I need to build a really really simple motor arm (crane?) that will do just a rotation movement at the base, and that will use an electromagnet to pick up different objects.
I need to be really really slow and precise, and as I said... as simple as possible. What kind of motor should I use and how should I attach it? I will attach two silly drawings I made to explain a bit better. Basically I will have objects on a plan and the arm will move above them and pick them up (or leave them down) with the magnet.
For slow and precise, I'd go with a stepper motor. You may want to put in some kind of 'home' indication, like a flag sticking out of the shaft that blocks an optical sensor. Add a micro-steping driver and you can get resolution better than one degree.
You could use a servo to do the rotation.There is a servo sweepsketch that has slow servo movement. Also, I think somebody has posted a library for having a servo move at slow speeds.
zoomkat:
You could use a servo to do the rotation.There is a servo sweepsketch that has slow servo movement. Also, I think somebody has posted a library for having a servo move at slow speeds.
With a servo I would worry about the requirement for precision. It's basically an analog device and the positions might shift slightly with environmental changes like temperature or humidity.
Most larger stepper motors have 200 steps per revolution and go 400 steps/rev with simple half-stepping. If less than 1 degree accuracy is enough then you could have the motor mounted strongly point vertically and have it be both shaft and bearing system and motor.
As mentioned you need a findable "zero" or "home" position that you can check for in software periodically.
With a servo I would worry about the requirement for precision. It's basically an analog device and the positions might shift slightly with environmental changes like temperature or humidity.
Just from the project description it doesn't seem like a "precision" project. I'd try an inexpensive servo like below to see if it will be accurate enough to do the desired positioning. I've done some "bamboo skewer" servo resolution tinkering with servos like the pix below, but haven't checked the position accuracy at each point in the ~190 deg rotation of the servo. A lot of "hobby" robotic arms use servos for base rotation. If mm precision is needed, then a geared stepper motor would be needed.
Even if it's a simple project I really need accuracy in the movement, especially it must be really slow as what the arm will do is to move around trying to pick up another arm under (hopefully using the electromagnet). The second arm it's in fact a turntable arm.
All this is to move a turntable arm in specif points of a record.... Maybe now it will make more sense! : )
Thanks for the links as well, but I've just received a ladyada motor shield so I think I will try to use that to drive a stepper (need to buy one) or a 360 servo which I used for a previous project (which as far as i know it's going far too fast).
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