NEWPORT Technical Reference - Tuning a PID (Three-Mode) Controller
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Controller
Operation
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Proportional
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Temperature
Control
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On/Off Control
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PID
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Controller
Operation
There
are three common types of Temperature/process controllers:
ON/OFF, PROPORTIONAL, and PID (PROPORTIONAL
INTEGRAL DERIVATIVE).
On/Off
CONTROL
An on-off controller is the simplest form of temperature control
device. The output from the device is either on or off, with
no middle state. An on/off controller will switch the output
only when the temperature crosses the setpoint. For heating
control, the output is on when the temperature is below the
setpoint, and off above the setpoint.
Although
capable of more complex control functions, the NEWPORT microprocessor
based MICRO-INFINITY ® AUTOTUNE PID 1/16 DIN Controller
can be operated as a simple On/Off Controller. The NEWPORT INFINITY
® series and INFINITY C ® series of highly accurate
microprocessor based digital panel meters can all function as
simple On/Off controllers.
With
simple On/Off control, since the temperature crosses the setpoint
to change the output state, the process temperature will be
cycling continually, going from below setpoint to above, and
back below. In cases where this cycling occurs rapidly, and
to prevent damage to contactors and valves, an on-off differential,
or "hysteresis," is added to the controller operations. This
differential requires that the temperature exceed setpoint by
a certain amount before the output will turn off or on again.
On-off differential prevents the output from "chattering" or
fast, continual switching if the temperature cycling above and
below setpoint occur very rapidly.
"On-Off"
is the most commonly used form of control, and for most applications
it is perfectly adequate. It's used where a precise control
is not necessary, in systems which cannot handle the energy
being turned on and off frequently, and where the mass of the
system is so great that temperatures change extremely slowly.
Backup
alarms are typically controlled with "On-Off" relays. One special
type of on-off control used for alarm is a limit controller.
This controller uses a latching relay, which must be manually
reset, and is used to shut down a process when a certain temperature
is reached.
Proportional
Control
Proportional control is designed to eliminate the cycling above
and below the setpoints associated with On-Off control. A proportional
controller decreases the average power being supplied to a heater
for example, as the temperature approaches setpoint. This has
the effect of slowing down the heater, so that it will not overshoot
the setpoint, but will approach the setpoint and maintain a
stable temperature.
This proportioning action can be accomplished by different methods.
One method is with an analog control output such as a 4-20 mA
output controlling a valve or motor for example. With this system,
with a 4 mA signal from the controller, the valve would be fully
closed, with 12 mA open halfway, and with 20 mA fully open.
Another
method is "time proportioning" i.e. turning the output on and
off for short intervals to vary the ratio of "on" time to "off"
time to control the temperature or process.
With
the analog output option, the NEWPORT INFINITY ® series
and INFINITY C ® series of 1/8 DIN digital panel meters
can function as proportional controllers. In addition, NEWPORT
offers models of "INFINITY C" for thermocouple and RTD inputs
featuring Time-Proportioning Control with its built in mechanical
relays.
With
proportional control, the proportioning action occurs within
a "proportional band" around the setpoint temperature. Outside
this band, the controller functions as an on-off unit, with
the output either fully on (below the band) or fully off (above
the band). However, within the band, the output is turned on
and off in the ratio of the measurement difference from the
setpoint. At the setpoint (the midpoint of the proportional
band), the output on:off ratio is 1:1; that is, the on-time
and off-time are equal. If the temperature is further from the
setpoint, the on- and off-times vary in proportion to the temperature
difference. If the temperature is below setpoint, the output
will be on longer; if the temperature is too high, the output
will be off longer.
The
proportional band is usually expressed as a percent of full
scale, or degrees. It may also be referred to as gain, which
is the reciprocal of the band. Note, that in time proportioning
control, full power is applied to the heater, but cycled on
and off, so the average time is varied. In most units, the cycle
time and/or proportional band are adjustable, so that the controller
may be better matched to a particular process.
One
of the advantages of proportional control is the simplicity
of operation. However, the proportional controller will generally
require the operator to manually "tune" the process, i.e. to
make a small adjustment (manual reset) to bring the temperature
to setpoint on initial startup, or if the process conditions
change significantly.
Systems
that are subject to wide temperature cycling need proportional
control. Depending on the precision required, some processes
may require full "PID" control.
