Need Help on RC Servo Ramp (speed)

arawana · Joined: 06 Apr 2008 Posts: 19 Location: SINGAPORE

Hi, all CCS member and Happy new year!!

I am a student at Singapore, I got a project about controlling 16 RCservo motors on PORTB & PORTC. Currently I am using 16F877A & 20MHz. I can control position but my problem now is how to control RC servo speed (ramp)?

Can PIC16F877A do a ramp program? Using which PIN? CCP? T0M1L Or T1M1L? Or it can only be done using DSPIC

Can anyone teach me the theory behind or could you just give me reference book or website. I have been searching on internet but cannot find it.

Thank you very much for your time and it is a big help if anyone can share some knowledge with me.

Ttelmah · Guest

The ramp won't involve a pin.
It will be down to how _you_ control your pulse widths.

For instance. You have a servo at one end of it's travel, with a pulse coming to it,of perhaps 0.6mSec. You want to move it to the other end of it's travel, so you change the pulse to 2.4mSec. The servo will move as fast as it can to this new position.
If instead, you change the pulse to 0.7mSec, count 100 loops of the servo pulse code, change it to 0.8mSec, count another 100 loops, and do this till the value reaches 2.4mSec, the servo will now take 1700+ cycles of your code, to move the distance.
So, you need to control the rate at which you change the pulse widths when moving. Down to some counting. Hw to do it, will depend on how your servo code is implemented, what the loop time is, what rates you want, etc. etc..

Best Wishes

arawana · Joined: 06 Apr 2008 Posts: 19 Location: SINGAPORE

ok

Because I have more than 2 RC servos, I have to generate PWM manually using loop.

Exp for 1 servo:

bungee- · Joined: 27 Jun 2007 Posts: 206

You can generate PWM using timer interrupt Wink

Look on forum I have posted an example for 8 servos on PIC18F4550 but it should not be a problem to recode it for your need.

Then you'll have to do ramping in your main code, but that will be easier because interrupt will take care of PWM for all the servos at "once"

John P · Joined: 17 Sep 2003 Posts: 331

I was all set to say that this is a tough little problem, but now I think I see the solution--you don't try to run the servo pulses all at once, but in sequence. First of all, make the servo refresh rate 32msec instead of the conventional 20 msec; apparently they still work OK if this is done. Dedicate the first 2msec of the 32msec to Unit 0, the next 2 msec to Unit 1 etc up to unit 15. That way, you only have to produce one pulse at a time, which means closely-matched intervals don't cause problems when they almost coincide, but not exactly. Just count through the 16 settings and generate a time interval for each. It seems workable. Use a proper timer with an interrupt and not that horrible delay() function.

I've only ever played with very cheap little servos, but what I found when I tried to control the speed was that they responded with a sequence of jerks rather than smooth motion. Maybe higher quality ones would be better, but you might have to live with less than perfectly smooth action.

Oh wait--the servo pulse never goes below 1msec. So maybe you could keep the 20msec cycle, but instead of having the units operate one after another, you could overlap the operating periods, starting each one when the previous one reached 1msec. That way, when Unit 0 ended (between 1msec and 2msec) Unit 1 would be running, but Unit 1 wouldn't end until a time between 2msec and 3msec, and by then you'd have started Unit 2, and so forth. You could use one timer to generate the repeated 1msec start times, and a second timer whose start and stop times would be interleaved with the first one, to produce the servo pulses. Would that be fun, or what?

arawana · Joined: 06 Apr 2008 Posts: 19 Location: SINGAPORE

Hi bungee-

I still don't understand your coding on http://www.ccsinfo.com/forum/viewtopic.php?p=115112#115112. Is it for remote control flight?

1) I have compile and try to simulate in proteus ISIS, but nothing happen? Do the program is waiting user to give input then it will move to certain position?

2)Is PPM same with PWM?

3) And how to do ramping program? Can you write down some program. Because I have try hard to understand your program.

Thanks

bungee- · Joined: 27 Jun 2007 Posts: 206

#1 - code there is correct, but the part where the servo's are moved is missing. If you want to move servo you just set the variable servo[x] to some value in the main program loop there where I put the delay_cycles(1). like: servo[3]=19;

#3 - So if you wan't to slowly move servo then you can do that in a loop in a main part of program like that: