error. In the circuit in Figure 7, the integrator is performing
the same task. It is integrating the error. It then provides a
correction signal to the motor. We can summarize integral
action in a few sentences:
1. An error must be present!
2. The integral section accumulates the error. A small error
can become a large correction over a period of time.
3. As the error is accumulated, the motor is forced to
correct the error.
4. Finally, the integrator will overshoot the set-point. It must
produce an error opposite of the original in order to
discharge the capacitor.
The final PID component is the derivative. Recall that
the output of the differentiator was proportional to the
slope of a wave. The same type of action is occurring in
this circuit. When the motor starts to turn, the voltage
measured by the resistor will be increasing or decreasing.
If we have a voltage changing over a period of time, we
have a ramp! The slope of this ramp changes with the
speed of the motor. If the motor is going fast, the slope is
high (i.e., voltage is changing fast for a given amount of
time). Consequently, the output of the derivative stage will
be high. The differentiator has the following attributes:
1. The motor must be moving!
2. The differentiator will have a high output voltage when
the motor is moving fast and a low voltage when the motor
is moving slow.
3. This signal is applied in such a way as to slow down the
4. If the motor is not moving, the differentiator has 0
The connections for the differentiator are different than
the proportional and integral sections. The differentiator
receives its input directly from the resistor. It, therefore, measures only the speed at which the motor is moving. It does not
care about the set-point. This is done to prevent large
derivative drive signals when the set-point is changed. Again,
the differentiator only responds to the speed of the motor.
Figure 8 contains a simplified schematic of a servo
motor PID control system. This schematic is an adaptation
of the PID controller presented by Professor Jacob in his
book, Industrial Control Electronics. This type of system
has the advantage of easy tuning. This circuit is also
16 bit analog development
with a PIC 16F876
RJL Systems has a 25 year history in medical devices
and is proud to announce an analog development board
for engineers and hobbyists who demand accurate
signal processing for display and communications.
Bipolar 16 bit ADC (+/- 1.5000 volts FS 100K SPS)
Isolated 8 channel analog mux (single ended)
Isolated power supplies (+/- 5.0V analog and 5.0V digital)
Isolated 9 digital PIC I/O pins with buffered LED indicators
Hard wired development area with access to all power
Isolated RS-232C communications (115.2 Kbps max)
4 line X 20 character (blue) display with white LED
In-circuit programming and debugging (RJ- 12 connector)
CCS PIC-C sample code and schematics provided (CD ROM)
Operates from any battery or bench 6 to 12 VDC power supply
Screw terminals for convenient wiring to external devices
RJL Systems, Inc.
33955 Harper Ave.
Clinton Twp, MI 48035