types of control systems in machine tools

1. CONTROL SYSTEMS IN
MACHINE TOOL AND OPEN
LOOP & CLOSED LOOP
CONTROL SYSTEM
By
k.Hari krishna

2. Why control system in machine tools :
The control systems of a machine tool are meant to
generate controlling movements which are essential for
carrying out a machining process in accordance with the
technical specifications of the part being machined.
A control system, in general, Consists of :
1. control member
2. transmission
3. an operative member

3. 1. control member or transmitter:
which initiates the controlling movement when it receives a
command from the feedback device
2. Transmission :
which transmits the controlling movement to the operative
member, the transmission
is generally a set of elements and mechanisms which
appropriately modify the initial controlling
movement to suit the requirements of the operative member
3. an operative member :
which finally executes the movement

4. Functions of control system :
I. changing speeds and feeds.
2. Providing the working and auxiliary motions in the
desired sequence necessary for machining a particular part.
Beside the above two, machine tools are provided with
safety controls to prevent damage to the machine tool and
cutting tool during execution of primary and auxiliary
motions.

5. The critical importance of control systems in the overall
machine tool performance dictates the following
requirements in their design:
1. Reliability against failure to respond; the greater the
degree of automation of the control system, the
higher should he its reliability.
2. Fast response to the information provided by the
feedback device; again, the greater the degree of
automation, the faster should be the control system
response.
3. Accuracy of the system, which depends upon the
accuracy of the feedback device, control member and
transmission.

6. Open-loop and closed –loop system
Open loop system :there is no feedback on the
accuracy of execution of the path magnitude commands.
Such systems are known as open-loop systems.
elements of an open-loop system are:
1.Program medium
2.Program reader
3.Buffer storage
4.Decoder
5.Amplifier
6.drive

8. Closed-loop system: Numerically controlled Systems
with a feedback device are known as closed-loop systems.
The feedback arrangement Consists of a transducer which
monitors the actual displacement of the operative member.
The actual displacement is compared with the programmed
displacement and the difference signal is employed to
actuate the drive motor until the command signal and actual
position coincide.
besides the six elements enumerated above, a
closed-loop system has two additional elements:
1. A displacement measuring device
2. A comparator.

10.  The displacement measuring device (transducer) and the
comparator may be analog or digital.
 For compatibility between the transducer signal and
comparator and between the transducer output and drive,
it is necessary to use appropriate analog-to-digital (AD)
and digitaltoaflal0g (DA) converters.
In general, four combinations of the transducers and
comparators are possible.
combination Converter ?1 Converter ?2
Analog comparator and analog transducer Nil Nil
Analog comparator and digital transducer DA Nil
Digital comparator and digital transducer Nil DA
Digital comparator and analog transducer AD DA

11. Machine control system :
The input devices, reading circuits. decoding circuits.
interpolator, comparator or controller, position control circuit,
velocity control circuit, deceleration circuit and auxiliary
functions constitute the machine control unit (MCU) of the
numerically controlled machine tool.
The MCU is housed in a separate cabinet and together with
the machine tool constitutes the NC machining system.

12.  Closed-loop systems are more reliable but also more
expensive. Numerically controlled machine tools have
traditionally been closed-loop systems. but in recent years,
the concept of open-loop control is fast catching up.
 The successful operation of open-loop systems is
governed by the reliability of stepping motors, i.e., their
ability to respond reliably to a pulse command.
 If a stepping motor were to respond unfailingly to
command signals, but not respond to spurious signals.
then there would he no need for the feedback
arrangement and the control systems would become
considerably simpler and cheaper.

13. At present. reliable stepping motors are available in a low
horsepower range only. Therefore,open-loop control is
generally employed only in positioning systems in which no
material removal takes place during the movement
controlled by the NC system.