
boost the current to a level sufficient
to drive the motor.
The individual P, I, and D components appear just as they were
presented earlier in this article. Each
of the terms has a variable resistor
to adjust its gains. The adjustment
(tuning) of this circuit is the topic for
Circle #99 on the Reader Service Card.
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Circle #104 on the Reader Service Card.
the next installment.
Component selection for this circuit
is not critical. The variable resistors
should be multiturn for ease of adjustment. General-purpose op-amps may
be used; however, U3 should be a FET
input type. The FET design is better
for the integrator, since it will not
self-charge the integrator capacitor. I
found a quad op-amp — such as the
LF347N — to be ideal for this application. Large capacitors are required for
the integrator and derivative circuits.
The large values necessitate that
electrolytic capacitors be used. The
electrolytic capacitor may be operated
as a non-polarized capacitor by
placing two capacitors in series, as
shown in the schematic.
A full schematic will appear in
next month’s installment. You may
download the CAD files (in Eagle)
from the Nuts & Volts website at
www.nutsvolts.com
Testing
Before we can test the PID circuit,
we need to know more about the
mechanical system. We need to know
how it responds to a command and
how the individual P, I, and D terms
interact. You will have to be patient
and wait for next month’s installment.
In the meantime, go ahead and
breadboard the circuit. You can use a
function generator to verify the individual stages. See how the individual
stages respond to sine, square, and
triangle waveforms. Remember to use
a low frequency — less than 10 Hz.
This frequency is approximately the
same as the servo motor system.
Stay tuned; next month, we will
learn how to tune the PID controller. We
will add additional circuitry to prevent a
condition called integral wind-up. Also,
keep a lookout for installment three,
where we will implement the PID on a
ZILOG Encore! microcontroller. NV
REFERENCE
Jacob, Michael.
Industrial Control Electronics.
New Jersey: Prentice Hall, 1988.
JANUARY 2005
