Are there any alternatives to an electric actuator?

[Austin.Loach]

What I'm looking for is something to do a "swing-like" motion (picture a human arm swinging a sword, for example). I was originally thinking a little motor with a gearbox to slow it down a whole lot would be good, but the idea of an electric actuator was brought to me. I watched a few videos but I need something faster than an electric actuator. Keep in mind I'm already spending a whole lot of money on this robot so please keep it as cheap as possible. Also, I'm rather new to robotics so if you could keep as many things simple that would be appreciated! lol Thank you in advance everybody!

[GeeTwo]

If you don't need to stop in mid-stroke, pneumatic cylinders are a good way to go for this sort of thing. One of the nice things about pneumatics is that most of the weight can be placed in the "main chassis" of your device, and the parts on the periphery are reasonably light-weight and simple. The really good thing is that you don't have to worry about "burning up motors" - pneumatic devices deliver a nice, steady force and don't consume any power or suffer any additional stress holding that force at the end of the stroke.

Edit: Also, IIRC, there are a number of non-cylinder actuators based on pneumatics and/or vacuum which are really light-weight and adaptable to plenty of "biomimicry" applications. They aren't legal in FRC, so I didn't look too deeply into them, but they may be worth it for you.

[pmangels17]

I was going to say a similar thing. One caveat to pneumatics is that they require an entire air system: storage tanks, tubing, gaues, solenoid valves, and a compressor all need to be added to the robot in order for you to use pneumatic actuators. A compressor isn't necessary to have on the robot, but you'll need some way to supply compressed air if you exclude one. You also have to think about a safety system like a pressure relief valve and quick release to vent all the air in the system.

That being said, if you have a thing that only needs to be in two (or three or four if you use multi-stage cylinders or multiple cylinders) positions, pneumatics are a simple and reliable way to achieve this. Motors and gearboxes add a different set of complications, namely making sure the motors and gears you select give you the correct amount of torque to get you where you want when you want to get there. You'll also need some sort of feedback sensors (limit switches, potentiometers/encoders, etc) and some sort of feedback control loop (not necessarily PID control, but probably more complex than the software for pneumatics). Both solutions have their advantages and disadvantages, and there are other solutions available as well, but this is what makes engineering challenging.

[Austin.Loach]

I'm actually trying to stay away from pneumatics mostly because I don't want the hassle of the air compressor, otherwise, those are very viable solutions. If you have any others that might work they would be greatly appreciated. Thank you anyway though!

[EricH]

You could make your own actuator, in a sense. It's a matter of linkages, though, and a slow-enough motor. I know a typical window motor can slap through 120 degrees in a second or less... but if you were to use a linkage on an arm off of that you could slow that way down.

Actually, this is where a speed controller can come in really handy. You can take a lot of the speed out in the speed controller if you want to, meaning a smaller (or no) gearbox.

[guniv]

Originally Posted by EricH

You could make your own actuator, in a sense. It's a matter of linkages, though, and a slow-enough motor. I know a typical window motor can slap through 120 degrees in a second or less... but if you were to use a linkage on an arm off of that you could slow that way down.

Actually, this is where a speed controller can come in really handy. You can take a lot of the speed out in the speed controller if you want to, meaning a smaller (or no) gearbox.

We use a DART this year with a linkage setup to pivot our wedge/arm/shooter. If we didn't, the DART would drop too low to provide pivot and would only push against it.

Pic: https://www.facebook.com/LNRobotics/...09258065893970

Don't know if this is relevant to what OP is trying to do.

[DonRotolo]

It depends a lot on the specifics of the application. I will take swinging a 5 lb sword as the need, in terms of speed, weight, needs to swing both ways, swing force and several swings needed but only one at a time, and 'cheap' as far as cost.

An electric solenoid doesn't have the force, but can do everything else. Get a cr srater motor and remove the solenoid, about 10 lbs force but a throw of only an inch.

A motor maybe, with a linkage to produce the swinging motion (and not driving the arm directly) - think a rotating disk with a linkage pinned near the outer diameter. A window lifter motor can go 60 RPM or more, so a swing there and back is a second. The problem here is a 'single' swing needs time for the motor to get up to speed.

Pneumatics offer it all, but cost is not 'cheap', even with an off-board compressor.

Linear (electrical) actuators are just too slow

A motor driving a lead screw could be good. Mechanical complexity is the disadvantage here, but perhaps the best choice of all so far.

[EricH]

Come to think of it...

I'm not sure you want a motor, per se.

Figure out how much torque you'll need to move what you're moving, then go down to your local hobby store that deals with large R/C stuff. You're looking for a good-sized servo. (Servo motors are technically motors, but they're set up to hold a given position rather than run at a given speed--unless you're making a continuous-rotation servo run at speed.)

The bonus here is that servos are rigged for linkages (say, a pivot) so you can really open up your options if you work it right. They don't rotate very fast, necessarily. And, they're relatively cheap. You'll just need a controller--the RoboRIO has servo slots, as I recall, and I know that Arduinos can handle 'em.

[BBray_T1296]

Originally Posted by EricH

Come to think of it...

I'm not sure you want a motor, per se.

Figure out how much torque you'll need to move what you're moving, then go down to your local hobby store that deals with large R/C stuff. You're looking for a good-sized servo. (Servo motors are technically motors, but they're set up to hold a given position rather than run at a given speed--unless you're making a continuous-rotation servo run at speed.)

The bonus here is that servos are rigged for linkages (say, a pivot) so you can really open up your options if you work it right. They don't rotate very fast, necessarily. And, they're relatively cheap. You'll just need a controller--the RoboRIO has servo slots, as I recall, and I know that Arduinos can handle 'em.

It depends on the scale. I think it will be difficult to find an out-of-the box servo solution for a man sized sword swinging arm.

Electric linear actuators might work however if you include a lever multiplier. If you apply the force near the pivot, the resulting end of the arm moves much faster. Again we aren't sure the scale, but if you actuator was attached 6 inches along a 3 foot arm, the end would travel 6 times faster and 6 times further than the actuator. Of course the limitation is 6 times less torque, but depending on the specific motor solution you may have way too much of that anyways.

[tr6scott]

This has been on my watch list for over a year...

Downsize this?

https://youtu.be/HN1xSIgnwDA

[efoote868]

Originally Posted by BBray_T1296

It depends on the scale. I think it will be difficult to find an out-of-the box servo solution for a man sized sword swinging arm.

On the contrary, they do this all the time with industrial robots. These would not meet the "cheap" criteria.

I wonder if a sketch by the OP might generate more specific ideas. Springs/Rubber Bands might be able to store enough energy for a swinging motion with smaller motors.

[Jon Stratis]

Depending on the forcespeed requirement, look at robot designs from Arial Assist (the 2014 game) - a whole lot of robots that year utilized a catapult to launch the ball, which is just an arm doing a swinging motion vertically instead of horizontally. Some were pneumatically driven, others utilized springs or surgical tubing/spear gun tubing with a motor to pull it back, and others used a motor to drive the arm directly. Even if that's faster than you want, you might get some useful ideas from those designs, all you'd need to do is slow them down!

[EricH]

Originally Posted by BBray_T1296

It depends on the scale. I think it will be difficult to find an out-of-the box servo solution for a man sized sword swinging arm.

This is true. However, given OP's other threads, a hobby-grade servo just might do it. The scale here is roughly FTC-scale, so it'd depend on the arm and the linkage used. I would say the human-ish motion is what's being looked for.

Admittedly, it might need to be a rather large servo of that general type, but some of the sailing ones might be powerful enough.