Comparison of torque-limiting products

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Comparison of torque-limiting products

Postby om_tech_support_JT » Wed Aug 21, 2013 11:20 am

Q: I am in the development of designing a new machine where the motor system needs to have torque-limiting capability with high accuracy. The accuracy of the torque limiting is important because my application consists of glass parts which are brittle. I noticed the BX brushless DC speed control system; the AR closed loop stepping motor system and the NX servo system all have torque limiting ability. How do the torque control accuracies of each system compare?

A: As you have pointed out, we offer several systems with torque limiting ability, which include the BX, BLE, BLV series brushless speed control systems, the AR series closed loop stepper motor systems and the NX series servo systems. Depending on the torque control accuracy requirement for your application, you may consider all three types of motor systems.

When the torque limiting function is used with brushless speed control products, such as the BX, BLE, and BLV series, the maximum fluctuation you may see between the preset torque-limit value and the actual torque generated can be as much as +/- 20%. This error may occur due to differences in tolerances between different electrical components such as power supplies and cables. Every electrical component connected to the system could have a difference in tolerance which can affect the result. To reduce this error, you can use a shorter extension cable (for example). If you kept the same operating conditions (such as same load, same speed, same cable, same driver, same power supply) and the same torque-limit setting, the repeatability would be much better at +/- 10%.

When using the torque limiting function (also known as "push motion operation") with our AR series closed loop stepping motor systems, the maximum error is reduced to +/-10% with a much better repeatability of +/- 2%. This is an improvement over the brushless DC speed control products because of differences in its motor's mechanical structure and in the driver circuitry. The AR motors are all equipped with built in resolvers. These resolvers constantly sends position and speed data back to the driver and provide much better (higher) resolution than the hall effect sensors we use on most of our brushless motors (except for BX series; it uses encoders). The AR series also offers better driver circuitry to monitor the amount of current the motor is receiving. Since current is proportional to torque, measuring the motor current is a good estimate of motor torque. The adoption of a higher resolution resolver, rotor teeth, and better current sensing circuitry allow the AR series stepper motor systems to position and limit torque better than the brushless motor systems.

The NX servo systems offer the best torque control accuracy available when comparing torque limiting accuracy specs across our products. When using the "torque control mode" with the NX series, the torque control accuracy is +/-5.6% with a repeatability of +/- 0.23%. The reason for the improvement is that the NX series servo motors has a built-in absolute encoder with even higher resolution than the resolver on the AR series products. In addition to this mechanical advantage, the NX series drivers also feature an electrical advantage with an improved control circuit design over the stepper motor and brushless motor drivers.

Torque control accuracy / repeatability comparison:
NX servo motor series: 5.6% / 0.23%
AR stepper motor series: 10% / 2%
BX, BLE, BLV brushless motor series: 20% / 10%

Before selecting a product, it is suggested to determine the torque control accuracy requirement as well as operational requirements for your application to help narrow down your options from our vast array of motion control products.

If you have any questions regarding any of our products, please contact our knowledgeable tech support group at 1-800-468-3982 or e-mail us at

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