Machinekit Documentation
HAL Component — AT_PID
INSTANTIABLE COMPONENTS — General
All instantiable components can be loaded in two manners
Using loadrt with or without count= | names= parameters as per legacy components
Using newinst, which names the instance and allows further parameters and arguments
primarily pincount= which can set the number of pins created for that instance (where applicable)
NAME
at_pid — HAL component that provides Proportional Integeral/Derivative control loops. It is a realtime component.
SYNOPSIS
at_pid
USAGE SYNOPSIS
loadrt at_pid
OR
newinst at_pid <newinstname> [ pincount=N | iprefix=prefix ] [instanceparamX=X | argX=X ]
( args in [ ] denote possible args and parameters, may not be used in all components )
DESCRIPTION
This file, 'at_pid.icomp', is a HAL component that provides Proportional/ Integeral/Derivative control loops. It is a realtime component.
In this documentation, 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.
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 relavent. 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 Derivative gain bias Constant offset on output FF0 Zeroth order Feedforward gain FF1 First order Feedforward gain FF2 Second order Feedforward gain deadband Amount of error that will be ignored maxerror Limit on error maxerrorI Limit on error integrator maxerrorD Limit on error differentiator maxcmdD Limit on command differentiator maxcmdDD Limit on command 2nd derivative maxoutput Limit on output value
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 usefull 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 cmdD Derivative of the command cmdDd 2nd derivative of the command
The PID loop calculations are as follows (see the code for all the nitty gritty details):
error = command - feedback if ( fabs(error) < deadband ) then error = 0 limit error to +/- maxerror errorI += error * period limit errorI to +/- maxerrorI errorD = (error - previouserror) / period limit errorD to +/- paxerrorD cmdD = (command - previouscommand) / period limit cmdD to +/- maxcmdD cmdDd = (cmdD - previouscmdD) / period limit cmdDd to +/- maxcmdDD output = bias + error * Pgain + errorI * Igain + errorD * Dgain + command * FF0 + cmdD * FF1 + cmdDd * FF2 limit output to +/- maxoutput
This component exports one function called '<name>.do-pid-calcs'
FUNCTIONS
at_pid.N.do-pid-calcs.funct ( OR <newinstname>.do-pid-calcs.funct (requires a floating-point thread) )
PINS
at_pid.N.deadband float in (default: 0.0) ( OR <newinstname>.deadband float in (default: 0.0) ) - Amount of error that will be ignored
at_pid.N.maxerror float in (default: 0.0) ( OR <newinstname>.maxerror float in (default: 0.0) ) - Limit on error
at_pid.N.maxerrorI float in (default: 0.0) ( OR <newinstname>.maxerrorI float in (default: 0.0) ) - Limit on error integrator
at_pid.N.maxerrorD float in (default: 0.0) ( OR <newinstname>.maxerrorD float in (default: 0.0) ) - Limit on error differentiator
at_pid.N.maxcmdD float in (default: 0.0) ( OR <newinstname>.maxcmdD float in (default: 0.0) ) - Limit on command differentiator
at_pid.N.maxcmdDD float in (default: 0.0) ( OR <newinstname>.maxcmdDD float in (default: 0.0) ) - Limit on command 2nd derivative
at_pid.N.bias float io (default: 0.0) ( OR <newinstname>.bias float io (default: 0.0) ) - Constant offset on output
at_pid.N.Pgain float io (default: 1.0) ( OR <newinstname>.Pgain float io (default: 1.0) ) - Proportional gain
at_pid.N.Igain float io (default: 0.0) ( OR <newinstname>.Igain float io (default: 0.0) ) - Integral gain
at_pid.N.Dgain float io (default: 0.0) ( OR <newinstname>.Dgain float io (default: 0.0) ) - Derivative gain
at_pid.N.FF0 float io (default: 0.0) ( OR <newinstname>.FF0 float io (default: 0.0) ) - Zeroth order Feedfoioard gain
at_pid.N.FF1 float io (default: 0.0) ( OR <newinstname>.FF1 float io (default: 0.0) ) - First order Feedforward gain
at_pid.N.FF2 float io (default: 0.0) ( OR <newinstname>.FF2 float io (default: 0.0) ) - Second order Feedforward gain
at_pid.N.maxoutput float io (default: 0.0) ( OR <newinstname>.maxoutput float io (default: 0.0) ) - Limit on output value
at_pid.N.tuneEffort float io (default: 0.5) ( OR <newinstname>.tuneEffort float io (default: 0.5) ) - Control effort for limit cycle.
at_pid.N.tuneCycles u32 io (default: 50) ( OR <newinstname>.tuneCycles u32 io (default: 50) )
at_pid.N.tuneType u32 io (default: 0) ( OR <newinstname>.tuneType u32 io (default: 0) )
at_pid.N.errorI float out ( OR <newinstname>.errorI float out ) - Integral of error
at_pid.N.errorD float out ( OR <newinstname>.errorD float out ) - Derivative of error
at_pid.N.commandD float out ( OR <newinstname>.commandD float out ) - Derivative of the command
at_pid.N.commandDD float out ( OR <newinstname>.commandDD float out ) - 2nd derivative of the command
at_pid.N.ultimateGain float out ( OR <newinstname>.ultimateGain float out ) - Calc by auto-tune from limit cycle.
at_pid.N.ultimatePeriod float io ( OR <newinstname>.ultimatePeriod float io ) - Calc by auto-tune from limit cycle.
at_pid.N.enable bit in (default: 0) ( OR <newinstname>.enable bit in (default: 0) ) - Enable/disabled the PID loop
at_pid.N.command float in (default: 0.0) ( OR <newinstname>.command float in (default: 0.0) ) - Commanded value
at_pid.N.feedback float in (default: 0.0) ( OR <newinstname>.feedback float in (default: 0.0) ) - Feedback input
at_pid.N.error float out ( OR <newinstname>.error float out ) - Current error
at_pid.N.output float out ( OR <newinstname>.output float out ) - Ouput value
at_pid.N.tuneMode bit in (default: 0) ( OR <newinstname>.tuneMode bit in (default: 0) ) - 0=PID, 1=tune.
at_pid.N.tuneStart bit io (default: 0) ( OR <newinstname>.tuneStart bit io (default: 0) ) - Set to 1 to start an auto-tune cycle. Clears automatically when the cycle has finished.
AUTHOR
John Kasunich
LICENCE
GPL v2