IIT NewsSummer 2002
UPDATE
  Newsletter of Industrial Research & Consultancy Centre

 
FOCUS
Low Cost Automation


Hydraulic Systems
 
Even though oil hydraulic systems are generally costlier than pneumatic systems, they are attractive where large forces are required along with fine control of speed. Their field of application is very wide, ranging from simple hydraulic brakes used in automobiles to the sophisticated actuating systems in aircrafts. They are used virtually in every industry, such as:

a.  Die casting machines in the casting      process
b.  Presses for heavy duty metal forming      processes 
c. Injection or transfer moulding machines for      plastics,       tractors, earth moving     equipment, fork lifts, machine       tools etc.

Electric Systems These consist of electric motors or electro-magnets for actuation. A variety of electric motors (AD, DC, servo, stepper etc.) are available as rotary actuators. Even though linear motors have also been developed, they are not yet commercially available. Today, electronic controls of motors have reached sufficiently high level of sophistication such that speed-torque characteristics can be controlled and adjusted. For linear actuators, conversion of rotation of motor through any mechanical means (rack and pinion, screw and nuts etc.) can be used.
     Sensors (transducers) also form a major part of electric LCA systems. A transducer converts a physical parameter (temperature, pressure, velocity etc.) to an electric signal which can then be used to control the actuator. This signal, generally being feeble, is amplified and then used. Switches, contactors, relays etc. are used to develop the circuit. The electric circuit consists of two parts: (A) the actuator part which consists of the electromagnets, motors, brakes etc. along with safety devices like overload protections, (B) the control part consisting of activating devices like start, stop push buttons, limit switches, relays, timers etc.

Hybrid Systems  Having seen the mechanical, pneumatic, hydraulic and electrical systems, it is easy to understand the utility of a hybrid system.

Fig. 3 Automatic Storage & Retrieval System

 In this, more than one type of system is used and interfaced appropriately with another so that the advantages of both are used, while ensuring that the disadvantages are not detrimental to the functional requirements. Depending on the systems that are coupled, these are called electro-mechanical, pneumo-hydraulic, electro-pneumatic etc. Fig. 3 shows a simple electro- mechanical system. There are storage shelves on both sides of a Stacker Crane. The crane moves on rails and can pick and place components on either shelf. By interfacing with a P.C. every transaction is

recorded so that inventory status and control is very easy.

Fig. 4 Hydro-Pneumatic Circuit

     Two typical examples are taken for clarifying the concept of a hybrid system. Fig. 4 shows a hydro-pneumatic circuit. The pneumatic system with a double acting reciprocating cylinder has its piston rods modified to act as plungers in cylinders of much smaller diameter, which are filled with oil. Due to the smaller area, the oil pressure becomes higher by the ratio of the area of pneumatic cylinder to the area of the piston rod. The output from the oil filled cylinders is diverted into a cylinder of large diameter. Since the pressure is acting on its large area, the force becomes equal to the pressure in the plunger-cylinder multiplied by area of the output cylinder. The piston rod of this cylinder will thus give a large force with corresponding reduction in displacement. This system has the advantage of simplicity of the pneumatic system coupled with the high forces developed by the hydraulic system.
     Consider again the pick-and-place unit shown in Fig. 5 which is operated with an electro-pneumatic system.  

Fig. 5 Electro- Pneumatic Device

The pneumatic part provides the movements for the arm of the pick-and-place unit whereas the electric circuit takes care of the control function. The device uses a pneumatic system for actuating motions, whereas the control system is electrical for quick and easy response. It picks parts and transports them to a magazine location, where it is dropped. These two examples indicate the variety of options available in developing a hybrid system – their inherent versatility makes it possible to build a system most appropriate for a given application.

Use of Microelectronics in LCA
The reliability of an LCA system can be significantly enhanced by using Programmable Logic Controllers (PLC) which can replace hardwired ladder logic. Use of a PC can further enhance the capabilities of the PLC system by storing data and using them as and when required. The input/output ports are connected to the sensors and actuating devices.

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