SYNOPSIS

DESCRIPTION

NAMING

FUNCTIONS

PINS

Additional auto tuning pins

PARAMETERS

BUGS

pid - proportional/integral/derivative controller with automatic tuning support

**loadrt pid
[num_chan=***num* **|
names=***name1***[,***name2...***]]
[debug=***dbg*]

**pid** is a
classic Proportional/Integral/Derivative controller, used to
control position or speed feedback loops for servo motors
and other closed-loop applications.

**pid**
supports a maximum of sixteen controllers. The number that
are actually loaded is set by the **num_chan** argument
when the module is loaded. Alternatively, specify names= and
unique names separated by commas.

The
**num_chan=** and **names=** specifiers are mutually
exclusive. If neither **num_chan=** nor **names=** are
specified, the default value is three. If **debug** is
set to 1 (the default is 0), some additional HAL parameters
will be exported, which might be useful for tuning, but are
otherwise unnecessary.

In the following description, it is assumed that we are discussing position loops. However this component can be used to implement other loops such as speed loops, torch height control, and others.

Each loop has a number of pins and parameters, whose names begin with ’pid.N.’, where ’N’ is the channel number. Channel numbers start at zero.

The three most important pins are ’command’, ’feedback’, and ’output’. For a position loop, ’command’ and ’feedback’ are in position units. For a linear axis, this could be inches, mm, metres, or whatever is relevant. Likewise, for a angular axis, it could be degrees, radians, etc. The units of the ’output’ pin represent the change needed to make the feedback match the command. As such, for a position loop ’output’ is a velocity, in inches/sec, mm/sec, degrees/sec, etc.

Each loop has several other pins as well. ’error’ is equal to ’command’ minus ’feedback’. ’enable’ is a bit that enables the loop. If ’enable’ is false, all integrators are reset, and the output is forced to zero. If ’enable’ is true, the loop operates normally.

The PID gains, limits, and other ’tunable’ features of the loop are implemented as parameters. These are as follows:

**Pgain**
Proportional gain **
Igain** Integral gain

Dgain

bias

FF0

FF1

FF2

FF3

deadband

maxerror

maxerrorI

maxerrorD

maxcmdD

maxcmdDD

maxcmdDDD

maxoutput

All of the limits (max____) are implemented such that if the parameter value is zero, there is no limit.

A number of internal values which may be useful for testing and tuning are also available as parameters. To avoid cluttering the parameter list, these are only exported if "debug=1" is specified on the insmod command line.

**errorI**
Integral of error **
errorD** Derivative of error

commandD

commandDD

commandDDD

The PID loop calculations are as follows (see the code in pid.c for all the nitty gritty details):

error = command
- feedback

if ( abs(error) < deadband ) then error = 0

limit error to +/- maxerror

errorI += error * period

limit errorI to +/- maxerrorI

errorD = (error - previouserror) / period

limit errorD to +/- maxerrorD

commandD = (command - previouscommand) / period

limit commandD to +/- maxcmdD

commandDD = (commandD - previouscommandD) / period

limit commandDD to +/- maxcmdDD

commandDDD = (commandDD - previouscommandDD) / period

limit commandDDD to +/- maxcmdDDD

output = bias + error * Pgain + errorI * Igain +

errorD * Dgain + command * FF0 + commandD * FF1 +

commandDD * FF2 + commandDDD * FF3

limit output to +/- maxoutput

This component
has a built in auto tune mode. It works by setting up a
limit cycle to characterize the process. This is called the
Relay method and described in the 1984 Automation paper
**Automatic Tuning of Simple Regulators with Specifications
on Phase and Amplitude Margins** by Karl Johan
ÃstrÃ¶m and Tore HÃ¤gglund
(doi:10.1016/0005-1098(84)90014-1),
https://lup.lub.lu.se/search/ws/files/6340936/8509157.pdf.
Using this method, **Pgain/Igain/Dgain** or
**Pgain/Igain/FF1** can be determined using the
Ziegler-Nichols algorithm. When using **FF1** tuning,
scaling must be set so that **output** is in user units
per second.

During auto
tuning, the **command** input should not change. The
limit cycle is setup around the commanded position. No
initial tuning values are required to start auto tuning.
Only **tune-cycles**, **tune-effort** and
**tune-mode** need be set before starting auto tuning.
Note that setting **tune-mode** to true disable the
control loop. When auto tuning completes, the tuning
parameters will be set, the output set to bias and the
controller still be disabled. If running from LinuxCNC, the
FERROR setting for the axis being tuned may need to be
loosened up, as it must be larger than the limit cycle
amplitude in order to avoid a following error.

To perform auto
tuning, take the following steps. Move the axis to be tuned
somewhere near the center of it’s travel. Set
**tune-cycles** (the default value should be fine in most
cases) and **tune-mode**. Set **tune-effort** to a
small value. Set **enable** to true. Set **tune-mode**
to true. Set **tune-start** to true. If no oscillation
occurs, or the oscillation is too small, slowly increase
**tune-effort**. Set **tune-start** to true. If no
oscillation occurs, or the oscillation is too small, slowly
increase **tune-effort** Auto tuning can be aborted at
any time by setting **enable** or **tune-mode** to
false.