PID
(Proportional Integral Derivative)
Processes with long time lags and large maximum rate of
rise (e.g., a heat exchanger), require wide proportional bands
to eliminate oscillation. The wide band can result in large
offsets with changes in the load. To eliminate these offsets,
automatic reset (integral) can be used. Derivative (rate) action
can be used on processes with long time delays, to speed recovery
after a process disturbance.
The
most sophisticated form of discrete control available today
combines PROPORTIONAL with INTEGRAL and DERIVATIVE or PID .
The
NEWPORT MICRO-INFINITY® is a full function "Autotune" (or
self-tuning) PID controller which combines proportional control
with two additional adjustments, which help the unit automatically
compensate to changes in the system. These adjustments, integral
and derivative, are expressed in time-based units; they are
also referred to by their reciprocals, RESET and RATE, respectively.
The proportional, integral and derivative terms must be individually
adjusted or "tuned" to a particular system.
It
provides the most accurate and stable control of the three controller
types, and is best used in systems which have a relatively small
mass, those which react quickly to changes in energy added to
the process. It is recommended in systems where the load changes
often, and the controller is expected to compensate automatically
due to frequent changes in setpoint, the amount of energy available,
or the mass to be controlled.
The
"autotune" or self-tuning function means that the MICRO-INFINITY
will automatically calculate the proper proportional band, rate
and reset values for precise control.
Temperature
Control
Tuning a PID (Three-Mode) Controller
Tuning a temperature controller involves setting the proportional,
integral, and derivative values to get the best possible control
for a particular process. If the controller does not include
an autotune algorithm or the autotune algorithm does not provide
adequate control for the particular application, the unit must
then be tuned using a trial and error method.
The
following is a tuning procedure for the NEWPORT® MICRO-INFINITY
® controller. It can be applied to other controllers as
well. There are other tuning procedures which can also be used,
but they all use a similar trial and error method. Note that
if the controller uses a mechanical relay (rather than a solid
state relay) a longer cycle time (10 seconds) is recommended
when starting out.
The following
definitions may be needed:
- Cycle
time — Also known as duty cycle; the total length of time
for the controller to complete one on/off cycle. Example:
with a 20 second cycle time, an on time of 10 seconds and
an off time of 10 seconds represents a 50 percent power
output. The controller will cycle on and off while within
the proportional band.
- Proportional
band — A temperature band expressed in degrees (if the input
is temperature), or counts (if the input is process) from
the set point in which the controllers' proportioning action
takes place. The wider the proportional band the greater
the area around the setpoint in which the proportional action
takes place. It is sometimes referred to as gain, which
is the reciprocal of proportional band.
- Integral,
also known as reset, is a function which adjusts the proportional
bandwidth with respect to the setpoint, to compensate for
offset (droop) from setpoint, that is, it adjusts the controlled
temperature to setpoint after the system stabilizes.
- Derivative,
also known as rate, senses the rate of rise or fall of system
temperature and automatically adjusts the proportional band
to minimize overshoot or undershoot.
A
PID (three-mode) controller is capable of exceptional control
stability when properly tuned and used. The operator can achieve
the fastest response time and smallest overshoot by following
these instructions carefully. The information for tuning this
three mode controller may be different from other controller
tuning procedures. Normally an AUTO PID tuning feature will
eliminate the necessity to use this manual tuning procedure
for the primary output, however, adjustments to the AUTO PID
values may be made if desired.
After
the controller is installed and wired:
1.
Apply power to the controller.
2.
Disable the control outputs. (Push enter twice)
3.
Program the controller for the correct input type (See Quick
Start Manual).
4.
Enter desired value for setpoint 1
5.
For time proportional relay output, set the cycle time to 10
seconds or greater.
- Press MENU until OUT1 is displayed.
- Press ENTER to access control output 1 submenu.
- Press MENU until cycle time is displayed.
- Press ENTER to access cycle time setting.
- Use MAX and MIN to set new cycle time value.
- Press ENTER when finished.
6. Set
prop band in degrees to 5% of setpoint 1. (If setpoint 1 = 100,
enter 0005. Prop band = 95 to 110). Note: Micro-Infinity takes
degrees ( if input is temperature) / counts (if input is process)
as Proportional Band value.
- If
ID is disabled: - Press MENU 1 time from run mode to get
to setpoint 1; confirm SP1 LED is flashing. - Use MAX and
MIN to set new setpoint value.
- If
ID is enabled: - Press MENU until Set Point is displayed.
