Hi
I wanted to design a system for a project where I would have a straight arm swinging clockwise 180 degrees and anticlockwise back to 0 degrees. The arm is to be an arms span in length and not carry any load and is rotating in plane parallel to the ground in 1 second either way. What would be the suitable motor for this project and the ideal circuit setup. Thanks

 

Not nearly enough information to comment.

1) make a drawing of the proposed idea, with some dimensions.

2) list the desired movement parameters; speed, acceleration, precision of stopping point, etc.

3) describe the CONTEXT of how this is used, what power source is available, etc.

 

That sounds like a windshield wiper to me. You may have to modify one to get 180 degrees. That is pretty fast to be flailing an arm back & forth. You better keep it light.

 

Like DNA said, sounds like a good place to use a "crank" to eliminate the need to reverse to motors direction.

 

The project is to build a device for boxing practice independently. Basically the bar will be at a persons face height (adjustable) and it will swing clockwise and the person will duck to dodge it and then anticlockwise and vice versa. It will of course face some impact when the person doesn't duck quick enough or may hit it intentionally with some force, but normally it should not face any force. The technique used in boxing is called bobbing and weaving. I've uploaded a video (requires speeding) of how it would like like. It would plugged in to a wall socket (230v) and be in operating for around around 1-2 hours a day. The bar should obviously swing from one end to another fast enough to imitate real boxing practice.

 

Something to start with. AC gearhead motor, 60 RPM, 230 V , 33 in. lb. intermittent but might add fan to exposed motor shaft, Item: 5-1373, US $ 5.95, Surplus Center, Lincoln NE.
Could use stepped pulley for step down & speed control.
Attach arm with collapsed spring to shaft. Use crank ( shortbus ) to convert continuous rotation to oscillation.

 

I too think a windscreen wiper motor is a good start. Add a length of swimming pool noodle to the arm so it is soft may be an option.
This is an easy way to get going.

 

Lincoln NE.

Was just there a week ago. Saturday night / Sunday morning.

 

Im planning on powering this though the mains power. What adapter specifications would be suitable for the 12v wiper motor.

 

I'd have to imagine a wiper motor is going to draw some amperage. Likely around 5 amps. The fuse protecting it is probably 10 amps but that's to protect the wiring from burning up should the motor go bad. Plus, at startup there's going to be a greater draw than the 5 amps; so I'd guess specs would probably call for 7 or 8 amps for startup and likely 5 amps to sustain. But these numbers are just guesswork and I could be way off.

To be safe, a 12 volt supply that is capable of at least 15 amps. Radio Shack sells a 13.8 volt supply that is rated for 19 amps. I'd be confident that can handle a wiper motor.

 

Thanks. Now to get hold of a wiper motor.

 

Breakers yard for a wiper motor but they're a lot more sophisticated nowadays than they used to be mainly because they communicate to the cars ECM when it comes to intermittent wipes. For that reason you may be best off looking for a rear wiper motor.
The other problem of using a crank is that it will get slower and slower as it approaches its limit of travel. I'm sure that any boxer worth his salt doesn't slow up just as he is about to hit his opponent. Maybe you will need to think about having your boxers head placed more in the middle of the arms travel.

 

I'm sure that any boxer worth his salt doesn't slow up just as he is about to hit his opponent.

And he also wouldn't be swinging like a girl. Rotating an arm doesn't sound like a good way to train to bob and weave, not for boxing anyway.

 

you will need to think about having your boxers head placed more in the middle of the arms travel.

I think he intends for the boxer to be in the mid point of the swing, where the arm will be moving at its peak speed.

Now, if you want to throw a fast ball at this, have the crank offset. That way when the arm is drawing backwards it draws back slowly, but when it swings forward it can really get moving, depending on the angle. I'll bang out a picture in a bit. Right now it's dinner time.

 

OK, let me see if I can explain the geometry of this thing: If the arm is connected at a 90˚ angle to the pulley then the arm will swing back and forth very uniformly, as in the left illustration. But if you change the angle so that it's on an acute angle then (in my illustration) on the down stroke the arm will have to change its position rapidly, move fast. But after the swing, as the crank is pulling on the up stroke it's pulling more in line with the slide (I used a slide in my illustration, but changing the angle will cause a rapid motion followed by a much slower return motion).

I'm not good at explaining this, but trust me. Take a stick and move it in a circular motion just like each illustration. You'll see how when you are pointing the stick almost straight down and pushing down on the stick, the motion at the end of the stick will be very rapid. But as you continue your circle you'll see that the stick is following the crank at nearly the same sped, much slower than when on the down stroke.

 

Not convinced that a wiper motor would be the best choice for this application.
I don't know much about boxing but a 1 second swing seems awfully slow to me. I think perhaps the OP should discuss needs with a few sparring partners first and maybe take some slow motion footage of the action to measure duration, speed, acceleration, direction etc. so that he is better informed of the requirements for his machine.
Electric motors can be reversed and braked very effectively but there are other methods of attacking the problem. Purely mechanical like a falling weight tied to the arm via a cord around a pulley (provides acceleration); An impulse to give the arm sufficient energy to carry on under it's own momentum (would not injure user); A large push/pull solenoid (electrical control easy); A spring with a ratchet and pawl (reset by user punching back at the arm). However the design is settled upon it is likely to need other things like micro switches, bump rubber stops and of course an inflatable Marigold glove

 

Ours is not to second guess what he should be doing. He's asked for a Do-ma-flachy and that's what we're helping him achieve. As for the rate at which the arm would swing, depending on the angle of the motor to the swing of the arm, it could be quite fast. And my wipers, when on high, they're typically sweeping across and back in a half second.

The TS wants to build an arm that a boxer can learn to duck a known and predictable swing. Personally, I think if a boxer ever gets into a fight with such a robot, he'd have the upper hand. But a real boxer, who can swing from the left, the right, from low down to up or straight punch - I'm not sure a boxer is going to learn anything of value ducking a pendulum. You know it's coming and you're ducking long before you need to. But hey! This is what he wants to build.

 

I'm envisioning what the TS was asking to make as a form of "quintain". I can't imagine any boxer swinging like that.
https://en.wikipedia.org/wiki/Quintain_(jousting)