The names for
pins, parameters, and functions are prefixed as: **
pid.N.** for N=0,1,...,num-1 when using

nameN.

The
**pid.N.** format is shown in the following
descriptions.

**pid.***N***.do-pid-calcs**
(uses floating-point) Does the PID calculations for control
loop *N*.

**pid.***N***.command**
float in

The desired (commanded) value for the control loop.

**pid.***N***.Pgain**
float in

Proportional gain. Results in a
contribution to the output that is the error multiplied by
**Pgain**.

**pid.***N***.Igain**
float in

Integral gain. Results in a
contribution to the output that is the integral of the error
multiplied by **Igain**. For example an error of 0.02
that lasted 10 seconds would result in an integrated error
(**errorI**) of 0.2, and if **Igain** is 20, the
integral term would add 4.0 to the output.

**pid.***N***.Dgain**
float in

Derivative gain. Results in a
contribution to the output that is the rate of change
(derivative) of the error multiplied by **Dgain**. For
example an error that changed from 0.02 to 0.03 over 0.2
seconds would result in an error derivative (**errorD**)
of of 0.05, and if **Dgain** is 5, the derivative term
would add 0.25 to the output.

**pid.***N***.feedback**
float in

The actual (feedback) value, from some sensor such as an encoder.

**pid.***N***.output**
float out

The output of the PID loop, which goes to some actuator such as a motor.

**pid.***N***.command-deriv**
float in

The derivative of the desired (commanded) value for the control loop. If no signal is connected then the derivative will be estimated numerically.

**pid.***N***.feedback-deriv**
float in

The derivative of the actual (feedback) value for the control loop. If no signal is connected then the derivative will be estimated numerically. When the feedback is from a quantized position source (e.g., encoder feedback position), behavior of the D term can be improved by using a better velocity estimate here, such as the velocity output of encoder(9) or hostmot2(9).

**pid.***N***.error-previous-target**
bit in

Use previous invocation’s target vs. current position for error calculation, like the motion controller expects. This may make torque-mode position loops and loops requiring a large I gain easier to tune, by eliminating velocity-dependent following error.

**pid.***N***.error**
float out

The difference between command and feedback.

**pid.***N***.enable**
bit in

When true, enables the PID
calculations. When false, **output** is zero, and all
internal integrators, etc, are reset.

**pid.***N***.index-enable**
bit in

On the falling edge of
**index-enable**, pid does not update the internal
command derivative estimate. On systems which use the
encoder index pulse, this pin should be connected to the
index-enable signal. When this is not done, and FF1 is
nonzero, a step change in the input command causes a
single-cycle spike in the PID output. On systems which use
exactly one of the **-deriv** inputs, this affects the D
term as well.

**pid.***N***.bias**
float in

**bias** is a constant
amount that is added to the output. In most cases it should
be left at zero. However, it can sometimes be useful to
compensate for offsets in servo amplifiers, or to balance
the weight of an object that moves vertically. **bias**
is turned off when the PID loop is disabled, just like all
other components of the output. If a non-zero output is
needed even when the PID loop is disabled, it should be
added with an external HAL sum2 block.

**pid.***N***.FF0**
float in

Zero order feed-forward term.
Produces a contribution to the output that is **FF0**
multiplied by the commanded value. For position loops, it
should usually be left at zero. For velocity loops,
**FF0** can compensate for friction or motor counter-EMF
and may permit better tuning if used properly.

**pid.***N***.FF1**
float in

First order feed-forward term.
Produces a contribution to the output that is **FF1**
multiplied by the derivative of the commanded value. For
position loops, the contribution is proportional to speed,
and can be used to compensate for friction or motor CEMF.
For velocity loops, it is proportional to acceleration and
can compensate for inertia. In both cases, it can result in
better tuning if used properly.

**pid.***N***.FF2**
float in

Second order feed-forward term.
Produces a contribution to the output that is **FF2**
multiplied by the second derivative of the commanded value.
For position loops, the contribution is proportional to
acceleration, and can be used to compensate for inertia. For
velocity loops, the contribution is proportional to jerk,
and should usually be left at zero.

**pid.***N***.FF3**
float in

Third order feed-forward term.
Produces a contribution to the output that is **FF3**
multiplied by the third derivative of the commanded value.
For position loops, the contribution is proportional to
jerk, and can be used to compensate for residual errors
during acceleration. For velocity loops, the contribution is
proportional to snap(jounce), and should usually be left at
zero.

**pid.***N***.deadband**
float in

Defines a range of
"acceptable" error. If the absolute value of
**error** is less than **deadband**, it will be
treated as if the error is zero. When using feedback devices
such as encoders that are inherently quantized, the deadband
should be set slightly more than one-half count, to prevent
the control loop from hunting back and forth if the command
is between two adjacent encoder values. When the absolute
value of the error is greater than the deadband, the
deadband value is subtracted from the error before
performing the loop calculations, to prevent a step in the
transfer function at the edge of the deadband (see
**BUGS**).

