PID: protect from division by zero because of dt

Co-authored-by: chfriedrich98 <125505139+chfriedrich98@users.noreply.github.com>

src/lib/pid/PID.cpp

@@ -44,8 +44,7 @@ void PID::setGains(const float P, const float I, const float D)
 float PID::update(const float feedback, const float dt, const bool update_integral)
 {
 	const float error = _setpoint - feedback;
-	const float feedback_change = std::isfinite(_last_feedback) ? (feedback - _last_feedback) / dt : 0.f;
+	const float output = (_gain_proportional * error) + _integral + (_gain_derivative * updateDerivative(feedback, dt));
-	const float output = (_gain_proportional * error) + _integral + (_gain_derivative * feedback_change);
 
 	if (update_integral) {
 		updateIntegral(error, dt);
@@ -63,3 +62,14 @@ void PID::updateIntegral(float error, const float dt)
 		_integral = math::constrain(integral_new, -_limit_integral, _limit_integral);
 	}
 }
+
+float PID::updateDerivative(float feedback, const float dt)
+{
+	float feedback_change = 0.f;
+
+	if ((dt > FLT_EPSILON) && std::isfinite(_last_feedback)) {
+		feedback_change = (feedback - _last_feedback) / dt;
+	}
+
+	return feedback_change;
+}

src/lib/pid/PID.hpp

@@ -50,6 +50,7 @@ public:
 	void resetDerivative() { _last_feedback = NAN; };
 private:
 	void updateIntegral(float error, const float dt);
+	float updateDerivative(float feedback, const float dt);
 
 	float _setpoint{0.f}; ///< current setpoint to track
 	float _integral{0.f}; ///< integral state