pid v0.1.0

Integer PID controller with integral clamping and output limits.

Application scenarios

• Motor speed or position control.
• Heater or fan temperature control.
• A current, voltage, or flow loop on a small MCU without floating point.

Contract

void pid_init(pid_t *pid, int32_t kp, int32_t ki, int32_t kd, uint8_t shift,
              int32_t integral_min, int32_t integral_max,
              int32_t output_min, int32_t output_max);
int32_t pid_step(pid_t *pid, int32_t setpoint, int32_t measured);
void pid_reset(pid_t *pid);

• Gains and outputs are integers; shift divides the final sum by 2^shift.
• The integral term is clamped to [integral_min, integral_max] (anti-windup).
• The output is clamped to [output_min, output_max].
• The derivative is error - previous_error; call on a fixed period.
• No allocation; state is about 36 bytes.

Parameters

• kp, ki, kd: tune in the same integer units as the error and output.
• shift: use it to keep gains in a comfortable range; 8..12 is common.
• Integral limits: set them from the expected steady-state output.
• Call period: PID assumes a fixed dt; the gains absorb it.

Memory and timing

• 36 bytes; O(1), a few 64-bit multiply/divide operations.

Integration

pid_t pid;
pid_init(&pid, 80, 4, 2, 8, -2000, 2000, 0, 1000);
int32_t duty = pid_step(&pid, target_rpm, measured_rpm);

When not to use

• A plant with a large dead time; add feed-forward or a different controller.
• A loop that must respect actuator rate limits; add clamp after the output.

Verification

make test covers proportional response, integral clamping, derivative action, output limits, reset, and shift scaling.

int32_t kp;
int32_t ki;
int32_t kd;
int32_t integral;
int32_t integral_min;
int32_t integral_max;
int32_t output_min;
int32_t output_max;
int32_t previous_error;
uint8_t shift;