Control System
A large variety
of components are manufactured in industries. Every component has its own geometric parameters, hence, different machining operations. For meeting the machining operations of different components a single
CNC system cannot give optimum results. So the CNC machines
are designed to meet specific
requirements to make them cost effective. CNC or NC machines
are classified on the basis of feedback
control and on the basis of control
system features. Based on feedback control,
the NC and CNC system is classified as open-loop and closed-loop control systems.
- Open-Loop Control System. Machine tool
controls in which there is no provision to compare the
actual position of the cutting tool or work piece with the input command value,
are called open-loop systems. In the open-loop control system, the electric motor
continues to run until the absence of power, from input command signal,
indicates that the programmed location has been attained. There is no
monitoring of the actual displacement of the machine slide. In the open-loop
control system, the control may indicate a movement of 50mm whereas actually
the slide may have moved only 49.8 mm. Figure "Block Diagram of an Open-Loop
System" below shows the block diagram of an open-loop control
system. In open-loop control system the
actual displacement of the slide may vary with change in external conditions
and due to the wear of components of the drive mechanism, i.e. backlash errors
in lead screw, etc. Since there is no provision of feedback in the control system
periodical adjustments are required to compensate for the changes due to
various factors. Although open-loop
systems are less accurate compared to a closed-loop system, these are adequate
for many applications, e.g. foundry work. Open-loop systems are less expensive
than closed-loop systems due to the absence of monitoring devices and their maintenance
is not complicated.
Block Diagram of an Open-Loop System
- Closed-Loop Control System. In a closed-loop control system, the actual output from the
system i.e. actual displacement of the machine slide is compared with the input
signal. The closed loop control systems are characterized by the presence of feed-back
devices in the system. In the closed-loop control system the displacement can be
achieved to a very high degree of accuracy because a measuring or monitoring device
is used to determine the displacement of the slide. The feedback from the monitoring
device is then compared with the input signal and the slide position is regulated
by the servo system until it matches with the desired position. Figure "Block Diagram of Closed-Loop System" below shows
a closed loop control system with a provision for feedback for the displacement
of position of machining slide. In order to measure the speed of the motor and to
compare the actual speed with the programmed speed, a velocity feedback system is
added to the system.
Block Diagram of Closed-Loop System
Servomechanism
In modern usage,
the term servomechanism or servo is restricted to feed back control system, in which
the controlled variable is mechanical position or time derivatives of position,
e.g. velocity and acceleration. A servo system is used to position a load shaft,
in which the driving motor is geared to the load to be moved. The output (controlled)
and desired (reference) positions are measured and compared by a potentiometer pair
whose output voltage is proportional to the error in angular position. The voltage
is amplified and is used to control the field current of a DC generator which supplies
the armature voltage to the driving motor.
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