**pid.***N***.maxoutput**
float in

Output limit. The absolute
value of the output will not be permitted to exceed
**maxoutput**, unless **maxoutput** is zero. When the
output is limited, the error integrator will hold instead of
integrating, to prevent windup and overshoot.

**pid.***N***.maxerror**
float in

Limit on the internal error
variable used for P, I, and D. Can be used to prevent high
**Pgain** values from generating large outputs under
conditions when the error is large (for example, when the
command makes a step change). Not normally needed, but can
be useful when tuning non-linear systems.

**pid.***N***.maxerrorD**
float in

Limit on the error derivative.
The rate of change of error used by the **Dgain** term
will be limited to this value, unless the value is zero. Can
be used to limit the effect of **Dgain** and prevent
large output spikes due to steps on the command and/or
feedback. Not normally needed.

**pid.***N***.maxerrorI**
float in

Limit on error integrator. The
error integrator used by the **Igain** term will be
limited to this value, unless it is zero. Can be used to
prevent integrator windup and the resulting overshoot
during/after sustained errors. Not normally needed.

**pid.***N***.maxcmdD**
float in

Limit on command derivative.
The command derivative used by **FF1** will be limited to
this value, unless the value is zero. Can be used to prevent
**FF1** from producing large output spikes if there is a
step change on the command. Not normally needed.

**pid.***N***.maxcmdDD**
float in

Limit on command second
derivative. The command second derivative used by **FF2**
will be limited to this value, unless the value is zero. Can
be used to prevent **FF2** from producing large output
spikes if there is a step change on the command. Not
normally needed.

**pid.***N***.maxcmdDDD**
float in

Limit on command third
derivative. The command third derivative used by **FF3**
will be limited to this value, unless the value is zero. Can
be used to prevent **FF3** from producing large output
spikes if there is a step change on the command. Not
normally needed.

**pid.***N***.saturated**
bit out

When true, the current PID output is saturated. That is,

**output** = ±
**maxoutput**.

**pid.***N***.saturated-s**
float out

pid.*N***.saturated-count** s32 out

When true, the output of PID
was continually saturated for this many seconds
(**saturated-s**) or periods
(**saturated-count**).

**pid.***N***.tune-mode**
bit in

When true, enables auto tune mode. When false, normal PID calculations are performed.

**pid.***N***.tune-start**
bit io

When set to true, starts auto tuning. Cleared when the auto tuning completes.

**pid.***N***.tune-type**
u32 rw

When set to 0,
**Pgain/Igain/Dgain** are calculated. When set to 1,
**Pgain/Igain/FF1** are calculated.

**pid.***N***.tune-cycles**
u32 rw

Determines the number of cycles
to run to characterize the process. **tune-cycles**
actually sets the number of half cycles. More cycles results
in a more accurate characterization as the average of all
cycles is used.

**pid.***N***.tune-effort**
float rw

The maximum output value used
during automatic tuning. Determines the effort used in
setting up the limit cycle in the process.
**tune-effort** should be set to a positive value less
than **maxoutput**. Start with something small and work
up to a value that results in a good portion of the maximum
motor current being used. The smaller the value, the smaller
the amplitude of the limit cycle.

**pid.***N***.ultimate-gain**
float ro (only if debug=1)

Determined from process
characterization. **ultimate-gain** is the ratio of
**tune-effort** to the limit cycle amplitude multiplied
by 4.0 divided by Pi.

**pid.***N***.ultimate-period**
float ro (only if debug=1)

Determined from process
characterization. **ultimate-period** is the period of
the limit cycle.

**pid.***N***.errorI**
float ro (only if debug=1)

Integral of error. This is the
value that is multiplied by **Igain** to produce the
Integral term of the output.

**pid.***N***.errorD**
float ro (only if debug=1)

Derivative of error. This is
the value that is multiplied by **Dgain** to produce the
Derivative term of the output.

**pid.***N***.commandD**
float ro (only if debug=1)

Derivative of command. This is
the value that is multiplied by **FF1** to produce the
first order feed-forward term of the output.

**pid.***N***.commandDD**
float ro (only if debug=1)

Second derivative of command.
This is the value that is multiplied by **FF2** to
produce the second order feed-forward term of the
output.

**pid.***N***.commandDDD**
float ro (only if debug=1)

Third derivative of command.
This is the value that is multiplied by **FF3** to
produce the third order feed-forward term of the output.

Some people
would argue that deadband should be implemented such that
error is treated as zero if it is within the deadband, and
be unmodified if it is outside the deadband. This was not
done because it would cause a step in the transfer function
equal to the size of the deadband. People who prefer that
behavior are welcome to add a parameter that will change the
behavior, or to write their own version of **pid**.
However, the default behavior should not be changed.

Negative gains may lead to unwanted behavior. It is possible in some situations that negative FF gains make sense, but in general all gains should be positive. If some output is in the wrong direction, negating gains to fix it is a mistake; set the scaling correctly elsewhere instead.