Create function from polyfit curve

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Megan Miller
Megan Miller 2020-9-10
评论： Ameer Hamza 2020-9-10
I'm trying to make an equation that correlates engine speed with pressure. My goal is to be able to input an rpm into a function and have it spit out a pressure. I have a plot of the data and a polyfit curve.
Here is the relevant snipit from my script:
[p, S, mu] = polyfit(Input_Speed, press, 7);
v = polyval(p, Input_Speed, S, mu);
figure(3)
plot(Input_Speed, press)
hold on
plot(Input_Speed, v)
xlabel("Speed (rpm)")
ylabel("Pressure (psi)")
legend('Filtered Data', 'Fit Curve')
What is the best way of doing this? A lookup table? Use poly2sym to turn the polyfit coefficients into an equation? I'm open to any suggestions. Thank you!
  1 个评论
KSSV
KSSV 2020-9-10
Already you have coefficients in hand..you can use polyval to get the output values for given input. poly2sym shows you a polynomial in x as symbolic class but you need to substitue and get the value again.

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采纳的回答

Ameer Hamza
Ameer Hamza 2020-9-10
An easy solution is to wrap the result of polyfit into an anonymous function
[p_coff, S, mu] = polyfit(Input_Speed, press, 7);
P = @(Input_Speed) polyval(p_coff, Input_Speed, S, mu);
Then you can call it like a function, without the need to specify p_coff, S, and mu each time
figure(3)
plot(Input_Speed, press)
hold on
plot(Input_Speed, P(Input_Speed))
xlabel("Speed (rpm)")
ylabel("Pressure (psi)")
legend('Filtered Data', 'Fit Curve')
  2 个评论
Megan Miller
Megan Miller 2020-9-10
That did the trick. I completely spaced making Input_Speed the variable instead of the default of x. I used p_coff, S, and mu just to get rid of a warning message saying the data was unfit for polyfit but your method got rid of that too. Thank you!!

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