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Frequently asked questions (FAQ)

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Servo Drives

My application consist of a Nextmove BXII controlling a FlexDrive-II. Should the flex drive be set to speed mode or current mode.

Speed Control
A drive in Speed mode can be used simply to control the speed of a motor, but more typically it is used in combination with a motion controller to control the position of the motor. It is most common to use Speed mode for position controlled systems as it provides a good dynamic response, a reduced risk of erratic behavior and makes the position loop is far easier to tune for the majority of cases.

A Speed control drive also contains a current controller. The drive receives a speed demand via its analog input, which is converted into a current demand by the speed controller. The current loop then drives current into the motor causing it to accelerate or decelerate.

The Speed controller is a closed control loop, which requires feedback from the motor. A transducer fitted to the motor generates a signal, which is used to determine the actual speed of the motor. Any difference between this and the demanded speed causes a demand on the current controller, which in turn causes the motor to react to close the gap. Each of these control loops has some associated gain terms, each of which require tuning to the system load conditions.

When a motion controller is added to the system an additional control loop has to be considered, the position loop. This also requires suitable feedback, which is generated by the motor transducer, fed back to the drive and then on to the motion controller. The position loop also has a set of gain terms that needs to be tuned.

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