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PID-CPP

This a C++ library implementing a PID controller intended for microcontrollers. It is a fixed point math implementation that was optimized for performance on the ARM Cortex M4 processors. The library was inspired by the excellent blog series written by Brett Beauregard and the PID library he put on Github (https://github.com/br3ttb/Arduino-PID-Library/).

This library is about a factor of 7-10 faster than the Arduino PID library!

Floating Point vs. Fixed Point Arithmetic

Floating Point Arithmetic

The library written by Brett Beauregard and numerous other implementations that can be found on the web do the calculations using floating point arithmetic. The advantage of this approach is, that one needs to know little about the input and output of the system as these datatypes have a large dynamic range (due to the use of the exponent bits). These libraries can therefore do their calculations in real units, which make the whole system more intuitive for newcomers.

The downside is, that without a dedicated floating point unit, both single precision and especially double precision floats are slow and the majority of all microcontrollers does not have an FPU. Also single precision floats are limited by their 24 bit precision (+1 sign bit).

Fixed Point Arithmetic

Fixed point arithmetic on the other hand is a more natural representation for the control loop. In real life most input data comes from components like an ADC or sensor, which outputs some binary value in its own system of units. The PID output will then be written to some DAC and is scaled to real units by the output stage. In any application that does not require huge amounts of dynamic range, the additional bits used by floats to scale the range (see the table below) can then be used for more precision.

The downside is, that we need saturating mathematical operations to make sure, that the calculations do not produce unexpected results. Some microcontrollers like the Cortex M4 support DSP instructions, which can do this in hardware.

The table below gives a summary of the different datatypes. Using single precision floats and ADC/DAC combinations with more than 12 bit is questionable, because the system 'gain' will be limited to (24 - DAC_resolution) due to the floating point precision.

Datatype Width Precision Comments
Single Precision Float 32 bit 24 + 1 sign bits Requires 32-bit FPU to be efficient
Double Precision Float 64 bit 53 + 1 sign bits Requires 64-bit FPU to be efficient
Q31 Fixed Point 32 bit 31 + 1 sign bits Requires 32-bit processor to be efficient

API

Constructor:

   enum FeedbackDirection : bool {
     feedbackPositive = 0,
     feedbackNegative = 1,
   };
   enum ProportionalGain : bool {
     proportionalToInput = 0,
     proportionalToError = 1,
   };

   PID(const uint32_t setpoint, const int32_t kp, const int32_t ki, const int32_t kd, const uint8_t qn, const FeedbackDirection feedbackDirection, const ProportionalGain proportionalGain);
   PID(const uint32_t setpoint, const int32_t kp, const int32_t ki, const int32_t kd, const uint8_t qn, const FeedbackDirection feedbackDirection);
   PID(const uint32_t setpoint, const int32_t kp, const int32_t ki, const int32_t kd, const uint8_t qn);

Arguments

  • setpoint [unsigned long] : The value in units of the input. It is the sensor value the controller will servo
  • kp [long] : The factor of the proportional term. The sign will ignored, but can be set by the feedbackDirection parameter
  • ki [long] : The factor of the integral term. This value must be scaled (ki * period), if the sampling period changes.
  • kd [long] : The factor of the derivative term. This value must be scaled (ki / period), if the sampling period changes.
  • qn [unsigned short] : The number of bits used to designate the fractional portion of the output.
    Set this number to be 32 - DAC_resolution
  • feedbackDirection [FeedbackDirection] : Choose whether a positive error causes a positve response (feedbackPositive) or a negative response(feedbackNegative).
    Default: feedbackNegative
  • proportionalGain [ProportionalGain] : Choose whether kp acts on the error (proportionalToError) or the deviation from the initial input (proportionalToInput). See here for more info.
    Default: proportionalToError

Methods

   uint32_t compute(const uint32_t input);

This function is the PID routine. It calculates the output based on the input and previous inputs. It needs to be called in regular intervals. In contrast to other libraries, this is not handled internally, because it gives more flexibility to the user.

Arguments

  • value [unsigned long] : The sensor input

Returns

  • [unsigned long] : The PID output. It will be scaled using the qn parameter, so that it does not contain the fractional part. It can be directly fed to the output hardware.

Setters

   void init(const uint32_t initialInput);

Call this function after switching from manual control to PID control or when initializing the PID controller.

Arguments

  • value [unsigned long] : This value initilizes the internal previous input variable used to calculate the derivative term
   void updateOutput(const uint32_t value);

Call this function whenever the output was manually changed to make the PID controller aware of the change. The PID controller needs to know the previous output when switching from off to on, to make sure there is no bump in the output due to a discontinuity.

Arguments

  • value [unsigned long] : This value initilizes the internal error sum of the integral term.
   void setOutputMin(const uint32_t value);
   void setOutputMax(const uint32_t value);

Arguments

  • value [unsigned long] : This value clamps the output to the the values chosen. Do not clamp the output afterwards, because using this function makes sure that the PID controller is aware of the limits and treats the integral term appropriately.
   void setTunings(const int32_t kp, const int32_t ki, const int32_t kd, const ProportionalGain proportionalGain);
   void setTunings(const int32_t kp, const int32_t ki, const int32_t kd);
   void setSetpoint(const uint32_t value);
   void setControllerFeedback(const FeedbackDirection feedbackDirection);

Arguments

See constructor

Getters

   uint32_t getKp();
   uint32_t getKi();
   uint32_t getKd();
   uint32_t getSetpoint();

Use these to query the internal state.

