2.3.3.1.50. NXpid_controller¶
Status:
base class, extends NXcomponent
Description:
A description of a feedback system in terms of the settings of a proportional-in ...
A description of a feedback system in terms of the settings of a proportional-integral-derivative (PID) controller.
Automated control of a physical quantity is often achieved by connecting the output of a sensor to an actuator (e.g. using a thermocouple to monitor the effect of a heater and influence the power provided to it). The physical quantity being operated on is typically referred to as the “Process Variable”, with the desired value being the “Setpoint” (which may vary as a function of time) and the “Error Value” is the time-varying function of the difference between the Setpoint value and the concurrent measurement of the Process Variable (Error Value = Setpoint - Process Variable).
A PID controller calculates an output value for use as an input signal to an actuator via the weighted sum of four terms: * Proportional: the current Error Value * Integral: the integral of the Error Value function * Derivative: the first derivative of the Error Value function * Feed Forward: A model of the physical system (optional)
The weightings of these terms are given by the corresponding constants: * K_p * K_i * K_d * K_ff
A classic PID controller only implements the P, I and D terms and the values of the K_p, K_i and K_d constants are sufficient to fully describe the behaviour of the feedback system implemented by such a PID controller. The inclusion of a Feed Forward term in a feedback system is a modern adaptation that aids optimisation of the automated control. It is not present in all PID controllers, but it is also not uncommon.
Note that the
NXpid_controlleris designed to be a child object of the actuator that its output is connected to. The parent object representing the actuator is likely to be represented by anNXactuatororNXpositionerbase class, but there is a wide variety of possible applications for PID controllers.
Symbols:
No symbol table
Structure:
description: (optional) NX_CHAR ⤆
Description of how the Process Value for the PID controller is produced by sen ...
Description of how the Process Value for the PID controller is produced by sensor(s) in the setup.
For example, a set of sensors could be averaged over before feeding it back into the loop.
setpoint: (optional) NX_FLOAT {units=NX_ANY}
The Setpoint(s) used as an input for the PID controller. ...
The Setpoint(s) used as an input for the PID controller.
It can also be a link to an
NXsensor.valuefield.K_p: (optional) NX_NUMBER
Proportional gain constant. This constant determines how strongly the output v ...
Proportional gain constant. This constant determines how strongly the output value directly follows the current Error Value. When this constant dominates, the output value is linearly proportional to the Error Value.
K_i: (optional) NX_NUMBER
Integral gain constant. This constant determines how strongly the output value ...
Integral gain constant. This constant determines how strongly the output value should react to an accumulated offset in the Error Value that should have been corrected previously. since the integral term is proportional to both the magnitude and persistence of the Error Value over time.
K_d: (optional) NX_NUMBER
Derivative gain constant. This constant determines how much the feedback syste ...
Derivative gain constant. This constant determines how much the feedback system should anticipate the future value of the Error Value function through adjustment of the output value that is proportional to the rate of change (i.e. derivative) of the Error Value. This term is important for damping oscillations in the feedback system.
K_ff: (optional) NX_NUMBER
Feed Forward gain constant. This constant determines how much the feedback sys ...
Feed Forward gain constant. This constant determines how much the feedback system should rely on a calculated output value to achieve the desired Process Variable value. A Feed Forward system uses a model of the physical system to calculate an appropriate output value to achieve a desired Setpoint value. A description of this model should be provided in the
feed_forward_modelfield.feed_forward_model: (optional) NX_CHAR
A description of the model used for the Feed Forward part of the feedback syst ...
A description of the model used for the Feed Forward part of the feedback system. Note that such models typically involve the Setpoint value, but not the Error Value. The simplest model is simply proportional to the Setpoint value. For example, the position (Process Variable) of a sample is measured by a a linear optical encoder (sensor) and manipulated by a piezoelectric scanning stage (actuator). The corresponding Feed Forward model could be that the output value (voltage applied to the piezo) is proportional to the Setpoint value (measured position of the sample).
A complex model could involve any number of input variables, mathematical functions, and coefficients in order to describe the physical system relevant to the PID controller.
control_action: (optional) NX_CHAR
The Error Value of PID feedback system is normally constructed in terms of the ...
The Error Value of PID feedback system is normally constructed in terms of the correction needed to bring the Process Variable towards a match with the Setpoint. This “direct” control action means that a measurement of the Process Variable that is lower than the Setpoint results in a positive Error Value and a generally positive control output that tells the actuator to push the value of the Process Variable upwards. In some implementations, the actuator will respond to a more positive control output by pushing the Process Variable towards lower values (e.g. a Peltier cooler) and so the output of the feedback system must be reversed to match the behaviour of the physical system. A feedback system may also be implemented with reverse action in order to ensure that failures (e.g. disconnected sensor output or actuator input) result in a safe state (e.g. a valve should be left open to release pressure).
Any of these values:
direct|reversepv_sensor: (optional) NXsensor
The sensor representing the Process Value used in the feedback loop for the PI ...
The sensor representing the Process Value used in the feedback loop for the PID.
In case multiple sensors were used, this NXsensor should contain the proper calculated/aggregated value.
Hypertext Anchors¶
List of hypertext anchors for all groups, fields, attributes, and links defined in this class.
