Linear Actuators

[pfreivald]

Originally Posted by Jon Stratis

But you don't really see or interact with lead screws very often, and it's not something that's already prevalent in FRC... all that combines to make it something that doesn't really come to mind when designing a robot.

When we used one to lift our robot in 2010 (our best year ever), people acted like it was magic and we were rather amazed at the number of people amazed by the mechanism. Many teams just don't even consider them, and a lack of familiarity may well be part of it.

Another part of it, though, is that pneumatics do a very similar job, don't stall and release magic smoke, and come in sufficient variety that if you're already using them (and thus have the compressor, etc already on-board), they're a better choice.

Lead screws are used in industry primarily for fine control, while FRC robots have to complete gross-control tasks. In 2010 it made sense for us because we had no other pneumatics on the robot--why add another system just for one mechanism? But if you're already using them, why use a slow mechanism specialized for fine control to execute tasks that don't require fine control?

[Tom Line]

I've been eyeing some of these models for some time:

http://www.servocity.com/html/12v_linear_actuators.html

I would love to have a drop-in electronic shifting option for gearboxes, and some of the lighter-duty short-throw actuators there have enough force to shift under load fairly quickly. They are pricey, however, and I haven't had time to find out what type of motor and shaft they are. I called servo city and they were unable to give me any more info than external dimensions, so someone would have to buy one to find out if any of our motors would mount in the same location.

[MrRoboSteve]

Originally Posted by Michael Hill

FYI, you are allowed to use electric solenoids <= 1" stroke

Yes, but the power limit is so low that they have pretty limited utility.

[JohnFogarty]

I can only think of one time in FRC I used a electric solenoid and that was 2012 when we designed our basketball elevator to no be able to allow the basketballs into our shooter until the solenoids were engaged then applying a clamping force on the outer wall of our elevator.

Let's just say that design worked....but the solenoids we had to use were absolutely terrible and ended up overheating and all sorts of things after continuous use during practice. That was because of the power limit among other things.

[Michael Hill]

Originally Posted by MrRoboSteve

Yes, but the power limit is so low that they have pretty limited utility.

Depends on how imaginative you are. I think it's POSSIBLE to shift with them (though not necessarily advisable). They can heat up quite quickly.

[JamesTerm]

Anyone mess with vex?

http://www.vexrobotics.com/276-1926.html

Is this kit legal?

[EricH]

Originally Posted by JamesTerm

Anyone mess with vex?

http://www.vexrobotics.com/276-1926.html

Is this kit legal?

I didn't see anything that would necessarily make it illegal (in 2014 at least--ask again next January).

However, given that it's a VEX item, and not a VEXPro item, I'd be very wary of using it with anything other than the VEX motors (which at least in 2014 were legal in FRC). Given that the VEX motors are far less powerful than, say, a CIM or 775, do your engineering on the power needed and available before committing.

On the other hand--the price ain't bad, and the VEX motors have decent power and as I recall were unlimited quantity items last year. Might have a winner if you can adapt it to shifting, or might not. Anybody want to try it during the offseason?

[Richard Wallace]

Originally Posted by Tom Line

I've been eyeing some of these models for some time:

http://www.servocity.com/html/12v_linear_actuators.html

I would love to have a drop-in electronic shifting option for gearboxes, and some of the lighter-duty short-throw actuators there have enough force to shift under load fairly quickly. They are pricey, however, and I haven't had time to find out what type of motor and shaft they are. I called servo city and they were unable to give me any more info than external dimensions, so someone would have to buy one to find out if any of our motors would mount in the same location.

These are intriguing, Tom.

The motors on the big ones (e.g., SDA6-263) look a lot like CIMs, so maybe they can be adapted to get FRC legal. The price is right at our present $400 single component BOM limit, and they weigh about 10 lb. -- but the prospect of lifting an entire alliance 6 inches in under 4 seconds is ... intriguing.

[Jared]

Lead screws are pretty cool, and they don't have to be slow. An 8 start 1/2" lead screw travels 1 inch per turn.

We used some of these (http://www.mcmaster.com/#99030a400/=sr50n0) on our climber in 2013, and they were decently fast. To travel 30 inches carrying a 150 lb robot took under 5 seconds. If we weren't carrying the weight, we could have done it faster.

[BBray_T1296]

Though our climber never came together by regional time in 2013, we were going to use rack/pinion to extend and retract our arms.

Mcmaster has a selection of racks in a good range of lengths, widths and pitches in both low load Nylon and heavy duty steel.

[JamesTerm]

I saw this link on you tube:
https://www.youtube.com/watch?v=GjfsPTJx_DY
It's not necessarily this HS-55 motor that intrigued me, but the attaching piece to the servo. While it's not truly a linear actuator it could serve that purpose with a little math. The thing I find most attractive about using servo's is the ease of setpoint operation use. The question then remains what servo's of what high torque are allowed.

I'd sure love to use something like this SDX-901 if it wasn't so expensive
https://www.youtube.com/watch?v=YDFMhyA4rHw

the HS-55 is only 18 oz-in (0.127 nm, 0.09 lb-ft)
while this SDX-901 is a whopping 402.7 oz-in (2.84 nm, 2.09 lb-ft)

[electroken]

Originally Posted by Jared

Lead screws are pretty cool, and they don't have to be slow. An 8 start 1/2" lead screw travels 1 inch per turn.

Our ball elevator used an 8-start, 5/8" diameter lead screw driven by a window motor. It was plenty fast for our needs. Over its 128 matches the thing survived an intense beating. Some of the stuff it was attached to... not so much.

[JamesTerm]

So I have a question for everyone who has used the lead screw technique. How did you do a set point operation... like if you want it to go a certain distance? Did you:

1. Use an encoder?
2. Use a potentiometer?
3. Use a limit switch?
4. String potentiometer?
4. Something else?


If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

[pfreivald]

Originally Posted by JamesTerm

If you used an encoder how do you ensure it remains calibrated (e.g. limit switch). I'd like to use this lead screw technique, but I haven't found reliable way to sense distance. (I have not tested the string potentiometer either).

We used encoders with limit switches on the ends for continuous calibration purposes. It worked quite well, and was very PID-friendly because you could be off by 50+ counts and still in the "right" position.

[Jon Stratis]

Obviously, it all depends on the design/setup. When you have an application with a limited operating range, I strongly recommend potentiometers. We haven't done it with a lead screw before, but we have done it with a winch, which is similar in that it turns rotary motion into linear motion. There are many design considerations to take into account first, though... like how many rotations are required. When we used it on our winch in 2013, the winch itself was designed for 7 rotations, and we had a 10-turn potentiometer available (It's always good to have a little extra rotation available, just in case).

In some applications, a potentiometer may not be appropriate. For example, it may require too many rotations to make an encoder practicable. In that case, an encoder with limit switches is great... especially when you can have limit switches at both extremes, and you travel to both extremes on a regular basis.

The only real downside to an encoder is that you have to keep track of where you are in the code - with a potentiometer, the value returned tells you where you are with nothing else needed.