Servos

[hauntedjack]

Hi, I am new to this forum and have a question.  I have worked with pneumatics for years and I am looking to branch out into servos.  My only problem is I know nothing about servos.  I have worked with several controllers that are on the market with pneumatics.  Does anyone have any books, links, or anything they recommend on servos so I can start educating myself on them?

[JonnyMac]

These links will get you started.

http://www.seattlerobotics.org/guide/servos.html
http://www.electronicsteacher.com/robotics/robotics-tutorial/intermediate-robotics/servo-motors.php

The important thing to remember is that analog servos are not "set and forget" -- the need to be updated every 20ms or so.  This can be a challenge for some programs, but if you look through the forums you'll see that we've done creative things in PBASIC to make the work.

[hauntedjack]

Ok, I think at least that I definately get the point now on what makes them work, and how much control you can have over them.  Just a few questions though.  Do servos have memory in them? Or do they need something controlling them?  Where can I find software to program servos so that I can start messing with them programming end?  If they do need something sending them this signal, are there controllers specifically designed for servos, or do they work with the same controllers (at least to an extent) as pneumatics?  Thanks for getting me this far.

[BigRez]

Looks like you and I are in for the same learning curve. ;)  I'm just now playing with a couple of servos connected to a prop-1 to see how they work. 

Do servos have memory in them? Or do they need something controlling them? 

They need that refresh signal that Jon mentions sent to them about ever 20ms.  For example, if you were to connect one to a prop-1, you could send it a signal to move to the clockwise-most position and it would do that. If you don't send it another signal to do the same thing (or move to another position) within 20ms, it could wander off to some other location by itself.

Where can I find software to program servos so that I can start messing with them programming end?

I got a couple of example programs from these message boards - just search for "servo".

If they do need something sending them this signal, are there controllers specifically designed for servos, or do they work with the same controllers (at least to an extent) as pneumatics?

Remember, that signal to the servo should be sent every 20ms so that means a lot of work for the prop-1/prop-2 to do and you've got to program it to do that.  Not a small task if other things are also going on.  Assuming that the pneumatics are fired from a valve of some sort, the main difference is that for the valve, you turn it on and off as needed without concern for the amount of time between. (Well, except for the reaction time of the valve.) 

I'm wondering the same thing though...  could a Parallax ServoPal (or similar) be used in-line between the prop-1 and the servo to free up the prop-1 from needing to perform the servo refresh function?

Another question I had about the refresh...  suppose I just needed the servo to move full CW or CCW and knew it took 10 pulses to get there.  If tension keeps the servo at that location, do I need to continue to refresh the servo after it moves to the location?

[JackMan]

Another question I had about the refresh...  suppose I just needed the servo to move full CW or CCW and knew it took 10 pulses to get there.  If tension keeps the servo at that location, do I need to continue to refresh the servo after it moves to the location?

The answer is yes. Tension on the servo horn won't keep it in a fixed position unless the tension is greater than the torque of the servo which is referred to as as "stall" (not good).

[JackMan]

Where can I find software to program servos so that I can start messing with them programming end?  If they do need something sending them this signal, are there controllers specifically designed for servos, or do they work with the same controllers (at least to an extent) as pneumatics?  Thanks for getting me this far.

Give this a look. I just ordered it a couple days ago but haven't received it yet. I'll post my results after I get a chance to play around with it.

http://www.rentron.com/Robo-Ware.htm

[BigRez]

The answer is yes. Tension on the servo horn won't keep it in a fixed position unless the tension is greater than the torque of the servo which is referred to as as "stall" (not good).

Maybe I wasn't clear...  if I program the servo with something like the following:

Code Select Expand

FOR looper = 1 to 50 
   PULSOUT Servo, pos
   PAUSE 18
NEXT

and then have more code following that, isn't there no power being sent to the servo following the loop and meaning the torque is zero?  If so, won't the servo stay in that position with only the slightest amount of tension on the linkage to the horn?

I hooked up a servo and tried this and it appears to work this way.  That is, once I get the servo to where I want it and exit the loop, the servo stays in that location without any other commands being sent to it.  Is this "bad" for the servo?  Is this just the behavior of my servo and others don't do this?

[livinlowe]

Bigrez-
Yes, your servo will stay at that position, and no it isn't bad. However, if you have a force that the servo is acting against, say a wieght hanging off a table and your using the servo to lift and drop that weight, after you exit the for next loop the servo won't stay where it is. The weight will move the servo into the down position. That could be bad for the servo depending on the weight. To make it stay in the up position, you would have to continually (every 20 mS) send a pulse to the servo saying - goto up position.

I hope that helps.

[BigRez]

Thanks Shawn - nice explanation and just as I experienced.

[JackMan]

Yep, that is correct. Misinterpretation of the "tension" statement. Actually there is still power to the servo (R), just no signal (W)  ;)