WHAT IS A SERVO?
What is a servo? This is not easily defined nor self-explanatory since a servomechanism, or servo drive, does not apply to any particular device. It is a term which applies to a function or a task.
The function, or task, of a servo can be described as follows. A command signal which is issued from the user's interface panel comes into the servo's "positioning controller". The positioning controller is the device which stores information about various jobs or tasks. It has been programmed to activate the motor/load, i.e. change speed/position.
The signal then passes into the servo control or "amplifier" section. The servo control takes this low power level signal and increases, or amplifies, the power up to appropriate levels to actually result in movement of the servo motor/load.
These low power level signals must be amplified: Higher voltage levels are needed to rotate the servo motor at appropriate higher speeds and higher current levels are required to provide torque to move heavier loads.
This power is supplied to the servo control (amplifier) from the "power supply" which simply converts AC power into the required DC level. It also supplies any low level voltage required for operation of integrated circuits.
As power is applied onto the servo motor, the load begins to move . . . speed and position changes.
As the load moves, so does some other "device" move. This other "device" is either a tachometer, resolver or encoder (providing a signal which is "sent back" to the controller). This "feedback" sig-nal is informing the positioning controller whether the motor is doing the proper job.
The positioning controller looks at this feedback signal and determines if the load is being moved properly by the servo motor; and, if not, then the controller makes appropriate corrections. For example, assume the command signal was to drive the load at 1000 rpm. For some reason it is actually rotating at 900 rpm. The feedback signal will inform the controller that the speed is 900 rpm. The controller then compares the command signal (desired speed) of 1000 rpm and the feedback signal (actual speed) of 900 rpm and notes an error. The controller then outputs a signal to apply more voltage onto the servo motor to increase speed until the feedback signal equals the command signal, i.e. there is no error.
Therefore, a servo involves several devices. It is a system of devices for controlling some item (load). The item (load) which is controlled (regulated) can be controlled in any manner, i.e. position, direction, speed. The speed or position is controlled in relation to a reference (command signal), as long as the proper feedback device (error detection device) is used. The feedback and command signals are compared, and the corrections made. Thus, the definition of a servo system is, that it consists of several devices which control or regulate speed/position of a load.
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