def _clamp(value, limits):
lower, upper = limits
if value is None:
return None
elif (upper is not None) and (value > upper):
return upper
elif (lower is not None) and (value < lower):
return lower
return value
[docs]class PID(object):
"""A simple PID controller."""
[docs] def __init__(
self,
Kp=1.0,
Ki=0.0,
Kd=0.0,
setpoint=0,
sample_time=0.01,
output_limits=(None, None),
auto_mode=True,
proportional_on_measurement=False,
differential_on_measurement=True,
error_map=None,
time_fn=None,
starting_output=0.0,
):
"""
Initialize a new PID controller.
:param Kp: The value for the proportional gain Kp
:param Ki: The value for the integral gain Ki
:param Kd: The value for the derivative gain Kd
:param setpoint: The initial setpoint that the PID will try to achieve
:param sample_time: The time in seconds which the controller should wait before generating
a new output value. The PID works best when it is constantly called (eg. during a
loop), but with a sample time set so that the time difference between each update is
(close to) constant. If set to None, the PID will compute a new output value every time
it is called.
:param output_limits: The initial output limits to use, given as an iterable with 2
elements, for example: (lower, upper). The output will never go below the lower limit
or above the upper limit. Either of the limits can also be set to None to have no limit
in that direction. Setting output limits also avoids integral windup, since the
integral term will never be allowed to grow outside of the limits.
:param auto_mode: Whether the controller should be enabled (auto mode) or not (manual mode)
:param proportional_on_measurement: Whether the proportional term should be calculated on
the input directly rather than on the error (which is the traditional way). Using
proportional-on-measurement avoids overshoot for some types of systems.
:param differential_on_measurement: Whether the differential term should be calculated on
the input directly rather than on the error (which is the traditional way).
:param error_map: Function to transform the error value in another constrained value.
:param time_fn: The function to use for getting the current time, or None to use the
default. This should be a function taking no arguments and returning a number
representing the current time. The default is to use time.monotonic() if available,
otherwise time.time().
:param starting_output: The starting point for the PID's output. If you start controlling
a system that is already at the setpoint, you can set this to your best guess at what
output the PID should give when first calling it to avoid the PID outputting zero and
moving the system away from the setpoint.
"""
self.Kp, self.Ki, self.Kd = Kp, Ki, Kd
self.setpoint = setpoint
self.sample_time = sample_time
self._min_output, self._max_output = None, None
self._auto_mode = auto_mode
self.proportional_on_measurement = proportional_on_measurement
self.differential_on_measurement = differential_on_measurement
self.error_map = error_map
self._proportional = 0
self._integral = 0
self._derivative = 0
self._last_time = None
self._last_output = None
self._last_error = None
self._last_input = None
if time_fn is not None:
# Use the user supplied time function
self.time_fn = time_fn
else:
import time
try:
# Get monotonic time to ensure that time deltas are always positive
self.time_fn = time.monotonic
except AttributeError:
# time.monotonic() not available (using python < 3.3), fallback to time.time()
self.time_fn = time.time
self.output_limits = output_limits
self.reset()
# Set initial state of the controller
self._integral = _clamp(starting_output, output_limits)
[docs] def __call__(self, input_, dt=None):
"""
Update the PID controller.
Call the PID controller with *input_* and calculate and return a control output if
sample_time seconds has passed since the last update. If no new output is calculated,
return the previous output instead (or None if no value has been calculated yet).
:param dt: If set, uses this value for timestep instead of real time. This can be used in
simulations when simulation time is different from real time.