Examples

This is an example, which runs on the Arduino platform. It is very simple and does not implement advanced features like setpoint changes, or a manual mode. It only implements regular calls to the compute() method.

#include <pid.h>    // Use quotation marks if the library is in the same folder as the sketch
unsigned long pidPeriod = 500;
unsigned long setpoint = 512;
int32_t kp = 1011120;
int32_t ki = 1320*1000;
int32_t kd = 5280 * 1000;
uint8_t qn = 22;    // Set QN to 32 - DAC resolution
Pid::PID pidController = Pid::PID(setpoint, kp, ki, kd, qn);

const unsigned short DAC_OUTPUT_PIN = 2;
const unsigned short ADC_INPUT_PIN = 0;


void setup() {
  Serial.begin(115200);
  pinMode(13, OUTPUT);
  pinMode(DAC_OUTPUT_PIN, OUTPUT);
  pinMode(ADC_INPUT_PIN, INPUT);

  pidController.setOutputMin(0);      // This is the default
  pidController.setOutputMax(1023);   // The Arduino has a 10-bit'analog' PWM output,
                                      // but the maximum output can be adjusted down.

  pidController.init(analogRead(ADC_INPUT_PIN));  // Initialize the pid controller to make sure there
                                      // are no output spikes

  // We will turn on the LED, once we are ready
  digitalWrite(13, HIGH);
}

void loop() {
  // The Teensy will lock up, if the input buffer overflows
  while (Serial.available()) {
   Serial.read();
  }

  static unsigned long lastTime = millis();    // Initialize lastTime *once* during the first loop iteration

  unsigned long currentTime = millis();
  if (currentTime - lastTime >= pidPeriod) {    // Only run the pid controller
    unsigned short inputValue = analogRead(ADC_INPUT_PIN);  // Read a new analog value
    unsigned short outputValue = pidController.compute(inputValue);   // Compute the PID output
    analogWrite(DAC_OUTPUT_PIN, outputValue);    // Write it to the DAC

    lastTime = currentTime;
    Serial.println("------------------");
    Serial.print("Analog input: ");
    Serial.println(inputValue);
    Serial.print("DAC output: ");
    Serial.println(outputValue);
  }
}

Performance Comparison

In terms of performance only two models of the Teensy platform were tested, because they have a wide range of 32-bit processors.

Model Processor Clock Speed in MHz Comments
Teensy LC Cortex M0+ 48 32-bit
Teensy 3.6 Cortex M4F 180 32-bit, FPU

The full benchmark sketch can be found here. It calculates the number of instruction cycles needed to run the compute() method.

For the floating point test, the code below was used, which was adapted from the Arduino PID Library. The LIKELY/UNLIKELY macro was used to tell the compiler about the default values and which conditional path was more likely. All benchmarks were compiled using the option Fastests with LTO (-O3 -flto):

The floating point code is simpler because overflow conditions only need to be checked at the end. It is therefore expected to run faster if there is a dedicated floating point unit available (that is single precision, 32-bit floats).

#ifdef DOUBLE_PRECISION_MATH
  typedef double myFloatType;
#elif defined(SINGLE_PRECISION_MATH)
  typedef float myFloatType;
#endif

static inline __attribute__((always_inline, unused)) myFloatType clamp(myFloatType value, myFloatType min, myFloatType max) {
  return (value < min) ? min : (value > max) ? max : value;
}

myFloatType __attribute__((noinline)) compute(const myFloatType input) {
   const myFloatType error = setpoint - input;
   const myFloatType dInput = (input - previousInput);
   outputSum+= (ki * error);

   if (UNLIKELY(proportionalGain == proportionalToInput)) {
      outputSum-= kp * dInput;
   }

   outputSum = clamp(outputSum, 0.00, 4055.00);

   myFloatType output = outputSum - kd * dInput;
   if (LIKELY(proportionalGain == proportionalToError)) {
      output += kp * error;
   }

   output = clamp(output, 0.00, 4055.00);

   previousInput = input;
   return output;
}

The spread in the number of Ops required to run one iteration of the compute() loop is the result of the branching required when saturating the output.

Algorithm Teensy LC Teensy 3.6
Fixed Point Arithmetic 380-392 Ops/Call 88-104 Ops/Call
Single Precision Arithmetic 1522-1558 Ops/Call 48-55 Ops/Call
Double Precision Arithmetic 2827-2845 Ops/Call 1107-1200 Ops/Call

In conclusion, it can be said, that unless you have an MCU with a dedicated FPU and do not need the precision of 32 bits, than it is recommended to use fixed point arithmetic for a 7-10x increase in performance.

Versioning

We use SemVer for versioning. For the versions available, see the tags on this repository.

Authors

License

This project is licensed under the GPL v3 license - see the LICENSE file for details

Acknowledgments

  • Brett Beauregard for his excellent work in explaining all the PID details and improvements