PID Control with
14. November 2008
Installing the PID Control Toolkit, also named Control Toolkit,
or the Real-Time Module adds the PID Palette
to the .
It is also avaible on the in LabVIEW.
The PID Palette has a number of functions for PID control applications.
It seems to me to be a need for some guidelines - both for
students and engineers - for using the PID control function(s) on the PID
Palette correctly. The purpose of this document is to give such guidelines.
This tutorial is based on LabVIEW 8.5.
Let us start with taking a look at the PID Palette.
The PID Palette
The figure below shows the functions of the PID Palette.
The PID Palette
A few comments to the functions on the Control Palette:
The PID function implements
a PID controller function on the summation (or parallel) form. The PID
function implements anti-windup. It has no lowpass filtering in the derivative
term, so the derivative term may give very high amplification of high-frequent
measurement noise. It can not be set in manual mode, making it a little
difficult to use in tuning and startup, and therefore I do not use this PID
function, but the next one.
The PID Advanced function
implements a PID controller function on the summation form, i.e. the P, I, and
D terms are summed, and with controller parameters Kc, Ti and Td. (More
precisely, the summation form realizes the so-called ideal form of the PID
controller function. The other summation form is the parallell form with
controller parameters Kc, Ki = Kc/Ti, and Kd = Kc*Td.) The PID function
implements anti-windup, and it can be set in manual mode. Options are
nonlinear gain and reduced setpoint weight in the proportional term (these
options are not well documented). It has no lowpass filtering in the
derivative term. (In applications I always use the
PID Advanced function, not the PID function, due to the lack of the manual mode option
in the latter.)
The PID Autotuning function
implements an autotuner based on the Relay method. (In the tuning phase a
Relay function acts as an On/Off controller, causing sustained oscilllations
in the control loop. Information about these oscillations are used to
calculate proper PID settings.)
The PID Setpoint Profile function
can be used to define a profile of the setpoint time function based on
piecewise constant and/or linear increasing/decreasing, i.e. ramped, time
functions. The ramps provides a smooth change between two setpoint values.
The PID Control Input Filter
function is a 5'th order FIR filter which can be used to smooth the
measurement signal. Of course, other filtering functions can be used, too, e.g.
the Butterworth Filter PtByPt function on the .
The PID Gain Schedule function
implements gain scheduling of the PID parameters based on a piecewise constant
interpolation of a given set of PID settings. You can use as many different
sets of PID settings as you want in the gain (or PID parameter) schedule.
The PID Output Rate Limiter
function limits the rate of change of the PID controller output, i.e.
the calculated control signal.
The PID Lead-Lag function
implements a transfer function with a first order numerator and a first order
denominator. It can be used in feedforward control, and possibly in a
decoupler in a multivariable controller.
The PID EGU to % function
can be used to scale a measurement signal from engineering unit to percent.
The PID % to EGU function
can be used to scale the calculated PID control signal from percent to
engineering unit, e.g. ampere or voltage.
Using the PID Advanced function
Here the PID Advanced function is described, not the PID function
which seems to be just a simpler version of the PID Advanced, lacking the
important feature of setting the controller in manual mode.
Below is the Front panel and the Block diagram of pid_control_labview.vi,
which is in the zip-file
pid_control_labview.zip (together with the lowpass filter
timeconstant_lowpass_filter.vi) which implements a
PID speed control loop for a simulated motor having gain 0.5 and time-constant 2
sec (the model is represented with a transfer function). This model includes the
speed sensor. In the simulator random measurement noise is included to make the
simulator more realistic. This simulator is ready to run if you have the LabVIEW
Simulation Module installed. Click on the figures to see them in full sizes.
Front panel of pid_control_labview.vi
(which is in the zip-file
Block diagram of pid_control_labview.vi
(which is in the zip-file
In Case Structure - Selecting simulated or real process
in the upper-right part of the block diagram the True case can be as shown in
the figure below (in the downloadable pid_control_labview.vi the True case
does not contain the DAQmx Write and DAQmx Read functions).
Here are comments to pid_control_labview.vi:
- On the PID Advanced function: Auto/Man? is used to set the PID
controller in either manual mode or automatic mode.
- u_man is the value of the
control signal when the controller is in Manual mode. The value has no effect
when the controller is in Automatic mode.
- Output range (a cluster) defines the
maximum and minimum value of the PID control signal. It is
important to set these values correctly, otherwise the important anti
windup feature will not work properly.
- Reinitialize controller is used to set the
internal variables of the PID function back to the default values. For
example, the integral term is reset.
- Setpoint is the desired value of
the process measurement signal.
- Sepoint range (a cluster) defines the
maximum and minimum value of the setpoint. It is important to set these values correctly, since they are
used in the implementation of the anti windup feature.
- PID gains (a cluster) contains the
three PID parameters. The input to the PID Advanced function assumes that Ti
(integral time) and Td (derivative time) are in minutes. However, in many
applications it is more convenient to let the user adjust Ti and Td
in seconds. The Block diagram implements the transformation from
seconds to minutes using the Cluster
PID Scaling from Sec to Min together with a Divide function. If you want to
keep minute as time unit just remove the latter cluster and the Divide function,
and wire PID gains directly to the PID
- Time step Ts is wired to the
dt of the PID Advanced block. It is the
time step of the controller, and it is one of the parameters of the function.
It has to have the same value as the actual time step of the execution of the
PID Advanced function, which in our VI means that it has to have the same
value as the While loop cycle time. If you do not wire any value to the
dt in put, the PID Advanced function is
actually able to detect the actual time step itself. However, I suggest that
you define the time step explicitly, as shown in the Block diagram. (When
using the PID Advanced function in a Simulation
Loop, as in the present VI, you have to define the dt input
explicitly to be sure the PID function works correctly.)
A measurement lowpass filter in the form of the sub-VI
timeconstant_lowpass_filter.vi is included to attenuate the inevitable high-frequent measurement
noise that exists in all practical control systems. (This noise will be transferred to the actuator via the controller, and
may cause excessive wear on.a mechanical actuator as a motor, a relay, etc.)
The PID control palette contains a measurement filter in the form
of a fifth order lowpass FIR filter, see the
Control Palette, but this filter can not be tuned.
LabVIEW has lots of filtering functions, but the advantages of
timeconstant_lowpass_filter.vi compared to these is that the filter parameter (time constant)
can be changed online without the filter output being reset to zero, and that
the initial filter output equals the initial filter input (thereby avoiding
the initial filter output to be zero).
- Polynomial Interpolation functions have been used to scale the
measurement signal (assumed to be voltage) to a scaled value (assumed to be
percent) and to unscale the PID controller ouput signal (assumed to be in
percent) to the raw or physical control signal (assumed to be voltage) to be
sent to the actuator. There are (un)scaling functions on the PID Control
palette, too, see end of PID Control Palette Section above. These
functions assume percent to be one of the units, while the Polynomial
Interpolation function can be used with any units.
In a practical application you may use the pid_control_labview.vi as the
starting point. You may keep the Simulation Loop, but if you want, you may
substitute the Simulation Loop with a While Loop or Timed Loop, but if you
do, you must remove the Simulation Module functions (as the Transfer
Function and the Simulation Time Waveform) since they can not be used other
loops than the Simulation Loop.
Note the following if you use the PID Advanced function in a
Simulation Loop: You must force the PID Advanced function to be
invoked only at the major time steps of the simulator, otherwise the
function may not behave correctly. This is done as follows: Right-click on
the PID Advanced function / Select SubVI Node Setup / Select
Continuous and deselect Include minor time steps.
[Finn's LabVIEW Page]