How to measure the rate of rise of a variable - c++

Question

I am reading in a temperature value every 1 second/minute (this rate is not crucial). I want to measure this temperature so that if it begins to rise rapidly above a certain threshold I perform an action.
If the temperature rises above 30 degrees ( at any rate ) I increase the fan speed.
I think I must do something like set old temperature to new temp and then each time it loops set old temp to the current temp of the engine. But I am not sure if I need to use arrays for the engine temp or not.

Answer 1

Of course you can store just one old sample, then check difference like in:
bool isHot(int sample) {
static int oldSample = sample;
return ((sample > 30) || (sample - oldSample > threshold));
}
It's OK from C point of view, but very bad from metrology point of view. You should consider some conditioning of your signal (in this case temperature) to smothen out any spikes.
Of course you can add signal conditioning letter on. For (easy) example look at Simple Moving Avarage: https://en.wikipedia.org/wiki/Moving_average
If you want control the fan speed "right way" you should consider learning a bit about PID controller: https://en.wikipedia.org/wiki/PID_controller
Simple discrete PID:
PidController.h:
class PidController
{
public:
PidController();
double sim(double y);
void UpdateParams(double kp, double ki, double kd);
void setSP(double setPoint) { m_setPoint = setPoint; } //set current value of r(t)
private:
double m_setPoint; //current value of r(t)
double m_kp;
double m_ki;
double m_kd;
double m_outPrev;
double m_errPrev[2];
};
PidController.cpp
#include "PidController.h"
PidController::PidController():ControllerObject()
{
m_errPrev[0] = 0;
m_errPrev[1] = 0;
m_outPrev = 0;
}
void PidController::UpdateParams(double kp, double ki, double kd)
{
m_kp = kp;
m_ki = ki;
m_kd = kd;
}
//calculates PID output
//y - sample of y(t)
//returns sample of u(t)
double PidController::sim(double y)
{
double out; //u(t) sample
double e = m_setPoint - y; //error
out = m_outPrev + m_kp * (e - m_errPrev[0] + m_kd * (e - 2 * m_errPrev[0] + m_errPrev[1]) + m_ki * e);
m_outPrev = out; //store previous output
//store previous errors
m_errPrev[1] = m_errPrev[0];
m_errPrev[0] = e;
return out;
}