Why are Hobby Servos built with low torque output
garyg
Posts: 420
Hi
I'm working on a Cherry Pitting machine.
It would seem like powering the pitter would be a relatively simple matter.
I'm needing to place 4 pounds pressure on my linear bearing pitting shaft.
Hobby servos generally have only about 50 inch ounces or less torque available.
I found a servo at Hobby King that has 400 inch ounces of torque I think,
but with only 60 degrees rotation.
While I can use a windshield wiper motor from an auto salvage yard,
Why do you guys suppose that the hobby and experimenter market uses very low
torque motors?
It would seem like people that build things would like very much more torque delivered from
servos and other hobby motors.
I realize I'm being very vague here, but just looking for comments or opinions
about Servo motor torque.
Thanks for any words of wisdom.
Garyg
I'm working on a Cherry Pitting machine.
It would seem like powering the pitter would be a relatively simple matter.
I'm needing to place 4 pounds pressure on my linear bearing pitting shaft.
Hobby servos generally have only about 50 inch ounces or less torque available.
I found a servo at Hobby King that has 400 inch ounces of torque I think,
but with only 60 degrees rotation.
While I can use a windshield wiper motor from an auto salvage yard,
Why do you guys suppose that the hobby and experimenter market uses very low
torque motors?
It would seem like people that build things would like very much more torque delivered from
servos and other hobby motors.
I realize I'm being very vague here, but just looking for comments or opinions
about Servo motor torque.
Thanks for any words of wisdom.
Garyg
Comments
Duane J
Are you sure about the 60 degree rotation? It's common for servos to list their speed in terms of how fast they can travel 60 degrees. Most servos can rotate at least 90 degrees.
This servo has 462 oz*in of torque. It's speed is listed as "0.18 sec / 60 degrees" but it looking down in the comments someone mentions it has 120 degrees of rotation at normal pulse settings and the rotation can be extended to 150 if you pulse it with longer and shorter pulses than the usual 1ms to 2ms limits.
Which servo were you looking at?
BTW, I'm currently working on a magnetic encoder project. My hope is to use magnetic encoders to read the position of hobby servos. I'm hoping to convert hobby servos to continuous rotation version while still maintaining position feedback.
I think the size of the servo puts a limit on how much torque the gears can withstand. As others have mentioned, they're original purpose was to move control surfaces on radio controlled airplanes.
There are some robot servos with higher torque than normal hobby servos but the cost more. Dynamixel makes some really nice (and really expensive) robot servos.
These guys have 1,189 oz*in of torque. You need to make sure and keep you fingers out the way when testing those (or so I hear, I don't have any myself).
I have used the AX-12+ servos (now they're called AX-12A). They're nice but again, expensive.
Another option would be to make your own servo by adding a potentiometer to the output shaft of your geared motor you can control the position of the motor in a similar fashion to a hobby servo. I've done this myself and it sure makes me appreciate the ready made servos.
If that's not a factor, check out Firgelli linear actuators. Perfect for your app and they can be controlled with servo commands.
-Phil
http://www.servocity.com/html/spg400a_top_mount.html
-Phil
That's the pits!
A double-acting steam (or air, if you must) piston with a uC-controlled valve would poke those pits out posthaste.
-Phil
Now a question for Garyg: how were you planning to orient the cherry in the pitter so that the pit gets poked out from "stem to stern" rather than sideways? Or does it matter?
-Phil
Typically one would build a demo using the cheapest available items, like hobby servos, and after you know how everything works, replace inadequate parts with something that meets the need better, for example wind shield wiper motors. You could try to spec out all the parts before hand, and buy exactly sized components right off the bat; but this seldom works out, one ended up designing for the (incorrect) parts on hand instead of designing to meet the task.
We either guess, or we work from experience. A guess is still just a guess no matter how carefully you plan. Experience means you built this exact item before. As you work from guess to the exact item, its often a good idea to use the cheapest item possible, and make the best guess possible.
Some people (and companies) make one guess (and often not a very good guess), then commit to production. While this is certainly one way to do it, its not the recommended way to minimize time, expense, and stress on all involved.
http://uk.rs-online.com/web/c/automation-control-gear/electric-motors-motor-controllers-peripherals/servo-motors/?cm_mmc=UK-PPC-0212-_-01_Electric_Motors_Motor_Controllers_and_Peripherals-_-Servo_Motors-_-servo_motor
Thanks for the heads up about maximum rotation angle, I was under the impression that the speed/60 degree was the standard max.
I'll be looking for a maximum rotation spec when I purchase my motor.
I'll be considering one of the Turnigy 1270HV motors. Looks like they may be a very good option and if it does not pit my cherries, I can
use it to index my indexing pitting wheel.
Erco,
I ran across the Servo City Gearboxes you linked to last week
Those may be a good solution.
Phil,
Ideally I'd like to pit from stern to stem.
My 1st mechanical prototype pitting head would allow the pit to move if the cherry were laying on it's side.
That would result with the Pit remaining in the cherry. Not Good.
My 2nd mechanical prototype pitting head uses heavy duty sewing machine needles bonded to 1/8" square tube.
The needles stick to the pit somewhat and the integrity of the cherry remains intact after piercing it.
The end result of that is that the pits fly out of the cherry and orientation of the cherry does not matter as much.
I'll need to build a 3rd pitting head that uses 5 sewing machine needles instead of the 4 needles.
The spacing of the 4 needles allows the pit to get stuck if the pit happend to be sideways.
I'll be watching the gearing specs of the Servo City gearboxes involving max speed.
I like that style of gearbox so I'll be investigating that further.
Thanks to all who replied to my servo torque question.
I always learn something new in this forum.
It's all relative. What you may consider low torque is plenty of torque to someone else. As has been said, hobby servos were designed for small RC vehicles. They serve their intended purpose well. As time goes on the demand has increased for stronger servos for giant scale RC, helicopters, steering for off road cars and trucks, and higher performance aircraft.
Have you considered using a solenoid? http://www.mcmaster.com/#solenoids/=nxqrqm
I'm always tempted to use a solenoid for linear short stroke applications.
BUT
In a past life, whenever I've used solenoids, they had tended to cause LOTS of electrical noise that I was not able to
compensate for.
Since I have not dealt with solenoids for a number of years, it's a possible solution for this particular project.
I'm also now considering a pneumatic solution.
Thanks for your comment
I believe my thinking is starting to move outside of my box.
A small pneumatic or hydraulic piston may be just the ticket for this application, although a motor and crankshaft assembly could also work.
-Tor
If you want to pit cherries, maybe something such as 1/10th hp motor is appropriate... that is still only about 75 watts of power, but huge in comparison to a hobby servo.
We could use input from a few good mechanical engineers