Problem
I have my motor (and optional feedback devices) properly configured and I am wondering if I can drive the motor in open-loop mode. Is the Open-loop mode of operation meant to be used in end-applications or does it have a specific purpose that I am not aware of?
Solution
The Open loop mode of operation has been created mostly for two main purposes which are: - Configuration and well-functioning verification: this includes determination of motor parameters, feedback device calibration, and verification that the motor and feedback devices are working properly and are not damaged in any way.
- For final operation: this includes applications that are driven in an open-loop mode without any taking into account any feedback information whatsoever.
From the two, the most commonly used is the first one but there are still some applications running in an open-loop mode so it might be worth explaining how one can have such application working correctly.
Open-loop for configurations and well-functioning verification
As we have just stated, the main purpose of the open-loop mode is for manual configuration and verification of the performance of the system. This can include:
- Determining motor parameters such as motor pair poles or pole pitch. Related articles to look for:
- Calibrating feedback devices. Related articles that dive deep into this:
- Check that the feedback devices are working correctly and are not damaged. An article explaining this in deep fashion:
- Check that the motor phases are working correctly and that there is no damage in the motor or the power stage of the drive. An article that explains this in detail:
Open-loop for the final operation
When using open-loop as the final mode of operation for an application, it is first very important to make the distinction depending on the type of motor used: brushless (BLAC and BLDC) motors with electronic commutation and brushed DC/VCA motors.
BLDC motors
Brushless motors can be driven in one of the 2 following particular open-loop modes:
The difference between the two is that the second one uses a certain feedback device for electronic commutation so that only the duty cycle (%) needs to be applied. The first one needs both the duty cycle and frequency (mHz) to function properly.
However, both options have a limited performance as far as the end application is concerned:
- You will need to send duty cycle and frequency commands at a very fast rate in order to get some decent commutation for the motor. Due to the limitations of the networks, it is almost certain that driving a motor with this mode of operation will not be an efficient and recommended way to do it.
- Even though you are using the commutation sensor, with open loop vector mode you encounter issues like running at different speeds depending on the direction of the movement for a fixed duty cycle. This is because of the construction of the own motor and there is not much that can't be done about it. Besides this, the standard update rate of 1kHz of networks like CANopen or EtherCAT might not be fast enough to ensure proper/desired control of the final movements of the motor.
If all these conditions don't affect or restrict your application in a significant way, then you can proceed to use open-loop as the final mode of operation.
Brushed DC motors and VCA
In the case of brushed DC and voice coil actuators, it works in a simpler way than the brushless motors due to the absence of an electronic commutator. With these motors, only the duty (%) voltage needs to be set in order to drive the motor.
When using this mode of operation it is very advisable to keep a proper track of the current that is being applied to the motor to make sure that no damages can appear due to a high increase in duty cycle without control. In addition to this, due to the own construction of the motor, it is more than possible that you see that the motor moves faster in a certain direction than the opposite for a fixed duty cycle %.
If all these conditions don't affect or restrict your application in a significant way, then you can proceed to use open-loop as the final mode of operation.
As a final disclaimer, we want to say that Open loop mode is not a recommended mode of operation for end-applications. It is recommended to use torque/velocity/position modes (either profile or cyclic) before open-loop any time that both options are possible. Open-loop should be mostly used for verification processes only.