Lynxmotion Tech Support

Okay, so my mind has wondered backwards.

Servos are rated in oz*in (or kg*cm). I measure an object using a table scale and it reads it to me as oz, this number however is a force because its actually measuring the objects weight... so, the servos ratings are also incorporating this weight factor (g, gravity term) into the number without revealing itself in the units. This is fine, but, when calculating dynamic torques, sometimes you need to factor in a 'g' term, and this is where my brain hit a wall. Do I factor in a 9.8m/s^2 or 32.15ft/sec^2 term in or what?

The reason is, so far I've been working with mg terms where the weight (in oz) has been easy to measure, but now working with moment of inertia and angular acceleration, I need to know the specifics of the units used to measure the torque characteristics of the servos and weights of my formulas.

Any help/insight into my problems is greatly appreciated, thanks!

Not sure what result you are wanting to achieve, but calculating *accurate* dynamic performance data of a servo could be quite involved. The mass of internal gears and motor rotor, friction losses, and the electrical characteristics of the motor itself at different rpms are just a few of the variables to consider. Might be more practical to the measure the tork available at various rpms (modify for continous rotation) and various applied voltages. You could make a small prony brake to gather the data.

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My goal is to calculate a ball parked number, for the max torque that I will need, so that I can purchase the right servos. I do understand the complexity of an accurate value, but I want to get the "bulk" torque numbers through calculations. My application is for a robot arm that lifts some mass at the end of a link. Thanks for the help, I will look into the prony brake, but this seems like a good device I can use after the arm is built to find max torque characteristics.

The biggest problem I see is the hobby servos max torque can't be tested without damaging the servo. In many of the torque tests the servo can heat up, as hobby servos are not designed to operate at anywhere close to stall torque. The duty cycle is also not 100% for a hobby servo. It can not sustain a load indefinitely.

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