Servo direction confusion question!

Bill Chennault

All--

Will you please set me straight on a question concerning servo-motor direction? I have two servos from Parallax, both are unmodified. I can make both move either clockwise or counter-clockwise. But, when I look at certain other servo-motor documentation, it appears that the device in question will only rotate in one direction, usually clockwise. To make it rotate counter-clockwise some (factory) modification is necessary.

Will such a servo-motor rotate both CW and CCW like the little 47oz-in motors I have from Parallax?

Here is an example . . .

http://www.servocity.com/html/hs-5945mg_digital_torque.html

Thanks!

--Bill

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You are what you write.

allanlane5

Well, you need to understand that the servo-motor combines an electric motor, reduction gears, a variable resistor connected to the gears, drive electronics for the motor, and a pulse-width-comparator device.

All of this was created to make a very simple to interface device to position flight-control surfaces in Radio Controlled (RC) airplanes. The control signal is normally low. It gets a high-going pulse of 1 mSec to 2 mSec width, repeated every 20 mSec to 50 mSec. The pulse width indicates to the servo the desired position of the servo.

The servo does this by comparing an 'internal' pulse generated with the variable resistor, then drives the internal motor to position the internal resistor to match the commanded pulse width. Voila, an output shaft with a controllable position through a certain number of degrees. Typically 180 degrees, but 90 degree servo's (like the one you call out) are also common.

A 'modified' servo has been 'modified' to disconnect the variable resistor from the gear train. The 'modification' also removes any mechanical stops that might prevent full rotation of the output shaft. The variable resistor is set at the middle of its range, or 1.5 mSec. Pulses longer than 1.5 mSec will cause the servo to 'run' forward continuously (trying to move the disconnected resistor, which it won't, but it will keep trying). Pulses shorter than 1.5 mSec will cause the servo to 'run' in reverse continuously. Pulses AT 1.5 mSec will make the servo think the commanded position already matches the Resistor position, so rotation stops.

So yes, the servo-motor you call out can be 'positioned' CW and CCW. Continuous rotation isn't going to happen, of course, because they're not 'modified' to do that.

Chris Savage

Bill,
·
·· If you could only move in one direction how would the servo return to its original position?· It can’t be spring return because servos are actually geared down.· I’m not sure where you have read this information but standard hobby servos move CW and CCW in a 90 to 180 degree range.· 1 mS to 2 mS pulse width with 1.5 mS being center.· Take care.

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Chris Savage
Parallax Tech Support

Bill Chennault

allanlane5--

Great explanation! Thank you very much!

--Bill

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You are what you write.

Bill Chennault

Chris--

Yeah . . . that return-thing kinda bothered me, too! [noparse]:)[/noparse]

It was all due to my misunderstanding of just exactly what a servo is designed to do. I have a better concept, now.

--Bill



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You are what you write.

Chris Savage

Bill,
·
·· You learn as you go.· In any event we have some tutorial information in our, “What’s a Microcontroller?” manual which you can download free from our website.· It would talk you through the servo experiments.· No problem asking though…Someone will try to answer.· Take care.

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Chris Savage
Parallax Tech Support

Bill Chennault

Chris--

Thanks!

WaM is great. I downloaded the document AND bought the manual (some time ago)·and bought a BUNCH of other stuff. Now, if I could only buy more HOURS IN THE DAY. <g>

I am in the midst of the servo experiments now and everything is going well. My real problem may have been more syntactical and grammatical (on the web site I mentioned above) than anything else. Or, it's just me, which I am perfectly willing to accept!

This web site and the support found hereon from forum members and Parallax employees are the reasons I am involved in Parallax microcontrollers and other Parallax products.

--Bill

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You are what you write.

PAR

Bill Chennault said...
...·when I look at certain other servo-motor documentation, it appears that the device in question will only rotate in one direction, usually clockwise. To make it rotate counter-clockwise some (factory) modification is necessary.

Will such a servo-motor rotate both CW and CCW like the little 47oz-in motors I have from Parallax?

Here is an example . . .

http://www.servocity.com/html/hs-5945mg_digital_torque.html

Bill, I believe what was confusing you is the HiTec ad·specification that gives only one direction (clockwise in this case) for the pulse timing spec. The device will go clockwise if it gets a timing pulse of between 1500 and 1900 usecs. (But, what the ad doesn't say is that the motor will also turn counterclockwise if given a pulse shorter than 1500 usecs.)
Direction: Clockwise/Pulse Traveling 1500 to 1900usec
HiTec (in the ad you cite) also can provide a motor that will turn counterclockwise if given that same timing pulse (and clockwise if given the pulse shorter than 1500 usecs). The ad refers to this as "reverse rotation".
You purchase (select, order) one model direction ("stock" rotation)·or the other direction ("reverse" rotation) ·(and one of the rotation types --90 deg, 180 deg, or continuous).
PAR

Bill Chennault

PAR--

You nailed my point of confusion perfectly.

Thank you.

--Bill

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You are what you write.

Chris Savage

PAR, thanks…I must admit I did not look at the ad or I may have seen the source of the confusion.· Take care.

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Chris Savage
Parallax Tech Support