Need: Very fast servo good for CR

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alasdair
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Need: Very fast servo good for CR

Postby alasdair » Sat Jun 16, 2012 8:16 pm

Hi there

I have designed a program and a PCB for a line following robot for the micromouse competition, and the drive is going to be a servo modified for continuous rotation. Therefore I am looking for a pair of very fast servos that can take 7.4v and are within a reasonable price range. They also need to be easily modifiable to have continuous rotation.

Anyone know anything good?

Cheers, Alasdair

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BeligerAnt
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Re: Need: Very fast servo good for CR

Postby BeligerAnt » Sat Jun 16, 2012 8:54 pm

Hi Alasdair

In my experience there is no such thing as a "really fast" servo. I used standard servos for my mini-sumo robot and they were really, really slow. One problem is that the servo control boards do not drive the motors to anything like full speed.

I ended up designing my own speed controllers and got much more speed out of exactly the same motor/gearbox combination than was possible with the servo boards.

One possibility would be to use the control boards from some cheap standard servos to drive some of the Chinese gearmotors. This would mean you still control the motors with R/C pulses but you get some decent performance on 7.4V. Both Oliver and I have used this set-up successfully on antweights.

Hope this helps.
Gary, Team BeligerAnt

alasdair
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Re: Need: Very fast servo good for CR

Postby alasdair » Sun Jun 17, 2012 5:04 pm

Thanks for your advice. I have looked into using some motor drivers and some gearmotors for it, and I will probably just adapt what I saw on a non contact wall follower robot using the same kind of setup. One of the main problems is that I am not fully aware of how you program motors on a picaxe. I am assuming if you want variable speeds you have to use a analogue input?

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peterwaller
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Re: Need: Very fast servo good for CR

Postby peterwaller » Sun Jun 17, 2012 6:12 pm

I have not used the pickaxe processors but I expect they are the same as the AVR processors I use.
You need a processor that has two PWM (pulse width modulation) outputs.
Then you just output a number from 0 to 255 to go from stop to fullspeed.
Once you have set the speed it will continue to output the correct on to off ration until you send it another number.
You will also need to output direction info usually on one or two digital outputs for each controller.
Image
In the diagram output PD5 and PD6 can be configured as PWM outputs OC0B and OC0A.
PD5 controls the speed of one motor and PD0/PD1 control its direction.
PD6 controls the speed of the other motor and PD2/PD3 control its direction.
To set the speed and direction is as simple as:
out........OCR0A,value(0 -255).......;set speed
mov.......portbout,direction1.........;get the direction
out........PORTB,portbout..............;output the direction control

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BeligerAnt
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Re: Need: Very fast servo good for CR

Postby BeligerAnt » Sun Jun 17, 2012 6:24 pm

So Mr Waller just beat me to it, and included a nice diagram...

(The Picaxe is based on a PIC microcontroller. The Atmel AVR series is similar and forms the basis of the Arduino.)

I too am not overly familiar with what the Picaxe environment provides. However, to drive a motor at a variable speed the usual technique is to use pulse width modulation (PWM). The Picaxe environment may disguise this as an "analog output" (I know the Arduino does this!). I assume you meant "analog output" rather than "analog input"?

The important point is that a DAC analog output will not drive a motor. PWM is definitely the way to go to drive variable speed (DC) motors.

The great advantage of using servo boards to drive gearmotors is that your software simply controls the speed by outputting a 1-2ms pulse every 20ms, as you would have to do to control modified servos anyway. I assume you had already figured this out as you were planning to use modified servos?
Gary, Team BeligerAnt

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olivers
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Re: Need: Very fast servo good for CR

Postby olivers » Tue Jun 19, 2012 10:51 am

Search on the picaxe forum for PWM and you get a few examples of controlling motors.

A better way is to look at the Picaxe with an L298 using 4 outputs for 2 motors, is in the Piacaxe manual. Think of 2 wires going to a motor and you set 1 pin high for the motor to go backwards and yuo set the other pin high to go forwards. No need to muck about with servo boards.

Depending on how fast the motors are you may not need PWM to begin with, just motors on/off. Once you start looking at PWM you start looknig at ramp up of motors and gets complicated very quickly (unless you are clever like Pete W/Gary).

I'd agree with the cheap Chinese Sanyo clones on ebay, someone else can add the link to them, I know Jim has had a few failures so get spares.

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olivers
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Re: Need: Very fast servo good for CR

Postby olivers » Tue Jun 19, 2012 5:45 pm

http://www.picaxeforum.co.uk/showthread ... nt+sensing

Link I didn't have at work explaining PWM on a Picaxe.

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earthwormjim
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Re: Need: Very fast servo good for CR

Postby earthwormjim » Thu Jun 21, 2012 12:07 pm

Cheap gearmotor. 400rpm.£5.20 with free shipping.What wheel diameter are you going to run?
http://www.ebay.co.uk/itm/6V-400RPM-Torque-Gear-Box-Motor-New-/330743629446?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4d01db4686
Cheers Jim :) (I have had everything APART from a gear motor fail!)

EDIT,If you want really fast,then go for 540rpm,but I would use very small wheels!! :)
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