Discussion:
Searching for servos.
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Henning Wangerin
2012-09-17 23:33:24 UTC
Permalink
Hi.

I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.

Most servos I have found either
- turns approx 170 degress, and that's not enough for my needs,
- or is continuously turning, witch is to no use as I need to be able to
move to a specific location.

What I need is a largely a device like this:
http://dx.com/p/mg995-tower-pro-servo-gear-for-r-c-car-plane-helicopter-148051?item=12
except that it a rotation-angle of at least 270 degrees. 360 or more
would be even better ;-)

Do you have an idea on where I can get such servos?

I would be happy for any hints on where to find such servos.

/Henning
Vance Howard
2012-09-18 00:11:22 UTC
Permalink
Post by Henning Wangerin
Hi.
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.
Most servos I have found either - turns approx 170 degress, and that's
not enough for my needs,
- or is continuously turning, witch is to no use as I need to be able to
move to a specific location.
http://dx.com/p/mg995-tower-pro-servo-gear-for-r-c-car-plane-
helicopter-148051?item=12
Post by Henning Wangerin
except that it a rotation-angle of at least 270 degrees. 360 or more
would be even better ;-)
Do you have an idea on where I can get such servos?
I would be happy for any hints on where to find such servos.
/Henning
***@hitecrcd.com
Have you contacted Hitec RCD to see if any of their servos could be
modified to do what you want? They have one robotic servo that can be
customized.
--
to reply via email: vhoward1122 at gmail dot com
Skywise
2012-09-18 01:18:15 UTC
Permalink
Post by Henning Wangerin
Hi.
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.
Most servos I have found either
- turns approx 170 degress, and that's not enough for my needs,
- or is continuously turning, witch is to no use as I need to be able to
move to a specific location.
My electronics background is screaming "stepper motor". But
it depends on the application specs, which we don't know.

Brina
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Sed quis custodiet ipsos Custodes?
Robert Roland
2012-09-19 19:14:24 UTC
Permalink
On Tue, 18 Sep 2012 01:33:24 +0200, Henning Wangerin
Post by Henning Wangerin
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.
There are sail winches (for sailboats) that work just like a servo.
They are usually slow and turn a few turns from end to end.

Here's an example:
http://www.hitecrcd.com/products/analog/boat/hs-785hb.html

You can increase the throw of a servo by adding resistors to the two
outer leads of the potentiometer, but you might not be able to reach
the full 270 degrees without overdriving the pot. I do not know if
there is gearing between the output shaft and the pot. If there is, it
could work.
--
RoRo
Tim Wescott
2012-09-22 21:28:00 UTC
Permalink
Post by Robert Roland
On Tue, 18 Sep 2012 01:33:24 +0200, Henning Wangerin
Post by Henning Wangerin
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.
There are sail winches (for sailboats) that work just like a servo. They
are usually slow and turn a few turns from end to end.
http://www.hitecrcd.com/products/analog/boat/hs-785hb.html
You can increase the throw of a servo by adding resistors to the two
outer leads of the potentiometer, but you might not be able to reach the
full 270 degrees without overdriving the pot. I do not know if there is
gearing between the output shaft and the pot. If there is, it could
work.
All the servos that I've taken apart put the pot right on the output
shaft (which makes sense -- you want to measure the output shaft, not
some gear train's opinion of it).

Most of the servo pots that I've seen would just barely read over 270
degrees -- you need to be able to reliably read position past the
intended ends of travel, or the servo will "run away" (most likely into a
mechanical stop). So if you could get 270 degrees out of it, it'd be
finicky.

Having said that, most servos have a mechanical travel limit (again that
works off the output shaft). There's usually a pin running in a groove
of limited extent, or a pin running into a vane. Cutting off the pin
gives you round-de-round mechanical travel, but doesn't solve the pot
travel problem.

All in all, I think you could reliably get more than 180 degrees, but I
don't think you could get up to 270, and you'd have to either know what
you're doing going in, or be smart enough so that you know what you're
doing coming out.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Henning Wangerin
2012-09-23 09:35:20 UTC
Permalink
Post by Tim Wescott
Most of the servo pots that I've seen would just barely read over 270
degrees -- you need to be able to reliably read position past the
intended ends of travel, or the servo will "run away" (most likely into a
mechanical stop). So if you could get 270 degrees out of it, it'd be
finicky.
One of my servos MUST be at least 225 degrees. I've been looking on
sail-winch servos.

Some of them need more to get maximum benefit, but wound work with less
that 270.

That may do the task.

Thanx

/Henning
RogerN
2012-09-23 16:22:04 UTC
Permalink
Post by Henning Wangerin
Hi.
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not been
able to find any.
Most servos I have found either
- turns approx 170 degress, and that's not enough for my needs,
- or is continuously turning, witch is to no use as I need to be able to
move to a specific location.
http://dx.com/p/mg995-tower-pro-servo-gear-for-r-c-car-plane-helicopter-148051?item=12
except that it a rotation-angle of at least 270 degrees. 360 or more would
be even better ;-)
Do you have an idea on where I can get such servos?
I would be happy for any hints on where to find such servos.
/Henning
You can find a servo that is capable of being modified for continuous
rotation and modify it by taking the pot out if possible. Now if you modify
the pot feedback with gears or pulleys, you could get 2X the output rotation
for 1X pot rotation. For example a 12T gear on the output shaft and a 24T
on the pot, then instead of 170 Deg full throw, you'd get 340 Deg full
throw. You don't have to use gears, you can use string and pulley's with a
spring for tension or something like that. If you read the pot ohms from
one endpoint to the other you could find a multi-turn pot that would give
the right change in resistance within a turn or so.

