newb question, could someone explain to me why the ServoPAL is necesary?

NewJerseyTN

Parallax is selling this device called the ServoPAL that sits inline between the basic stamp and a servo continually sending pulses.· So one pulse is sent from the Basic Stamp and the ServoPAL keeps sending that pulse to the servo until another pulse is received from the Basic Stamp.· I was under the impression that with regular servos one pulse was sufficient to set it to a position AND maintain that position until another pulse was recieve.· I guess this might be usefull with a continuous rotation servo since those turn a certain amount with each pulse.· I guess my question is if I want to have a regular servo maintain a certain position against load do I need to keep pulsing or is one pulse sufficient.

Steve Joblin

you will still need to send pulses with an unmodified servo... the pulses are what tells the servo what position it should have... as soon as the servo starts to move from that position, it will detect a difference and work to correct it... this constant correction is what helps the servo maintain its position. As soon as you stop sending pulses, the servo will "relax" and you can easily rotate the servo by hand, while you are sending pulses, the servo will fight to keep its position if you try to rotate it by hand.

NewJerseyTN

Thanks Steve, I read another page just after I posted that and realized I had misunderstood what I had read about "a servo will maintain position". So my code will have to either loop in under 20ms or I need to get a servoPAL for each discrete line to a servo. For the project I'm looking at some of the servos won't have alot of load so I'm wondering if I can get away without it on those servos.

Mike Green

The main issue is what else you're trying to do between these servo pulses. 20 ms is not a huge amount of time. If you're trying to control 3 servos, the pulses themselves will take up to 7-8ms out of that leaving 12 ms. That leaves about 48 PBasic instructions and most PBasic statements take more than one instruction. You can't do much computation during that time. If your program is simple, that's fine. If you want to read several sensors and search a table for information, you may be out of luck ... hence the servoPAL and other external servo controllers. There are plenty of examples of object avoidance and other fancy stuff using just a BS2 in a BOEBOT so it can be done without the separate servo controller.

crgwbr

NewJerseyTN said...
I guess this might be usefull with a continuous rotation servo since those turn a certain amount with each pulse.

Unfortunately, thats not quite accurate. On a continuous rotation servo, the pulse length tells the servo a direction and a speed, not the amount to turn. For that, you'll need an wheel encoder.

Regards,
Craig

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NewJerseyTN

Ok, related question. Does anyone know if the Parallax serial servo controler that you can connect to the BOE sends constant pulses like the servoPAL?

Mike Green

The Parallax Servo Controller works differently, but it does send servo pulses about every 20ms just like the servoPAL.

Phil Pilgrim (PhiPi)

NewJerseyTN said...
So my code will have to either loop in under 20ms or I need to get a servoPAL for each discrete line to a servo.

Actually, one ServoPAL will control two servos.

-Phil