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Digital servo ok (and FEZy) to use?


#1

A friend of mine is into RC cars and helicopters.

He said he’d give me a servo to use in my project. Looking at them I noticed they were digital.

Are these the same as what’s sold on this site (the Parallax)? I thought they were PWN and therefore analog.

http://www.wheelspinmodels.co.uk/car/item/TRX-2075-Servo-digital-high-torque-ball-bearing-waterproof__4502/
http://www.servodatabase.com/servo/traxxas/2075

In the servo database page, under “Modulation” it has digital.

Thanks


#2

All servos are digital and use pwm


#3

Oh ok. Great news. For some reason I though PWM was analog.

I need to re-read the eBook, again :wink:

Thanks.


#4

Digital versus analog has to do with the control loop within the servo. The electrical interface to both is the same.


#5

Well after having it my possession for exactly for 1 hour I think it’s shot.

I hooked it up to Di10 in my Domino ( :P) and used a PWM object to control it - using the code from the eBook. It would make a noise like it was moving then nothing, except a low sound. I tried different timings and the same.

Eventually I got the hardware error when trying to deploy my code and it wouldn’t make any noise or move or anything.

It was a Traxxas 2075, with black, red and white wires coming out of it. I cut the end of and connected the wires to a JST connector, matching up the colors.

Any suggestion on what to try next?

The only thing I could think of, as on 1 site selling them they listed the rate of movement as x sec/60 degrees. Does that mean it’s range of motion is 60 degrees and I was trying to send it to 90 and then 180 degrees?

Thanks for any reply!


#6

what were you using to supply the power. Not sure what the amp draw on the servo you are controlling, but you may want to try a external power source for it. Like a 4 pack AA battery holder or something of the type.


#7

As Bstag says an external power source might be the cure.
Anyway you should not draw power from the same source as the micro controller. Servos and any other motor should have their own power source. Connect the ground so they have a common ground and that will take the load of the voltage regulator on your board.


#8

I was using the USB supplied power to drive it.

Since I’m using the component shield, am I correct in assuming that I can connect the +/- to the battery and still use the white wire from the JST?

Thanks


#9

Servos draw a stack of power, digital servos more so. They can pull an amp with just a slight load on them no problems! You really need a higher power external supply, something in the 3-6A range would be ideal.


#10

Well I tried with 4 1.2v; 1300mhA, NiCd C cells and absolutely nothing. I’m guessing it’s done with.


#11

Digital Servo’s can be strange beasties if one is new to them.

My best bet is to purchase a cheap analog servo from eBay and have a play. You get get them for about $10 or so (even less).

With Digital servo’s - because the internals are a digital computer rather than just a feedback circuit - a lot of additional functionality can be programmed into them.

For example - a Hitec servo that I use can be programmed to have an automatic mid point, and min and max movement point etc. When I first started to play around with it I could not get it to work because I was asking it to move to a range that was outside of it’s user-assigned settings so it just sat and hummed at me without moving…

The other big difference with digital servo’s is the number of times that it updates its position. An analog servo will check its position about 20 times a second withc can lead to inaccuracies when the servo is under strain. A digital servo checks 300 times a second so it will resist strain a lot more.

Finally the Traxxas 2075 is a BIG servo and it will want to draw a lot of power, and its also a 6V servo.

Hope this helps…
Dave


#12

Thanks for all the feedback.

This is essentially what was happening to me. So I figured I’ll shelf this part of my project until I’ve got the rest of it working ok.

I’m planning to mount my IRs on a servo, so for now having them fixed is more “KISS” compliant anyways.