180 degree motion servo that can take a hit

Question

I was looking into servos and I need some help choosing the proper one. I need a servo that has complete 180 degree motion but it's going to get hit by dogs a lot so it needs to hold it's position against a good deal of force. My biggest problem with this however is the fact that it needs to be tiny. I was thinking I might have to go with a locking gear system what do you recommend.

I am attempting to make self adjusting weave poles like these:

The problem is dogs will be hitting them constantly and there is very little room for these servos. Especially with the fact that they will have to be operated using rechargeable batteries. In order to ensure that they are properly adjustable, they need to hold shape. Also I am designing a GUI that will allow them to be remote controlled so they can be set to different angles depending on the dogs skill level. Anyone have some ideas?

Answer 1

I would approach this as a mechanical problem, not a servo-selection problem. Design a mechanism which can take a hit and keep the servo isolated, then you only need a normally rated servo (one that can move the object, rather than being strong enough to cut a dog in half).

I would look at adding some spring, rubber, a cush-drive, overload or slip clutch (or dog clutch, ha ha!), or just slack in the mechanism to allow the movement/impact.

Personally I'd look closely at other actuation methods, for example small pneumatic cylinders are easily actuated with solenoid valves and a £5 12v car tyre inflator can provide enough air, and being pneumatic they will take having their pistons moved by force and return naturally to where they're set.

Answer 2

Well, this is going to be tricky, and probably expensive. Your main problem is a dire lack of space. I assume you want to get the servos inside that central pole on the ground. But why do you need to confine yourself to the exact size and shape of the poles you have? Would it be so terrible if the bottom pole was double the width? What if it wasn't a pole at all, but a box, much wider than the pole, but the same height. That would give you much more room to fit your solution, and allow you to use much less expensive motors and gearboxes.

The first thing you'll need to do to spec the mechanical components you need is to do some calculations to determine the amount of mechanical load they must endure. How much torque will be generated at the gearbox of the servo by a dog running into it at full speed? Let's do a rough calculation, and assume a dog is just sitting on a horizontal pole.

Torque = force x distance
= weight of dog x length of pole
= 300N x 1m
= 300Nm

That's a lot of torque. Here's a typical servo motor which can produce 300Nm torque:

This is a beast of a motor. It'll weigh a hell of a lot, cost a lot and consume a lot of energy.

What you may need to do is separate the ability to move from the ability to survive an impact. Make a brake, which disengages when you want to move the poles, and re-engages when you want to hold them still. This way, you can use a much lower torque servo.