In general, a Machine Control Unit consists of following elements
- Input reading unit
- Memory unit
- Processing unit
- Output channels
- Feedback unit
Input reading unit
It is a electromagnetic device useful in reading the NC programs in the form of instructions.
The main functions of a input reading unit are
- To accept the input media like punched cards, punched tapes, magnetic tapes and floppy disks
- To send them into a reading unit to extract the information stored in them
- To process the collected information
- To collect the input media to be used again
Different types of reading equipments are used to extract the stored data from punched cards and punched tapes. They may be of mechanical, optical or pneumatic type. Magnetic tape readers are used to retrieve the data from magnetic tapes and disks.
A block of information received from the input media consists of words and codes. Sequential informations are arranged in a desired order as separate blocks. These blocks of informations are stored in a temporary memory known as buffer memory. The memory unit will provide the information blocks to the processing unit in the desired sequence.
The processing unit serves as a link between the memory unit and output channels. The processor does the duty of co-ordinating and controlling the other units of the MCU. The informations received from the buffer are processed here and appropriate signals are given to various units at particular points of time. Until a few years ago, processors made use of vacuum tubes and transistors. But recently, when the era of Integrated Circuits (IC) has dawn, processors are driven by ICs.
Mostly output channels are the pins found on ICs or the wires coming out of processors. These channels emit electric pulses of very low voltage / current. The pulses are the result of processed information based on the NC program. In order to drive or actuate various slides of the machines, the pulses are amplified by means of electronic or electromagnetic amplifiers or thyristors.
Feedback unit consists of some electrical or electronic hardware. This is used for converting physical quantities like displacement or velocity into electrical pulses. This system is used in a NC system to check whether the operations are carried out in accurate manner.
The system of operation of a NC machine mainly depends upon the accurate dis- placement of slides and proper speed of driving units. The actual quantity of these factors during the operation is measured, converted as electrical pulses and sent back to processing unit for checking.
The devices used to convert one form of physical quantity into electrical pulses is known as transducers. Transducers used in a NC system are of two types
- Velocity transducers
- Position transducers
Velocity transducers:- They are used to measure spindle speed and slide velocity. An electric tachometer can be used as a velocity transducer.
Position transducer :- They are used to measure slide displacement.