Torque isn’t just torque when we’re talking about motion control. It’s another one of those subtleties of the field of mechatronics that requires consideration when you are doing a new project. With retrofits there are different rules. It is possible to use the motor as a sensor by monitoring the current over time which will reliably tell what is going on at the load.

The most important aspect of torque is the proper description of torque from the mechanical perspective. Torque has three mechanical aspects, the torque needed to overcome the friction of the load, the torque needed to keep the inertia mass moving, and the torque of acceleration.

Each one of these has to be considered. They also have to be considered in terms of their relationship to the total amount of torque required from the motor and drive combination. Generally we consider the frictional torque to be a small fraction of the requirement and sometimes we can ignore it altogether. But this is a mistake. I have had some complex conveyor and material handling fixtures in which the friction was the most significant part of the load. And it was ignored and caused all of the servo sizing to have to be increased significantly. A very expensive mistake.

What makes the torque requirement so critical is that the torque needed to accelerate the load is a complex calculation that has the change in the time in the denominator of the formula. This means that as the time required for move decreases, the torque required increases arithmetically. This is why acceleration is generally the main consideration in sizing servo systems.

The electrical component of torque is that torque is current. So the amount of current that is required from the drive amplifier must be correctly sized or there won’t be enough torque available to power the load. The calculation uses the torque constant of the motor which is expressed as ounce inches of torque per ampere of current. But this calculation doesn’t consider the rate at which torque needs to be applied to the load in order to achieve the desired move times.

Coincidentally, the rate at which power can be added to the load is also the breakdown condition of the power transistors that are used for drive amplifiers. In very high speed applications, this value also has to be considered in terms of the thermodynamic implications. Using a bigger amplifier will allow you to push more current through the motor, i.e. more torque, but you have to have dwell time for the motor to dissipate the heat that is generated. So there’s no free lunch here.

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