Hello. I’m hoping to get some advice on a simple project.
Basically, I need to move a straight arm from an initial (vertical) position, through 90 degrees to a (horizontal) position, to be activated by a switch and a 12v dc power source. The arm will pivot or rotate at the bottom:
initial position: |
ending position: __
the arm needs to reverse or toggle between the two positions with each activation of the switch as follows:
initial arm position: vertical
on switch:arm horizontal
on switch:arm vertical
etc.
The arm will have no more than five pounds of weight attached to the end opposite the motor/pivot point.
Questions:
What type of motor should be used?
How do I determine the required torque rating of the motor?
Any external control electronics necessary?
How to vary the speed of the motor (vary speed of arm movement)?
Any other considerations?
It’s that simple, but I’m a complete newbie, so any help is appreciated.
>>How do I determine the required torque rating of the motor?
Attach a foot long pole attach it to the end of the motor shaft. attach the other end of the pole to a pull scale. Now run the motor until it stalls, record the weight. That’s how much torque the motor has, measured in foot pounds
>>Any external control electronics necessary?
If its not necassary. You can just use a switch battery and 2 limit shitches to turn off the power when it reaches its specified position.
Ok, but don’t I need logic functions from external gates and flip flops to accomplish the toggling of the power and motor rotation?
For example, using a two-position switch:
with the operator switch in position one, the motor pwr is latched “on” and the arm rotates to the horizontal position…when the (horizontal) limit switch toggles the pwr off.
with the operator switch manually changed to position two, the motor pwr is toggled to “on” again, and the motor polarity is reversed, returning the arm to the vertical position…until the (vertical) limit switch toggles the pwr off again.
Just do a simple 2 circuit design. You need the arm to move up while a switch and limit switch are closed (this is assuming reverse logic with limit switch, can be accomplished mechanically). If either are open the motor does not operate. Repeat the same thing for the second circuit, but reverse the leads on the motor (assuming the motor reverses with current, I have been told some do not).
i would recommend a motor used in the automotive field (window motor, windshield washer, door) all are commonly available and cheap but powerful
How do I determine the required torque rating of the motor?
take the force (5lbs) at the point of most stress (45 deg) and
[multiply the distance from the orgin * the sin(45deg) * the force (5lbs) ]]]this will give you the moment about the rotation of the motor which is the exact torque needed at that point. so get a motor that is at least that strong
Any external control electronics necessary?
not really you can do this with relays and limit switches. but if you need any external feedback of any sort (exact position, angle, torque, etc) you will need some external electronics and i would recommend a micro controller (oopic is a good one) easy to program and interface
How to vary the speed of the motor (vary speed of arm movement)?
in the simple circuit do this with resisters (less voltage=slower speed) but you loose torque that way so be careful. you can also do this with gearing but for your application a potentiometer(variable resister )would work fine
Any other considerations?
just plan it out before you build anything and remember we are here to help if you need it
Woah Greg, hold on a minute …
The maximum torque required will be at the point where the force normal to the arm is maximum, which would be in the horizontal position. Max torque required for this application will be the 5lbs X arm length <edit>PLUS the mass of the arm multiplied by the distance from its cg to the pivot point</edit>.
Also Erik, you should make sure your motor has enough power for the speed that you desire. If you multuply the max torque number from above (in foot-pounds) by the maximum angular speed that you desire (in radians/sec) <edit>then divide by 550</edit> you will get the power required in horsepower.
Of course these torque and hp numbers would be the minimum required motor ratings assuming perfect (frictionless) conditions so I would suggest to also increase these numbers by a significant safety factor, maybe 50% (multiply by 1.5).
I generally design so that the max torque on the motor is 25% of stall torque. If the motor has a fairly stright torque vs RPM curve, it should be turning about 75% of free speed. In this condition, it is turning plenty fast enough for adequate cooling and the motor isn’t working too hard. Also if there is a hang up, the motor starts producing more power to break through. If you’re running anywhere close to max torque then there is no power left to give.
Make life as easy on your motors as you can. Electric motors like to run at high rpm and light load.