"""
if not self.auto_mode:
return self._last_output
now = self.time_fn()
if dt is None:
dt = now - self._last_time if (now - self._last_time) else 1e-16
elif dt <= 0:
raise ValueError('dt has negative value {}, must be positive'.format(dt))
if self.sample_time is not None and dt < self.sample_time and self._last_output is not None:
# Only update every sample_time seconds
return self._last_output
# Compute error terms
error = self.setpoint - input_
d_input = input_ - (self._last_input if (self._last_input is not None) else input_)
d_error = error - (self._last_error if (self._last_error is not None) else error)
# Check if must map the error
if self.error_map is not None:
error = self.error_map(error)
# Compute the proportional term
if not self.proportional_on_measurement:
# Regular proportional-on-error, simply set the proportional term
self._proportional = self.Kp * error
else:
# Add the proportional error on measurement to error_sum
self._proportional -= self.Kp * d_input
# Compute integral and derivative terms
self._integral += self.Ki * error * dt
self._integral = _clamp(self._integral, self.output_limits) # Avoid integral windup
if self.differential_on_measurement:
self._derivative = -self.Kd * d_input / dt
else:
self._derivative = self.Kd * d_error / dt
# Compute final output
output = self._proportional + self._integral + self._derivative
output = _clamp(output, self.output_limits)
# Keep track of state
self._last_output = output
self._last_input = input_
self._last_error = error
self._last_time = now
return output
def __repr__(self):
return (
'{self.__class__.__name__}('
'Kp={self.Kp!r}, Ki={self.Ki!r}, Kd={self.Kd!r}, '
'setpoint={self.setpoint!r}, sample_time={self.sample_time!r}, '
'output_limits={self.output_limits!r}, auto_mode={self.auto_mode!r}, '
'proportional_on_measurement={self.proportional_on_measurement!r}, '
'differential_on_measurement={self.differential_on_measurement!r}, '
'error_map={self.error_map!r}'
')'
).format(self=self)
@property
def components(self):
"""
The P-, I- and D-terms from the last computation as separate components as a tuple. Useful
for visualizing what the controller is doing or when tuning hard-to-tune systems.
"""
return self._proportional, self._integral, self._derivative
@property
def tunings(self):
"""The tunings used by the controller as a tuple: (Kp, Ki, Kd)."""
return self.Kp, self.Ki, self.Kd
@tunings.setter
def tunings(self, tunings):
"""Set the PID tunings."""
self.Kp, self.Ki, self.Kd = tunings
@property
def auto_mode(self):
"""Whether the controller is currently enabled (in auto mode) or not."""
return self._auto_mode
@auto_mode.setter
def auto_mode(self, enabled):
"""Enable or disable the PID controller."""
self.set_auto_mode(enabled)
[docs] def set_auto_mode(self, enabled, last_output=None):
"""
Enable or disable the PID controller, optionally setting the last output value.
This is useful if some system has been manually controlled and if the PID should take over.
In that case, disable the PID by setting auto mode to False and later when the PID should
be turned back on, pass the last output variable (the control variable) and it will be set
as the starting I-term when the PID is set to auto mode.
:param enabled: Whether auto mode should be enabled, True or False
:param last_output: The last output, or the control variable, that the PID should start
from when going from manual mode to auto mode. Has no effect if the PID is already in
auto mode.
"""
if enabled and not self._auto_mode:
# Switching from manual mode to auto, reset
self.reset()
self._integral = last_output if (last_output is not None) else 0
self._integral = _clamp(self._integral, self.output_limits)
self._auto_mode = enabled
@property
def output_limits(self):
"""
The current output limits as a 2-tuple: (lower, upper).
See also the *output_limits* parameter in :meth:`PID.__init__`.
"""
return self._min_output, self._max_output
@output_limits.setter
def output_limits(self, limits):
"""Set the output limits."""
if limits is None:
self._min_output, self._max_output = None, None
return
min_output, max_output = limits
if (None not in limits) and (max_output < min_output):
raise ValueError('lower limit must be less than upper limit')
self._min_output = min_output
self._max_output = max_output
self._integral = _clamp(self._integral, self.output_limits)
self._last_output = _clamp(self._last_output, self.output_limits)
[docs] def reset(self):
"""
Reset the PID controller internals.
This sets each term to 0 as well as clearing the integral, the last output and the last
input (derivative calculation).
"""
self._proportional = 0
self._integral = 0
self._derivative = 0
self._integral = _clamp(self._integral, self.output_limits)
self._last_time = self.time_fn()
self._last_output = None
self._last_input = None