I've thought of doing similar but the servo's pot would be on my actuator
and the servo would be used like a gear motor to move the actuator. If you
wanted to get more control you could do something with a continuous servo
and feedback to an Arduino, the application I'm interested in involves an
airsoft gun aiming device, the arduino can be programmed to stop sending
pulses when no move command is being sent, that way the servo won't drift
and jump around from pot or electrical noise. The servos would turn screws
or worm & wheel to point the airsoft gun. The Arduino, or similar, approach
could be used to open the door to all kinds of possibilities even
controlling industrial servos, CNC machines, & Robotics from a hobby radio
control transmitter.

RogerN
Tim Wescott
2012-09-25 05:15:56 UTC
Permalink
Post by RogerN
Post by Henning Wangerin
Hi.
I'm looking for propotional servos for a small robotic projoct, where I
need small servos that can operate minumum 270 degress, but have not
been able to find any.
Most servos I have found either - turns approx 170 degress, and that's
not enough for my needs,
- or is continuously turning, witch is to no use as I need to be able to
move to a specific location.
http://dx.com/p/mg995-tower-pro-servo-gear-for-r-c-car-plane-
helicopter-148051?item=12
Post by RogerN
Post by Henning Wangerin
except that it a rotation-angle of at least 270 degrees. 360 or more
would be even better ;-)
Do you have an idea on where I can get such servos?
I would be happy for any hints on where to find such servos.
/Henning
You can find a servo that is capable of being modified for continuous
rotation and modify it by taking the pot out if possible. Now if you
modify the pot feedback with gears or pulleys, you could get 2X the
output rotation for 1X pot rotation. For example a 12T gear on the
output shaft and a 24T on the pot, then instead of 170 Deg full throw,
you'd get 340 Deg full throw. You don't have to use gears, you can use
string and pulley's with a spring for tension or something like that.
If you read the pot ohms from one endpoint to the other you could find a
multi-turn pot that would give the right change in resistance within a
turn or so.
I've thought of doing similar but the servo's pot would be on my
actuator and the servo would be used like a gear motor to move the
actuator. If you wanted to get more control you could do something with
a continuous servo and feedback to an Arduino, the application I'm
interested in involves an airsoft gun aiming device, the arduino can be
programmed to stop sending pulses when no move command is being sent,
that way the servo won't drift and jump around from pot or electrical
noise. The servos would turn screws or worm & wheel to point the
airsoft gun. The Arduino, or similar, approach could be used to open
the door to all kinds of possibilities even controlling industrial
servos, CNC machines, & Robotics from a hobby radio control transmitter.
RogerN
That's not a bad idea.

I'm not sure if you're confused or me, but the pot in a servo isn't just
a variable resistor -- it's a real potentiometer, that gets 0V and servo
power, and reads the servo position as a voltage.

If you could find a 3-turn pot _and_ you could figure out how to extend
the output shaft out the bottom of the servo, you'd get a 360 degree
rotation servo out of the deal.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
RogerN
2012-09-27 00:24:56 UTC
Permalink
<snip>.
Post by RogerN
RogerN
That's not a bad idea.
I'm not sure if you're confused or me, but the pot in a servo isn't just
a variable resistor -- it's a real potentiometer, that gets 0V and servo
power, and reads the servo position as a voltage.
If you could find a 3-turn pot _and_ you could figure out how to extend
the output shaft out the bottom of the servo, you'd get a 360 degree
rotation servo out of the deal.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
When I started in R/C you could buy left hand and right hand servos, most of
the sets I saw came with 3 right hand servo's and 1 left hand servo. I used
to take my servos apart and reverse the pot leads and motor leads to reverse
servos. After doing this, I had to re-center the servo. Because of this I
believe servo center isn't pot center and servo rotation isn't necessarily
end to end on the pot. So if a servo uses maybe 120 Degrees of a 180 Degree
pot, maybe using a (180 X 3) degree turn pot might give a (120 X 3) 360
degree output on the shaft. Or if you made the pot external you could do
the ratio you wanted. I guess one could use a string pot and a drum to wind
string to make a multi-turn output shaft.

RogerN
Tim Wescott
2012-09-29 04:11:09 UTC
Permalink
Post by RogerN
<snip>.
Post by RogerN
RogerN
That's not a bad idea.
I'm not sure if you're confused or me, but the pot in a servo isn't just
a variable resistor -- it's a real potentiometer, that gets 0V and servo
power, and reads the servo position as a voltage.
If you could find a 3-turn pot _and_ you could figure out how to extend
the output shaft out the bottom of the servo, you'd get a 360 degree
rotation servo out of the deal.
--
Tim Wescott Control system and signal processing consulting
www.wescottdesign.com
When I started in R/C you could buy left hand and right hand servos,
most of the sets I saw came with 3 right hand servo's and 1 left hand
servo. I used to take my servos apart and reverse the pot leads and
motor leads to reverse servos. After doing this, I had to re-center the
servo. Because of this I believe servo center isn't pot center and
servo rotation isn't necessarily end to end on the pot. So if a servo
uses maybe 120 Degrees of a 180 Degree pot, maybe using a (180 X 3)
degree turn pot might give a (120 X 3) 360 degree output on the shaft.
Or if you made the pot external you could do the ratio you wanted. I
guess one could use a string pot and a drum to wind string to make a
multi-turn output shaft.
Nominal center definitely isn't real center, and if you take one of the
pot elements out completely you'll see that it has a bit more than 270
degrees of element available for sensing rotation.

I reversed a couple of HS-55 servos a few years ago -- I don't know if
I'll try that again, but I'm pretty sure I won't try it with anything
smaller!
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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