Push-motion function to increase torque at standstill

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Push-motion function to increase torque at standstill

Post by om_tech_support_JT » Tue Nov 18, 2014 5:55 pm

Q: I am using your Oriental Motor AR stepper motor and pulse input type driver packages for X, Y and Z axes on my CNC machine. The motors run very well at high duty cycles, however, sometimes when the motor is at standstill and the machine applies a force to the motor that's greater than the motor's holding torque, the motor will move a little bit. Once the motor senses the force has been moved, it returns to its original position. However, this motion introduces errors in my system that I need to avoid. I have talked to your tech support team, and they recommended using a larger motor or an electromagnetic brake motor which offer more holding torque. However, at this point we can not change the motor. Is there another way for me to increase my holding torque? Can I disable the automatic current cutback function, which cuts current to 50% at standstill?

A: Our best recommendation would be to re-size the motor taking into account the automatic current cutback function. Since we know that the automatic current cutback function will reduce atleast 50% of the motor's current during standstill, I would double your holding torque requirement and use that as operating torque to select a motor. A correctly sized motor would be the most ideal option. An oversized motor may introduce power inefficiency as well as vibration and noise. If this is not possible, there is one other way to increase the holding torque of the motor during standstill.

However, it is important to understand the goal of the automatic current-cutback function and also understand problems that may arise if this is bypassed. The automatic current cutback function is used to reduce temperature of the system; it allows the driver and the motor to cool down at standstill. From Step Motor theory, we know that motor service life is proportional to motor temperature, so the automatic current cutback function also increases service life of the motor. If the temperature of the motor is very high, it can reduce the service life or damage the motor. This is why we recommend a larger motor or electromagnetic brake.

Push-motion operation is a type of operation where pulses are input to pressurize the load continuously. In push-motion operation, a desired current between 0 to 100% can be selected. Push motion operation is activated when the T-MODE input is turned on, then our BCD inputs M0, M1, and M2, allows you to select between 7 preset "torque limits". The current value set in the parameter will be used to limit the output torque. The current value is directly proportional to output torque, so 50% current equals 50% of the rated output torque. This operation also works at standstill. The motors will heat up quicker than before when used this way therefore we don't recommend it.

To enable the Push-motion operation in the AR, you would need to have access to the extended functions by using the OPX-2A hand-held pendant or MEXE02 GUI software.

For more information, please refer to section 12.2 Push-motion operation of the AR manual.
http://www.orientalmotor.com/products/p ... 60156E.pdf

Do not perform the push-motion operation with geared types. Doing so may cause damage to the motor or gearhead.

Monitor the temperature of the driver and motor. Use fans to blow air into the driver and motor to keep it cool and improve ventilation. Do not let the internal temperature of the driver exceed 85 °C. An alarm will trigger if the internal temperature of the driver exceeded 85 °C. The motor surface temperature should not exceed 100 °C. The driver has an overheat protection function, but the motor has no such feature.

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