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Author Topic: Calculating Modified Servo Speed (RPS)  (Read 7825 times)

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Offline pete2009Topic starter

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Calculating Modified Servo Speed (RPS)
« on: June 22, 2009, 03:21:02 AM »
Hi Guys,

I've just started looking at building a robot and am having some calculation issues.

If I look at the U$50 robot I see I need to modify the servo to allow continuous rotation.
My question is what RPM will I get once the servo is modified.

I'm hoping my calculations here are wrong, but if I look at the recommended HS-311 servo I get the following:

From website: operating spped 0.15sec/60deg no load
So lets assume a perfect world an no load.

so to get a full circle we divide 360/60 = 6
so it takes 0.15s * 6 = 0.75s to do a full rotation
so we get a RPS of 1.3

Now to me that seems pretty low so the two options here are
1) I calculated incorrectly and you get a higher RPS
2) 1.3RPS is plenty for a robot

Can someone please advise me here as I am a bit lost.
Thanks

Offline Ro-Bot-X

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Re: Calculating Modified Servo Speed (RPS)
« Reply #1 on: June 22, 2009, 05:22:33 AM »
A regular servo converted to continuous rotation will give you around 60-70 RPM. That is a bit slow for a speed competing robot. You may find faster servos that will raise the RPM to 80 or a bit above, but still under 100 RPM. For a speedy robot, you need to raise the RPM around 200 or more. If you use PWM, it is better to have a higher RPM motor to have enough speed when needed, then slow it down for regular use. I wanted to get a a motor with built in encoders with around 200 RPM at a reasonable price for a hobbyst, but what I found available is not much faster than a fast servo (80 RPM), so, to increase the speed I need to use big diameter wheels, like Admin is using on his ERP robot. At 80 RPM (at 6V), using the Lego motorcycle wheels, I get 34 cm per second. If I will also raise the voltage to 12V (120 RPM), I will get 51 cm per second which is fine for a house roaming robot.
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Offline pete2009Topic starter

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Re: Calculating Modified Servo Speed (RPS)
« Reply #2 on: June 22, 2009, 06:12:06 AM »
I've been looking at the different servos avaliable and I have found this one which gives a decent amount of torque and also a decent price.
http://www.servocity.com/html/s3305_std__metal_gear_bb.html

The problem is that I don't know how to calculate the (no load) RPMs on this sucker.

Offline Ro-Bot-X

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Re: Calculating Modified Servo Speed (RPS)
« Reply #3 on: June 22, 2009, 04:16:56 PM »
So it's 0.20 sec for 60 degrees rotation, that means 1.2 seconds for a full 360 degrees. That makes 0.83333333333333333333333333333333 RPS. Multiplied by 60 gives us 50 RPM.

For a fast and cheap standard servo, check this one out: http://www.junun.org/MarkIII/Info.jsp?item=90
Runs at 83 RPM at 6V. For $10.50, it's a great servo, easy to modify for continuous rotation!
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