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Command
Description
AF
Analog Input for feedback
AZ
Analog Output for control
KP
Proportional Gain
KD
Derivative Gain
KI
Integral Gain
IL
Integrator Limit
DB
Deadband
CL
Control Loop Update Rate
PS
Commanded Setpoint
TE
Tell Error
AQ
Analog Input Range
DQ
Analog Output Range
*Note – All PID parameters are burnable except PS, DB, AQ, and DQ. If you issue a BN with the PID’s
enabled the default values for PS,DB,AQ, and DQ will be in effect upon power up.
To understand how a Process Control Loop works on the RIO, consider an example where it is
desirable to control the temperature of an oven. The key items needed to do this are a heater, a
temperature sensor, the oven itself, and a RIO unit to control the process. As shown in the diagram
below, the heating element is coupled to the “System” which in this case is the oven. The
temperature sensor provides feedback to the RIO in the form of an analog input. The RIO unit then
compares the desired set-point (entered by the PS command) with the temperature sensor. The
difference between the two is called the error “E”. The error goes through a PID digital filter and
then through a Digital to Analog Converter (DAC) which outputs a control voltage to the heater to
close the loop.
The example program below uses analog input 0 as the feedback from the temperature sensor and
analog output 0 as the control voltage to the heater. An update rate of 25msec was set using the CL
command, but a slower update rate could have been chosen due to the slow nature of temperature
response. The PID values entered were experimentally found to provide optimum results based on
the system. The desired set-point was chosen as 1V. A dead-band of 0.1V was added in order to
prevent the system from responding to minor disturbances of the sensor.
#PCL
CL 25;
'25msec update rate
AF 0;
'analog input 0 as feedback
AZ 0;
'analog output 0 as control
KP 1;
'proportional gain to 1
KD 10;
'derivative gain to 10
KI 0.5;
'integral gain to 0.5
DB 0.1;
'deadband of 0.1V
PS 1.8;
'set-point at 1.8V
Chapter 5 Programming ▫ 80 RIO-47xxx Rev. 1.0r
Figure 5.1: Process Control Loop