Make an acceleration/deceleration ramp : any ideas?

[pxidr84]

I don't want to use a PID routine or a lookup table (because a lookup table is hungry of program memory space). It's a open-loop inverter.

The user changes the frequency reference by turning a potentiometer connected to a ADCIN port.

Exemple :

Note the acceleration and deceleration ramp of the output frequency, that changes the speed of the AC motor.

I like to have a routine (in a low-priority interrupt for exemple, to have the most accurate timings) that increments a value from the actual output frequency (for exemple 20Hz) to the frequency reference (for exemple 60Hz, defined by the user) in x seconds (for exemple 10 seconds, can be modified in real-time), and that without using any "pause" command (because my PIC has a lot things to do during this time).