Servo Drives & Controllers Defined

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What is the difference between a servo controller & a drive?

Controllers and drives are two separate servo system components. With many different original servo equipment manufacturers, you can easily decipher a drive from a controller, such as with Fanuc and Indramat drives and controllers. However, Allen Bradley is an exception when defining controllers and drives, because Allen Bradley has named their servo component, which is equivalent to a drive, an AC servo controller.

What is a controller?

A controller takes signals from different machine monitoring devices (e.g. tachometers, linear scalesencoders, proximity switches, etc.), and the controller figures out mathematically where and how the servo motor needs to move.  The monitoring devices providing are both external to the servo equipment, and some of the devices are part of the servo equipment pieces, such as servo motors have encoders and resolvers built into them and spindle motors have encoders and speed sensors.

The majority of controllers are used for programming offsets, diagnosing, machine status, and part counts.

What is a drive?

In general, a drive takes a signal from a controller; the signal from the controller tells the drive what to compensate for in respect to the servo and/or spindle motor to achieve programming.

A drive can also tell a motor’s status by letting you know if the motor is overheating or if it is being overworked.

Allen Bradley: The exception 

Though drives and controllers can be technically defined, Allen Bradley has been using the terminology AC servo controller for their servo components that are reminiscent of servo drives. An example of an Allen Bradley servo component equivalent to a drive is the Allen Bradley 1391 series of AC servo controllers. The 1391 series AC servo controllers are actually drives despite the name, because the Allen Bradley 1391 AC servo controllers control the servo or spindle motors’ movements.

Allen Bradley does make servo equipment called drives now, such as the PowerFlex series drives. The fairly new PowerFlex series drives, however, are still dissimilar to a regular servo drive. Allen Bradley PowerFlex drives are variable frequency drives (VFDs), which the VFDs do not require a controller, because the controller is built into the drive.

Other Manufacturers

Manufacturers such as Fanuc and Indramat have the traditional separate servo drive and servo controller system. A couple well known series for Fanuc drives and controllers are the Alpha series and the C series; for each type of series, there is both a drive and a controller, which work together in a servo system. A well-known series of Indramat servo drives and controllers are the Diaxo series drives and controllers, which also work together in a complete servo system; the common Indramat Diaxo controllers are the BTV controllers.

Though, there is a clear difference between servo drives and servo controllers for most original equipment manufacturers, Allen Bradley has set its own standards for what makes up a servo drive and servo controller and how they work. Allen Bradley is the exception when defining servo controllers and servo drives.

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