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  • November 2013
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Servo Drives & Controllers Defined

Find your servo drive and controller today

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.

Industrial Monitor Types: What is the difference?

CRT, HMI, Operator Interface, Industrial Computers

CRT, HMI, Operator Interface, and Indusrtial Computer Explained

Many terms exist for seemingly similar pieces of servo system equipment: Operator interface, HMI, CRT Monitor, Industrial Computer. So, what do the different terms mean, and which servo component terms have similar or different meanings?

Operator Interface

Operator interface refers to a control system a person uses to communicate with the multiple linked servo system components. Operator interfaces include both hardware and software allowing a person to interact with the machinery. Operator interfaces are manufacturer specific, which requires you to use the same manufacturer for all servo equipment parts.


HMI stands for Human Machine Interface. Like an operator interface, an HMI is made up of both software and hardware and allows a user to communicate with the machinery; however, HMIs are part of one machine or a piece of equipment, whereas operator interfaces are part of linked servo equipment. The user communicating with the machinery can manipulate the machines system and view the effects of the users’ manipulations. The HMI is manufacturer specific to the machinery and servo system you are using.

CRT Monitor

Cathode Ray Tube (CRT) monitors are used to display faults, positioning, and machine status. Some CRT monitors do work as Operator Interfaces; for example, Fanuc’s CRT monitors are made to both display and allow a person to communicate with their machinery. Cathode Ray Tube refers to the glass tube screen of the monitor; today CRT monitors are outdated and inefficient compared to LCD monitors; however, CRT monitors can easily be updated to LCD monitors through LCD retrofitting. CRT monitors are manufacturer specific to the servo system and machinery you are using.

Industrial Computer

An industrial computer is a programming terminal; it is used to program the machinery. The industrial computer can be used to change the settings to pick up different faults, and it can be used to save programming for different machinery. Industrial computers are a necessity for older machinery, which rely on the industrial computer for programming. The positive aspect with industrial computers is the computers are not manufacturer specific, so if your machinery still requires an industrial computer you are able to replace it with any other industrial computer if necessary.
Industrial computers are not used in newer machinery systems, because new machines’ servo components have programming specific parts built into the equipment, such as drives, which makes the industrial computer obsolete in newer machinery.