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MECHATRONIC UNIT-3 (ACTUATION SYSTEMS)

Fluid Based Actuation: Concept of Hydraulic and Pneumatic Actuation system, Oil and Air preparation unit, Direction Control Valve, Pressure Control Valve, Single and doubly actuated systems, Actuators and Accumulators.

Electrical Actuation Systems: Introduction to Switching devices, Concept of Electro Mechanical Actuation, Solenoids and Solenoid Operated Direction Control Valves, Principle of working of DC and 3 Phase Induction Motor, Stepper motors and Servo Motors with their merits and demerits.

(PART-1)

ACTUATION SYSTEM:

The actuation systems are the elements of the control system and they are responsible for transforming the output of a microprocessor into a controlling action on a machine or device.

Actuators produce physical changes such as linear and angular displacement.

There are four types of actuators.

I. Mechanical actuators.

2. Electrical actuators.

3. Hydraulic actuators.

4. Pneumatic actuators.

Example:
In a CNC milling machine, there may be an electrical signal output from the CNC controller to move the milling table in the x direction for a certain length. There you need an actuation system.

PNEUMATIC AND HYDRAULIC SYSTEMS:

1. Power from one point to another point can also be transmitted using air as medium called pneumatic transmission or liquid as medium called hydraulic transmission.

2. In case of hydraulic system, liquid, which may be water or hydraulic oil is pressurized to 20 to 250 atm pressures and transmitted through pipe line.

3. The pressurized liquid is made to actuate rotary or linear actuator through control valves to get required function.

4. In hydraulic actuation system, the hydraulic signals are used to control device but are more expensive than pneumatic system. Oil leak is another problem in hydraulic system.

5. The high power density of this system allows for high thrust, and fast operating speeds. Electrohydraulics tend to have a high initial price. Routine maintenance involves periodic replacement of seals, O-rings, etc

6. Some of the applications of hydraulic system are hydraulic presses, fork lifts, hydraulic jacks and hydraulic shaper etc.

Hydraulic power supply:
Hydraulic systems are design to move large loads

The basic components of a hydraulic system are: in a hydraulic system, pressurized oil is provided by a hydraulic

pump driven by an electric motor. The hydraulic pump pumps the oil from a sump through a non return valve and an accumulator to the system. A pressure relief valve is circulated to release the pressure when

it rises above the safe level. The non return valve is to prevent the oil returning back to the pump.
The accumulator is a reservoir in which the oil is held under pressure.
The accumulator is used to store the oil and provides a smooth
drive during any short term fluctuation in the output oil pressure.

PNEUMATIC SYSTEMS:

In pneumatic control system. the moister should be separated, to avoid presence of free moisture during expansion. Besides, this
moisture will pose problems in line especially in pilot operator solenoid
valves. Pneumatic system is fast comparable to hydraulic system. But
positioning and speed control is difficult because of compressibility of
air.
Of the three types of actuators, pneumatics tend to have the lowest initial price.

However, operating costs are high due to the need to generate clean, dry, compressed air.

Maintenance requirements are also the highest of the three actuator types.

Pneumatic Power supply:
The basic components of a pneumatic system are,
In a pneumatic power supply an‘ electric motor drives an air
compressor.
Before the air enters the compressor, it passes through a filter
and a silencer.
In the filter all the dust particles present in the inlet air is
removed.

In the silencer the noise level is reduced.
A pressure relief valve is provided to protect the system in case
of pressure rises above the safe level.
Since the air compressor increases the temperature of the air, a
cooler is provided to reduce the temperature of air.
In the filter and water trap, the water from the air and other
unwanted particles in air are removed.
An air receiver increases the volume of air in the system and
smoothens out any short term pressure fluctuation.

DIRECTION CONTROL VALVES:
The direction control valves are used in the pneumatic and
hydraulic system to direct the flow of liquid through a system. They are
used for varying the rate of flow of liquid. They are either completely
open or closed.
There are two types of direction control valves. They are.
1. Spool valve.
2. Poppet valve.

1. Spool Valve:

The device is connected to port 2, and device is pressurized.
In fig. b when the spool is moved to the left, the air supply is cut
off.
Port 2 and port 3 are connected.
So the air in the system connected to port 2 is allowed to go out
to the atmosphere through port 3.
In fig. a air is allowed to flow into the system.
In fig. the air is allowed to flow out of the system.

2. POPPET VALVE:

This valve is normally in the closed
condition.
2 The port 1 is connected to pressure
supply.
The Port 2 is connected to the system,

Initially there is no connection between
N port 1 and port2.
Here balls, discs, or cones are used as a
valve to be seated in the valve seat to
control the flow.
Here a ball is used as shown in fig.
When the push button is depressed,

the ball is pushed out of its
seat
This allows the flow from port 1 connected to port 2.
When the button is released, the spring forces the ball back to its
seat and so closes off the flow.

3. Direction valve:

Figure shows a simple direction control valve and its symbol.
Free flow can occur in one direction through the valve. that which
results in the ball being pressed against the spring. Flow in the other
direction is blocked by the spring forcing the ball against its seat.
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The symbol used for control valves consists of a square for each
of its switching positions.
A two position valve will have two squares; a three position valve
will have three squares.
The arrow headed lines are used to indicate the direction of flow
in each of the position.
The blocked-off lines indicate the flow is closed.
In the fig the valve has four ports.
The ports are labelled by a number or a letter according to their
function.
The ports are labelled 1 (or P) for pressure supply.
The ports are labeled 3 (or T) for hydraulic return port, 3 or 5 (or
R or S) for pneumatic exhaust port and 2 or 5 (or B or A) for
output ports.
Ports are located on the manifold to which the directional control valve is mounted, and are used as connection points to the system. Typically the ports or labelled with a single letter:

P, pressure, or supply

Supply of high-pressure working fluid to be supplied to actuator by the valve.

T, tank, drain, or return

Working fluid from actuator returned to be repressurised from the valve.

A, actuator

Working fluid through the valve for the action of the actuator.

B, actuator, opposing

Working fluid through the valve to opposing action of actuator.

X, external operation

Working fluid to operate the valve.

Y, external operation, opposing

Working fluid to operate the valve in opposition, or to return to pilot valve.

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Basics of Air Preparation

Having a steady supply of clean and dry air is a requirement for protecting pneumatic components and ensuring their proper operation.

good quality air flow for the required pressure is essential and must be designed into the system.

Preparing the air before it is used will help to ensure your machine gets the best possible protection and a better possible service life.

Filters

Compressed air filters should receive the supply air first and are necessary to reduce contaminates and moisture in the compressed air at the machine.

They are available in different sizes depending on the needs of your application.

Also, the filter element should be controlled from time to time, for Coalescing Filters a PDI (pressure drop indicator) is used to see the rate of contamination.

Regulators

After the air has been through the filter, a pressure regulator will take that clean, dry air and control the pressure downstream.

They are operated by turning a valve to determine the pressure – turning it clockwise will allow greater pressure to pass.

Lubricators

A lubricator should be added to a system to provide downstream pneumatic components with a constant supply of oil lubrication by introducing a small amount of oil to the compressed air stream.

The oil will be dissolved and transported by the airstream to the point of use, in the form of fog. This technology is important to guarantee the best lubrication of your components, keeping your machine operating at maximum efficiency.

Today’s pneumatic devices sometimes don’t require lubrication but high-speed pneumatic power tools often do. Some are pre-lubricated and don’t require it.

Air preparation systems