This is where the gear system inside a servomechanism comes into
the picture. The gear mechanism will take high input speed of the
motor (fast) and at the output, we will get an output speed which
is slower than original input speed but more practical and widely
applicable. Say at initial position of servo motor shaft, the
position of the potentiometer knob is such that there is no
electrical signal generated at the output port of the
potentiometer. This output port of the potentiometer is connected
with one of the input terminals of the error detector amplifier.
Now an electrical signal is given to another input terminal of the
error detector amplifier. Now difference between these two signals,
one comes from potentiometer and another comes from external
source, will be amplified in the error detector amplifier and feeds
the DC motor.
This amplified error signal acts as the input power of the DC motor
and the motor starts rotating in desired direction. As the motor
shaft progresses the potentiometer knob also rotates as it is
coupled with motor shaft with help of gear arrangement. As the
position of the potentiometer knob changes there will be an
electrical signal produced at the potentiometer port. As the
angular position of the potentiometer knob progresses the output or
feedback signal increases. After desired angular position of motor
shaft the potentiometer knob is reaches at such position the
electrical signal generated in the potentiometer becomes same as of
external electrical signal given to amplifier. At this condition,
there will be no output signal from the amplifier to the motor
input as there is no difference between external applied signal and
the signal generated at potentiometer. As the input signal to the
motor is nil at that position, the motor stops rotating. This is
how a simple conceptual servo motor works.
0.85 to 15 kW servomotors require servomotor-end connector and
brake power supply connector. The servomotor-end connector is the
same as is used for servomotors without holding brakes.
To order a brake power supply connecter kit (0.85 to 15 kW) with
the order no. below, contact your Yaskawa representative. Connector
Type S: Straight plug A: L-shaped plug Bush Size*1 S : Size S (4.0
dia. to 6.0 dia.) M : Size M (6.0 dia. to 9.0 dia.) L : Size L (9.0
dia. to 11.6 dia.) Contact Pin Size S2: Soldered C3: Crimped type*2
*1: A size-M connector kit is available as a standard equipment.
*2: A crimp tool (model: 357J-50448T) by DDK Ltd. is required.
Configuration of Servo System
The following diagram illustrates a servo system in detail:
(1) Controlled system: Mechanical system for which the position or
speed is to be controlled.This includes a drive system that
transmits torque from a servomotor.
(2) Servomotor: A main actuator that moves a controlled system. Two
types aravailable: AC servomotor and DC servomotor.
(3) Detector: A position or speed detector. Normally, an encoder
mounted ona motor is used as a position detector.
(4) Servo amplifier: An amplifier that processes an error signal to
correct the difference between a reference and feedback data and
operates theservomotor accordingly. A servo amplifier consists of a
comparator, which processes error signals, and a power
amplifier,which operates the servomotor.
(5) Host controller: A device that controls a servo amplifier by
specifying a positionor speed as a set point.
Servo components (1) to (5) are outlined below:
(1) Controlled system
In the previous figure, the controlled system is a movable table
for which the positionor speed is controlled. The movable table is
driven by a ball screw and is connected tothe servomotor via gears.
So, the drive system consists of:
Gears + Ball Screw
This drive system is most commonly used because the power
transmission ratio(gear ratio) can be freely set to ensure high
positioning accuracy. However, play in thegears must be minimized.
The following drive system is also possible when the controlled
system is a movable
table:
Coupling + Ball Screw
When the power transmission ratio is 1 :1, a coupling is useful
because it has noplay.
This drive system is widely used for machining tools.
To develop an excellent servo system, it is important to select a
rigid drive system that has no play. Configure the controlled
system by using an appropriate drive system for the control
purpose.
Timing Belt + Trapezoidal Screw Thread
A timing belt is a coupling device that allowsthe power
transmission ratio to be set freelyand that has no play.
A trapezoidal screw thread does not proviexcellent positioning
accuracy, so can be treated as a minor coupling device.