- Press ENTER to access setpoint 1 setting. - Use MAX and
MIN to set new setpoint value.
- Press
ENTER to stored setting when finished.
7.
Set reset and rate to 0.
- Press
MENU until OUT1 is displayed.
- Press
ENTER to access control output 1 submenu.
- Press
MENU until autopid is displayed.
- Press
ENTER to access autopid setting.
- Press
MAX to disable autopid; press ENTER when done.
- Press
MENU until Reset Setup is displayed.
- Press
ENTER to access Reset setting.
- Use
MAX and MIN to set Reset to 0; press ENTER to store the
new setting.
- Display
advances to Rate Setup.
- Press
ENTER to access Rate setting.
- Use
MAX and MIN to set Rate to 0; press ENTER to store the new
setting.
- Press
MIN 2 times to return to run-mode. Should the unit reset,
press ENTER twice to put it into stand-by mode.
NOTE:
On units with dual three-mode outputs, the primary and secondary
proportional parameter is independently set and may be tuned
separately. The procedure used in this section is for a HEATING
primary output. A similar procedure may be used for a primary
COOLING output or a secondary COOLING output.
A.
TUNING OUTPUTS FOR HEATING CONTROL
- Enable
the OUTPUT (Press Enter) and start the process.
- The
process should be run at a setpoint that will allow the
temperature to stabilize with heat input required.
- With
RATE and RESET turned OFF, the temperature will stabilize
with a steady state deviation, or droop, between the setpoint
and the actual temperature. Carefully note whether or not
there are regular cycles or oscillations in this temperature
by observing the measurement on the display. (An oscillation
may be as long as 30 minutes). 3. The tuning procedure
is easier to follow if you use a recorder to monitor the
process temperature.
- If
there are no regular oscillations in the temperature, divide
the PB by 2 (see Figure 1). Allow the process to stabilize
and check for temperature oscillations. If there are still
no oscillations, divide the PB by 2 again. Repeat until
cycles or oscillations are obtained. Proceed to Step 5.
- If
oscillations are observed immediately, multiply the PB by
2. Observe the resulting temperature for several minutes.
If the oscillations continue, increase the PB by factors
of 2 until the oscillations stop.
- The
PB is now very near its critical setting. Carefully increase
or decrease the PB setting until cycles or oscillations
just appear in the temperature recording.
- If
no oscillations occur in the process temperature even at
the minimum PB setting skip Steps 6 through 15 below and
proceed to paragraph B.
- Read
the steady-state deviation, or droop, between setpoint and
actual temperature with the "critical" PB setting you have
achieved. (Because the temperature is cycling a bit, use
the average temperature.)
- Measure
the oscillation time, in minutes, between neighboring peaks
or valleys (see Figure 2). This is most easily accomplished
with a chart recorder, but a measurement can be read at
one minute intervals to obtain the timing.
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Now,
increase the PB setting until the temperature deviation,
or droop, increases 65%. The desired final temperature deviation
can be calculated by multiplying the initial temperature
deviation achieved with the CRITICAL PB setting by 1.65
(see Figure 3). Try several trial-and-error settings of
the PB control until the desired final temperature deviation
is achieved.
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You
have now completed all the necessary measurements to obtain
optimum performance from the Controller. Only two more adjustments
are required — RATE and RESET.
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Using
the oscillation time measured in Step 7, calculate the value
for RESET in repeats per minutes as follows:
RESET
= (5/8 ) x To
Where
To = Oscillation Time in Seconds. Enter the value for RESET
in OUT 1 (follow the same procedure as outlined in preparation
section, step 7 to set RESET).
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Again
using the oscillation time measured in Step 7, calculate
the value for RATE in minutes as follows:
RATE
= To 10
Where
T = Oscillation Time in Seconds. Enter this value for RATE
in OUT 1 (follow the same procedure as outline in preparation
section, step 7 to set RATE).
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If
overshoot occurred, it can be reduced by increasing the
proportional band and the RESET time. When changes are made
in the RESET value, a corresponding change should also be
made in the RATE adjustment so that the RATE value is equal
to:
RATE
= (4/25) x RESET
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Several
setpoint changes and consequent Prop Band, RESET and RATE
time adjustments may be required to obtain the proper balance
between "RESPONSE TIME" to a system upset and "SETTLING
TIME". In general, fast response is accompanied by larger
overshoot and consequently shorter time for the process
to "SETTLE OUT". Conversely, if the response is slower,
the process tends to slide into the final value with little
or no overshoot. The requirements of the system dictate
which action is desired.
