SparkFun Forums

[liveitnerd]

Hey,

I'm looking at modifying a digital NTP clock to analogue and was thinking of using a couple of servos to do the job. I've never used servos on an Arduino and am wondering if I can use a full rotation servo and tell it to move 6 degrees at a time and if I can reset it to 0 one power on so I know where the clock hands are?

Thanks,
LiveItNerd

[skimask]

Not really. You can't tell it to move "6 degrees at a time" when a servo has been modified for continuous rotation. You might be able to make a sort of "home position" switch for the clock hands (say 12'o'clock position), then give it a value which will make it move slowly and you can guess where it will end up, but it won't be the same place every time. Or, use multiple position sensors, one master at 12'o'clock, then a 11 more, one at each number, then you're smaller movements will be guesses, ending up at each individual sensors.
If you want repeatable positional accuracy, you'll be looking at using stepper motors along with suitable drivers. Not hard to use at all.

[Duane Degn]

skimask if correct about the lack of position feedback with a CR servo.

While a stepper is a good option, you could still use the CR servo if you also used some sort of encoder.

I've used these sensors to make my own optical encoder.

http://www.sparkfun.com/products/9454

If you use two of these sensors you could use make a quadrature encoder which would allow you to sense both motion and direction.
I think there are striped disks available to make printing the encoder disk easier. You can even get by with hand drawing the encoder disk (I've done this myself).

IMO, it's kind of a toss up between the CR servo/encoder and stepper motor as to which method is easier. If it were me, I would probably go the CR servo route since I've had more experience with CR servos and encoders than I have with stepper motors.

[liveitnerd]

Thanks for the tips. The more I look, the more a stepper motor seems to be the way to go and seeing that I have no experience with either it'll be a learning curve either way. I'll just need to use a light break sensor to detect when both arms are at midnight in case of a power reset. Thanks for the suggestions.