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I am modelling an Automatic Cruise Control model in Simulink. I used the PID block and tuned the parameters by trial and error. Now the tuned PID controller works for input speed ranges from 40 kph to 165 kph. The problem is that when I enter the desired speed value to be less than 40 kph, it no longer works!

What could be the problem? Also, as I am using blocks from simscape to build my vehicle model, I don't know the transfer function.

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    \$\begingroup\$ Sounds like a job for Keanu Reeves and Sandra Bullock... \$\endgroup\$ – Brian Drummond Apr 24 '15 at 10:07
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You may need to detune it (make the response more sluggish) at the higher speeds, or apply the concept of gain scheduling. It behaves now like the closed-loop gain is higher at low speeds, which will tend to reduce the stability.

Real cruise controls are generally disabled at very low speeds anyhow, for safety reasons.

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