RC Servo Question

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LizzieB

Senior Member
I need about 270 degrees rotation, in 30 or 45 degree steps. +/- 3 degrees accuracy would be good enough. Very little torque required, rotates a sensor horizontally. Can't rotate > 360 deg. Needs to be able to be made weather proof (within reason). Speed is not important. Will scan maybe ea 15 minutes, for 12 hours a day.

I thought this would be a good 08 + RC servo application (never used one before) but I guess the max rotation is less than 270. Can someone tell me what the real world max rotation is? I'm not seeing that on the datasheets (probably means I'm blind).

Assuming the servo won't do it what other 08 driven suggestions do folks have? I don't really want to hassle with a stepper and some kind of limit/0 position sensor, but if I must, I must.
 
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BeanieBots

Moderator
Most "standard" servos will only sweep about 90deg.
However, you can use mechanical gain. This can be with push-rods or gears if you don't mind amplifying the backlash.
Alternatively, have a browse for servos at a hobby shop. There many different types. The used in sail-boats for example (sometimes titled "winches") can do all manner of rotational arcs. Many even do N*360.
Also, landing gear servos often have > 180 deg.
They should all clearly indicate the angle of rotation.

eg
http://www.servoshop.co.uk/index.php?pid=FUTS5801&area=Servo

or maybe this if you don't want to spend so much.
http://www.sussex-model-centre.co.uk/shopexd.asp?id=22160

As with everything, you get what you pay for.
 
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LizzieB

Senior Member
Should I be able to find gears that are designed to go directly on the servo shaft?

Whoa, pricey. I was also thinking I would be spending a few $ for one..
 
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boriz

Senior Member
Idea:

Use two servos. One has it’s base fixed onto the output of the other. Piggy back. Most servos I have played with go about 180 degrees, so two piggybacked should easily manage 270. Use a standard servo for the base, a mini servo for the upper.
 
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Andres Rodriguez

New Member
The 2 servo, the gears, and/or the belts will work. But, won't it be easier and less expensive with a stepper motor?
 
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premelec

premelec

Senior Member
You Need position sense too I think for this app... you could hack a servo for continuous rotation and then put an external pot on it that would approximate the hacked out pot - the servo doesn't need to 'know' where the control signal is coming from... There are surplus servo pots that have 360 deg rotation but your ordinary pot could probably do the 270deg.

Stepper or DC motor could work - with code wheel feedback [see codewheel generation program referenced on this forum... as usual too many ways to do it - but which one?? :)
 
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BeanieBots

Moderator
With a "standard" servo the limitation is the hard stops. These are usually +/-90 ie 180 full swing. Many can be easily removed with a scalpel but it depends on the specific servo how easy/hard that is to do.
Then there's the POT which is limited (as are most) to around 270 deg.
If the POT limitation is OK for your app, then to actually get the full possible 270 degree turn using "standard" servo signals, you will also need to add a resistor in series with EACH side of the POT to make it less sensitive and hence force the motor to drive it further for the same amount of feedback.
 
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Michael 2727

Senior Member
If you don't want to spend big bucks just make wooden/plastic gears/wheels and a rubber band
or if you want to get fancy then cut a groove on the rim of the wheels and use a belt
off a Cassette recorder or VCR etc.
You could use the old Transistor Radio (Tuner) style string method where you wrap
the string around each wheel 3 or 4 times then the join has a small spring for tension.
this will prevent the wheels slipping out of sync.
The calculations should only take 2 mins on any calculator.
 
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LizzieB

Senior Member
The servo is attractive due to the simple electronic control and especially the ease of making a mechanical connection to the shaft. I've been frustrated in a couple of projects recently by my inability to achieve a good, solid, centered connection to the diamond hard, mirror finish, 3 mm shafts of small motors. (I'm very tempted to get one of those $500 mini lathes from Harbor Freight so I can make my own parts.)

I'm going to look into the "modify for extended rotation" idea. Even if that doesn't work out I can still use the servo modified for continuous rotation (I've seen plenty of instructions for that) with some sensing arrangement (opto, reed) for the detent positions.

Thanks for all the suggestions.
 
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Wrenow

Senior Member
For gears. belts, pulleys, mounting plates, etc. that fit servos, many are available from http://servocity.com

In fact, they make a geared servo system that allows you to gear the output down while using andn external pot for position control (the output gear uses the pot for its axle). Yes, for weatherproofing, you could use an oil-filled waterproof pot, methinks.

You will also find servos there with specs, some with up to or over 270 degrees (including sail winch servos with 720 degrees).

You might want to be aware that Traxxas now makes some nice waterproof servos that might be a good idea in your application.

Another source you might find useful is http://lynxmotion.com/ - several servo mount and accessory mounting options I imagine you would find useful.

Cheers,

Wreno
 
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