Window-wiper motion

We’re a rookie team (6070) and we are trying to design a mechanism to raise an arm from a horizontal position to a roughly vertical position and back again, like a window-wiper. Ideally this is with the push of a button. Next we want to be able to retract the arm back to the starting position, with another button.

The force needed should be relatively low, as we are simply raising a square-extruded piece of aluminum.

Do we use a motor with some kind of shut-off switch that is tripped once the arm is in the desired position? Or should we use a pneumatic actuator? Or is there a servo that can do this for us?

Any kind of direction is appreciated.


How many times per match do you expect to raise/lower it?

Pneumatics would be pretty easy, as you can use a hard stop as opposed to using encoders. However, pneumatics would weigh more because you have to add on tank(s) and a compressor (depending on how often you plan to raise/lower the arm per match)

Hope this helps :smiley:

It wouldn’t be very many times per match. One or two?

Generally, if you use pneumatic s you want to use it for multiple things, in order to justify all the stuff that comes along with a pneumatic system.

Doing it with a motor would be fairly easy- and AndyMark PG motor/gearbox es you a nice output shaft you can use, or run that to a separate shaft using a chain and sprocket. With something like this, it’s always a good idea to have a hard stop at both extremes, along with a limit switch at both extremes to tell you when you get there. Your code can then recognize that when one of the limit switches is pressed, it shoud stop moving the motor in that direction.

You could use a VersaPlanetary gearbox, probably with a 100:1 reduction (as high as they go) and a BAG motor. Then you cold use limit switches at the top and bottom of the movement so that the arm hits the limit switch right where you need to stop. I’m not a programmer but from what I understand this is easier to program and set up than using encoders for a rookie team. Pneumatics could be an option, but if this is all you would use it for personally I would not want do have to deal with all that extra stuff just for one arm.

You can use a motor (with gearbox) and limit switches to stop motion at predefined points.

You can use a motor (with gearbox) and an encoder to allow you to stop at any point.

You can use pneumatics and the end points could be hard stops.

Don’t bother with a servo as the restrictive FIRST rules on servos make them largely useless.

If you want to use a motor, look at window wiper linkages for inspiration. A four-bar linkage with uneven bar lengths can be made to do some interesting tricks, including this reciprocating motion - with the motor just going forwards.

Then again, in college I drove a 1970 Hornet with pneumatic (vacuum) wipers.

If it will only have 90 degrees of rotation, I would look at a potentiometer over an encoder. They are cheaper and can be used with any motor/gearbox combo pretty easily.

We have used single turn and ten turn pots pretty much every year to control simple systems.

How much force do you need? What kind of speed are you looking for? I like window motors for simple motion since they’re cheap and easy to come by, but they are hard to interface with. You could also use a window wiper motor…

Actually, the servo rules this year are much more open than in previous years. It still wouldn’t be the best choice to drive an arm with, though.

Think raising a flag–that sort of purpose. Although I should mention that there are two of these "flags"


I am student of Glenforest on the Robotics Club. We have this windshield wiper motor (see link) and we don’t know how to use it. There seems to be a electric plug that goes in which we don’t own and we don’t seem to have any gears that are compatible with it. Are there any teams which have used a similar motor that can help us out?