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When
satisfactory tuning has been achieved, the cycle time should
be increased to save contactor life (applies to units with
time proportioning outputs only. Increase the cycle time
as much as possible without causing oscillations in the
measurement due to load cycling.
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Proceed
to Section C.
B.
TUNING PROCEDURE WHEN NO OSCILLATIONS ARE OBSERVED
- Measure
the steady-state deviation, or droop, between setpoint and
actual temperature with minimum PB setting.
- Increase
the PB setting until the temperature deviation (droop) increases
65%.
- Set
the RESET in OUT1 to a low value (50 secs). Set the RATE
to zero (0 secs). At this point, the measurement should
stabilize at the setpoint temperature due to reset action.
- Since
we were not able to determine a critical oscillation time,
the optimum settings of the reset and rate adjustments must
be determined by trial and error. After the temperature
has stabilized at setpoint, increase the setpoint temperature
setting by 10 degrees. Observe the overshoot associated
with the rise in actual temperature. Then return the setpoint
setting to its original value and again observe the overshoot
associated with the actual temperature change.
- Excessive
overshoot implies that the Prop Band and/or RESET are set
too low, and/or RATE value is set too high. Overdamped response
(no overshoot) implies that the Prop Band and/or RESET is
set too high, and/or RATE value is set too low. Refer to
Figure 4. Where improved performance is required, change
one tuning parameter at a time and observe its effect on
performance when the setpoint is changed. Make incremental
changes in the parameters until the performance is optimized.
Figure 4 Setting RESET and/or RATE PV
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When
satisfactory tuning has been achieved, the cycle time should
be increased to save contactor life (applies to units with
time proportioning outputs only.). Increase the cycle time
as much as possible without causing oscillations in the
measurement due to load cycling.
C.
TUNING THE PRIMARY OUTPUT FOR COOLING CONTROL
The
same procedure is used as defined for heating. The process should
be run at a setpoint that requires cooling control before the
temperature will stabilize.
D.
SIMPLIFIED TUNING PROCEDURE FOR PID CONTROLLERS
The
following procedure is a graphical technique of analyzing a
process response curve to a step input. It is much easier with
a strip chart recorder reading the process variable (PV).
- Starting
from a cold start (PV at ambient), apply full power to the
process without the controller in the loop, i.e., open loop.
Record this starting time.
- After
some delay (for heat to reach the sensor), the PV will start
to rise. After more of a delay, the PV will reach a maximum
rate of change (slope). Record the time that this maximum
slope occurs, and the PV at which it occurs. Record the
maximum slope in degrees per minute. Turn off system power.
- Draw
a line from the point of maximum slope back to the ambient
temperature axis to obtain the lumped system time delay
Td (see Figure 5) . The time delay may also be obtained
by the equation: Td = time to max. slope – (PV at max. slope
– Ambient)/max. slope
- Apply
the following equations to yield the PID parameters: Pr.
Band = Td x max. slope Reset = Td/0.4 secs. Rate = 0.4 x
Td minutes
- Restart
the system and bring the process to setpoint with the controller
in the loop and observe response. If the response has too
much overshoot, or is oscillating, then the PID parameters
can be changed (slightly, one at a time, and observing process
response) in the following directions: 5. Refer to
figure 4, vary the proportional band, the Reset value, and
the Rate value to achieve best performance.
Example:
The chart recording in Figure 5 was obtained by applying full
power to an oven. The chart scales are 10°F/cm, and 5 min/cm.
The controller range is -200 - 900°F, or a span of 1100°F.
Maximum slope = 18°F/5 minutes = 3.6°F/minutes. Time
delay = Td = approximately 7 minutes.
Proportional
Band = 7 minutes x 3.6°F / minutes = 25.2°F.
Note:
Prop Band in Micro-Infinity is set in degrees/ counts. Reset
= 7/.04 minutes = 17.5 min. or 1050 secs. Note: Reset in Micro-Infinity
is specified in seconds Rate = 0.4 x 7 minutes = 2.8 min. or
168 secs.
Set
Prop Band to: 025.0; Set Reset to: 1050 Set Rate to: 168 Follow
step 6 and 7 of the preparation section to set new values for
Prop Band, Reset, and Rate.
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