Hi Hongxiao,
The first argument of the step function is an independent variable, which is generally time in most cases. However, you are taking the relative angular velocity in the first argument. The same is the case with the AKISPL spline function, which generally needs an independent variable in the first argument, like time. I also noticed that you didn't close the bracket for the second argument of the step function.
Please check out the information on functions in motion document and try to build simple functions first.
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