User Guide

Command Syntax
Siemens Building Technologies, Inc. 3-67
ig Integral gain. The contribution of integral control action to the
total control action of the loop. Integral action minimizes offset
(the difference between the process variable and the
setpoint). Adding integral gain can increase the time required
to tune the loop.
When using integral gain, be sure to reduce the proportional
gain so that the total gain of the loop is not high enough to
cause instability and cycling. Begin with a low integral gain
and increase it slowly, if required. A recommended starting
point is 2% of the proportional gain.
• This parameter can be a point name, integer,
decimal, or local variable.
• If using an integral gain, use the following calculation:
ig = pg × .02
• If not using an integral gain, set ig to 0.
dg Derivative gain. The time rate of response of the control
system. Derivative gain is usually applied to fast responding
systems. Adding derivative gain can increase the time
required to tune the loop.
• This parameter is entered as an integer, decimal,
point name, or local variable.
• When derivative gain is not used, enter a zero.
st Sample time. How frequently (in seconds) the process
variable (pv) is sampled.
• The minimum sampling time allowed is 1 second.
• This parameter is entered as an integer, decimal,
point name, or local variable.
bias Bias is the value (in engineering units) of the control output
(cv), in a proportional only loop, when the measured variable
(pv) equals the setpoint (sp).
• This parameter entered as a decimal number, integer,
point name, or logical point.
• The bias value should always be between the high
and low value.
Bias is calculated by adding 50% of the output control span to
the low limit. For example, you would do the following to
calculate the bias for a valve with a 3 to 8 psi spring range:
• Find one-half of the spring range. (5 psi spring range
÷ 2 = 2.5)
• Add that value to the low limit of the valve (3 psi).
• The bias for this device is 5.5 psi.
APOGEE PPCL User’s Manual
3-68 Siemens Building Technologies, Inc.
lo Low limit of the loop output. The low limit should be set to
match the low end range of the controlled device.
• This parameter is entered as a decimal, integer, point
name, or local variable.
hi High limit of the loop output. The high limit should be set to
match the high end range of the controlled device.
• This parameter is entered as a decimal, integer, point
name, or local variable.
0 Not used. Enter zero.
Example
2000 C CONTROL LOOP STATISTICS
2002 C DIRECT CONTROL LOOP
2004 C INPUT = RM100 OUTPUT = HVALVE
2006 C SETPOINT = HSETPT
2008 C PROPORTIONAL GAIN = PGAIN-NO I OR D GAINS
2010 C SAMPLE TIME = 1 SECOND
2012 C BIAS = 5.5
2014 C LOW = 3.0
2016 C HIGH = 9.0
2018 LOOP(0,RM100,HVALVE,HSETPT,PGAIN,0,0,1,5.5,3.0,
9.0,0)
Use
This command performs closed loop control by using any
combination of proportional, integral, and derivative control actions in
either direct or reverse acting modes.
LOOP monitors an input point (process variable), compares it with a
desired value (the setpoint), and adjusts an output (control variable)
to bring the input closer to the setpoint.
• The LOOP command is the software counterpart to a pneumatic
receiver controller.
• Anti-windup is automatically prevented for the integral action
once the high or low limit is reached.
See also
ADAPTM, ADAPTS