Make an acceleration/deceleration ramp : any ideas?

[pxidr84]

Hi,

I'm currently making a 3-phase VFD for an AC motor.
The program is 70-80% done, but I've to make an acceleration/deceleration ramp.

In other words, I start with a minimum value defined by the user (for exemple : 10), and I reach the maximum value defined by the user (for exemple 1200) linearly in x seconds (again defined by the user).

Here an exemple :


So any ideas how I can do that simply?
-With a low-priority interrupt?
-With a for...next loop?
-etc.

Thanks for your future help.

[Jerson]

If you do not intend running any tasks in the background while doing the acc/deceleration, the following idea may work

Rate = ((Max-Min) *1000)/ Time   ' the 1000 is since I assume a mS time base

Here the Time variable is specified in seconds.

Now, using a 1mS time base, you can do this

for x = 0 to rate
   Output = Output + Rate  ' increase the output by a fraction we calculated
   pause 1           ' this is the 1mS time base I talked of
next

Rate could be -ve to decelerate.

[pxidr84]

If you do not intend running any tasks in the background while doing the acc/deceleration, the following idea may work

Rate = ((Max-Min) *1000)/ Time   ' the 1000 is since I assume a mS time base

Here the Time variable is specified in seconds.

Now, using a 1mS time base, you can do this

for x = 0 to rate
   Output = Output + Rate  ' increase the output by a fraction we calculated
   pause 1           ' this is the 1mS time base I talked of
next

Rate could be -ve to decelerate.

Hello,

Thanks for your idea, I will try this into my program, but unfortunately I think that the "pause" command will make problems, because a lot of things are running in the background during this acc/deceleration (like the sine PWM generation high priority interrupt, using timer 1).

I think use this in a low frequency and low priority interrupt (with timer 0), maybe the "pause" command will be useless?

Note I'm using Darrel Taylor's interrupts.

[J. Mark Wolf]

My approach would be to generate the "points" you need for the line equation, given your slope and time parameters, in Excel. Then print a CSV file of the results, then copy the CSV file into your program as a table. It'll take some minor editing to generate your table but no big deal, and get the values from the table with Lookup or Lookup2. You can even change the slope of your ramp easily by adjusting the dwell time at each point, etc.

I do this sort of thing a lot for thermistor temperature tables, etc. Saves the time of executing the extensive formula for thermistor values.

[mark_s]

Can you give more info?

What is the motor being used for?

How does the user change the speed?

Typical servo systems use a PID routine.

Henrik Olsson posted a great PID program example on the forum. As I recall the thread mentioned motors and DT interupts.

[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).