A fast fixed-point PID controller for Arduino

This PID controller is faster than alternatives for Arduino becuase it avoids expensive floating point operations. The PID controller is configured with floating point coefficients and translates them to fixed point internally. This imposes limitations on the domain of the coefficients. Setting the I and D terms to zero makes the controller run faster.

• Kp - P term of the PID controller.
• Ki - I term of the PID controller.
• Kd - D term of the PID controller.

The computation pipeline expects 25 bit coefficients. This is controlled by PARAM_BITS and cannot be changed without breaking the controller. The number of bits before and after the decimal place is controlled by PARAM_SHIFT in FastPID.h. The default value for PARAM_SHIFT is 15.

The parameter domain is [1023 to 0.00006103515625] inclsive

The input and the setpoint are an int16_t this matches the width of Analog pins and accomodate negative readings and setpoints. The output of the PID is an int16_t. The actual bit-width and signedness of the output can be configured.

• bits - The output width will be limited to values inside of this bit range. Valid values are 16 through 1
• sign If true the output range is [-2^(bits-1), -2^(bits-1)-1]. If false output range is [0, 2^bits-1]

If you're using an unsigned type as an output be sure to cast the output so you don't inadvertantly get a negative value.

Time is computed automatically each time step() is called based on millis(). Only PID controllers that use a Ki and Kd term rely on time information. You must control the rate at which the controller is called. Unlike ArduinoPID calling step() will always do a calculation, no matter how much time has passed.

#include <FastPID.h>

#define PIN_INPUT     A0
#define PIN_SETPOINT  A1
#define PIN_OUTPUT    9

float Kp=0.1, Ki=0.5, Kd=0;
int output_bits = 8;
bool output_signed = false;

FastPID myPID(Kp, Ki, Kd, output_bits, output_signed);

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  int setpoint = analogRead(PIN_SETPOINT) / 2; 
  int feedback = analogRead(PIN_INPUT);
  uint8_t output = myPID.step(setpoint, feedback);
  analogWrite(PIN_OUTPUT, output);
  Serial.print("sp: "); 
  Serial.print(setpoint); 
  Serial.print(" fb: "); 
  Serial.print(feedback);
  Serial.print(" out: ");
  Serial.println(output);
  delay(100);
}