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Fixed-Point PID Algorithm
// Fixed-Point PID Algorithm
// Ported from: https://gist.github.com/bradley219/5373998
// Author: Li "oldrev" Wei <oldrev@gmail.com>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#define FIXED32_Q (16)
#define FIXED32_FMASK (((Fixed32)1 << FIXED32_Q) - 1)
typedef int32_t Fixed32;
typedef int64_t Fixed64;
Fixed32 Fixed32_FromInt(int32_t n) {
return (Fixed32)((Fixed32)n << FIXED32_Q);
}
int32_t Fixed32_Frac(Fixed32 a){
return a & FIXED32_FMASK;
}
// Optional
Fixed32 Fixed32_FromFloat(float f) {
return (Fixed32)((f) * (((Fixed64)1 << FIXED32_Q) + ((f) >= 0 ? 0.5 : -0.5)));
}
// Optional
Fixed32 Fixed32_ToFloat(float T) {
return (float)((T)*((float)(1)/(float)(1 << FIXED32_Q)));
}
Fixed32 Fixed32_Mul(Fixed32 a, Fixed32 b) {
return (Fixed32)(((Fixed64)a * (Fixed64)b) >> FIXED32_Q);
}
Fixed32 Fixed32_Div(Fixed32 a, Fixed32 b) {
return (Fixed32)(((Fixed64)a << FIXED32_Q) / (Fixed64)b);
}
typedef struct {
Fixed32 Dt;
Fixed32 Max;
Fixed32 Min;
Fixed32 Kp;
Fixed32 Kd;
Fixed32 Ki;
Fixed32 PrevError;
Fixed32 Integral;
} FixedPid;
void FixedPid_Init(FixedPid* self) {
memset(self, 0, sizeof(FixedPid));
}
Fixed32 FixedPid_Calculate(FixedPid* self, Fixed32 setpoint, Fixed32 pv) {
// Calculate error
Fixed32 error = setpoint - pv; //setpoint - pv;
// Proportional term
Fixed32 Pout = Fixed32_Mul(self->Kp, error); // self.Kp * error;
// Integral term
self->Integral += Fixed32_Mul(error, self->Dt); // self.Integral += error * self.Dt;
Fixed32 Iout = Fixed32_Mul(self->Ki, self->Integral);
// Derivative term
Fixed32 derivative = Fixed32_Div(error - self->PrevError, self->Dt);
Fixed32 Dout = Fixed32_Mul(self->Kd, derivative);
// Calculate total output
Fixed32 output = Pout + Iout + Dout;
// Restrict to max/min
if(output > self->Max) {
output = self->Max;
}
else if(output < self->Min){
output = self->Min;
}
// Save error to previous error
self->PrevError = error;
return output;
}
int main() {
FixedPid pid;
FixedPid_Init(&pid);
pid.Dt = Fixed32_FromFloat(0.1);
pid.Max = Fixed32_FromFloat(100);
pid.Min = Fixed32_FromFloat(-100);
pid.Kp = Fixed32_FromFloat(0.1);
pid.Kd = Fixed32_FromFloat(0.01);
pid.Ki = Fixed32_FromFloat(0.5);
float float_val = 20;
Fixed32 val = Fixed32_FromFloat(float_val);
printf("max=%d min=%d dt=%d\n", pid.Max, pid.Min, pid.Dt);
for(int i = 0; i < 100; i++) {
Fixed32 inc = FixedPid_Calculate(&pid, 0, val);
float float_inc = Fixed32_ToFloat(inc);
float_val = Fixed32_ToFloat(val);
printf("%d - val:% 7.8f inc:% 7.8f\n", i, float_val, float_inc);
val += inc;
}
return 0;
}
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