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When installing a servo or other actuator, I measure the force needed to perform whatever action is needed and find an actuator that can generate more than the necessary force. However, I recently wondered if there's a rule of thumb or guideline for how much overhead is useful, before it becomes a waste.

For a (perhaps oversimplified) example, say I have a lever to lift something, and the force needed is 100 Newtons. An actuator that can manage 100 N maximum will have problems with this and stall, with any sort of friction or other imperfections. I would use an actuator that can produce 150 or 200 N - whatever is available and fits the design and budget. After testing, it may become apparent that 200 is overkill, 120 is sluggish, but 150 is good. Other than trial and error, is there a way to measure this, or a rule of approximation?

I realize that variables in the mechanics and construction can significantly alter what force might is needed to be considered ideal, but is there a commonly accepted value for simple applications? Something like "If you need x force, install an actuator with x + 20% force."

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    $\begingroup$ I had to look up what kgf means. Although not relevant for your question per se, maybe using Newton would make it more generally readable? $\endgroup$ – Jakob Jul 31 '14 at 6:45
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    $\begingroup$ @Jakob Point taken, I've converted the units. $\endgroup$ – JYelton Jul 31 '14 at 7:17
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When selecting an actuator, you need to consider not just the force but also the power.

Power = force x speed

The faster you need to lift the lever, the more power you'll need. Companies like Maxon provide detailed graphs showing exactly how much torque you can expect at various speeds for their motors. Other companies may just state the motor's wattage.

Maxon torque graph

You will also need to consider acceleration.

Force = mass x acceleration

When you're lifting your lever, you need to add its weight and the force required for acceleration. If you want to maintain maximum acceleration right up to the maximum speed, then the total power is:

power = (weight + mass x acceleration) x maxspeed

or:

power = mass x (9.8+acceleration) x maxspeed

Next check the efficiency of the gears. Again, companies like Maxon specify the gear efficiency, so divide your calculated power by the gear efficiency. If you're unsure of the efficiency, then assume it's about 60% efficient.

Lastly, you need to estimate how efficient the rest of your mechanism is. A tendon driven robot might be only 50% efficient in its power transmission, whereas attaching your lever directly to the motor shaft will be 100% efficient.

After all that, if you allow another 20% - 50% power on top of that, you should be OK.

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I always add 100-120% overhead (normally this just means the next bigger unit than exactly what I need) but also this comes with experience, a motor that can just barely lift your load will do so very slowly.

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