Add the FOPDT and PID modules

The First Order Plus Delay Time module is a model of a
controlled system that is useful in modelling and
simulating process control systems.

The Proportional Integral Differential module is a control
system that is useful in controlling process control
systems.

This is not fully functional yet.

site_ansto/instrument/util/pid.py

@@ -0,0 +1,119 @@
+#!/usr/bin/env python
+# vim: ft=python ts=8 sts=4 sw=4 expandtab autoindent smartindent nocindent
+#
+# Class for device simulation using: Proportional-Integral-Derivative (PID)
+#
+# This recipe gives simple implementation of a Discrete
+# Proportional-Integral-Derivative (PID) controller.
+#
+# PID controller gives output value for error between desired reference input and
+# measurement feedback to minimize error value.
+#
+# More information: http://en.wikipedia.org/wiki/PID_controller
+#
+# cnr437@gmail.com
+#
+####### Example #########
+#
+# p=PID(3.0,0.4,1.2)
+# p.setPoint(5.0)
+# while True:
+#     pid = p.update(measurement_value)
+#
+#
+"""
+TODO:
+* Look into making the update time based because the logic seems to assume constant (1 second) time
+  - Derivator should be dE/dT but is just dE
+  - Integrator should be sigma(dT*E) but is just sigma(E)
+* Look into making the Derivator based on changes in the PV instead of the Error
+"""
+
+class PID:
+    """
+    Discrete PID control
+    """
+
+    def __init__(self, P=2.0, I=0.0, D=1.0, Derivator=0, Integrator=0, Integrator_max=500, Integrator_min=-500):
+
+        self.Kp=P
+        self.Ki=I
+        self.Kd=D
+        self.Derivator=Derivator
+        self.Integrator=Integrator
+        self.Integrator_max=Integrator_max
+        self.Integrator_min=Integrator_min
+
+        self.set_point=0.0
+        self.error=0.0
+
+    def update(self,current_value):
+        """
+        Calculate PID output value for given reference input and feedback
+        """
+
+        self.error = self.set_point - current_value
+
+        self.P_value = self.Kp * self.error
+        # TODO: check the sign is correct
+        self.D_value = self.Kd * (current_value - self.Derivator)
+        self.Derivator = current_value
+
+        self.Integrator = self.Integrator + self.error
+
+        if self.Integrator > self.Integrator_max:
+            self.Integrator = self.Integrator_max
+        elif self.Integrator < self.Integrator_min:
+            self.Integrator = self.Integrator_min
+
+        self.I_value = self.Integrator * self.Ki
+
+        PID = self.P_value + self.I_value + self.D_value
+
+        return PID
+
+    def setPoint(self,set_point):
+        """
+        Initilize the setpoint of PID
+        """
+        self.set_point = set_point
+        self.Integrator=0
+        self.Derivator=0
+
+    def setIntegrator(self, Integrator):
+        self.Integrator = Integrator
+
+    def setDerivator(self, Derivator):
+        self.Derivator = Derivator
+
+    def setKp(self,P):
+        self.Kp=P
+
+    def setKi(self,I):
+        self.Ki=I
+
+    def setKd(self,D):
+        self.Kd=D
+
+    def getPoint(self):
+        return self.set_point
+
+    def getError(self):
+        return self.error
+
+    def getIntegrator(self):
+        return self.Integrator
+
+    def getDerivator(self):
+        return self.Derivator
+
+if __name__ == "__main__":
+#p=PID(3.0,0.4,1.2)
+#p.setPoint(5.0)
+#while True:
+#     pid = p.update(measurement_value)
+    p = PID(3.0, 0.4, 1.2)
+    p.setPoint(20.0)
+    pv = 20
+    for i in range(0,61):
+        print p.update(pv)