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PLC output card

I = leakage current (mA)

		load	Vac
	PLC		power
	controlled	Von
			supply
	TRIAC
		R
			neut.
		R < Von/I

15.1.1.1 - Relays

15.1.1.2 - Transistors

• BJT, FET, darlington, etc.

15.1.1.3 - Triacs

15.2 TYPES

15.2.1 Solenoids

• These use a coil of wire to attract a ferrous core when actuated. When the coil is deenergized a spring will pull the core back out of the coil.

current off

current on

• These are particularly bad electrically. There is a large actuation current and when deenergized

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there will be a large inductive spike.

• A very popular application for solenoids is valves. These effectively use the solenoid to drive pistons that open/close ports on a cylinder.

The solenoid has two position and when actuated will change the direction that fluid flows to the device. The symbols shown here are commonly used to represent this type of valve.

solenoid

exhaust out power in

solenoid

power in

exhaust out

15.2.2 Hydraulic

•Very powerful and well suited to slower speeds with higher forces.

•Use of fluids, and high pressures can make this method awkward, messy, and noisy when improperly applied.

•Becoming less common in smaller force applications, as electrical systems mature

•Typical hydraulic systems use a simple solenoid valve to actuate devices, some newer developments use more sensitive devices

•A hydraulic actuation system is shown below

•Higher maximum accelerations that D.C. motors

•small time constants giving smooth operation

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•time constant of hydraulic servo valve is about 5ms

•cylinders have small motions

•rotary motions with gearing are most common

15.2.3 Hydraulics

•Incompressible fluids are used to transmit volume and pressure changes throughout a system.

•Pascal’s law basically describes these systems,

F

P = --

A

If a pressure is applied to a non-moving (static) fluid, the pressure (hydrostatic pressure) the fluid exerts on all surfaces that it touches are the same.

• Hydrostatic force/motion multiplier,

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F1

∆ x1

P, V2

∆ x2

P, V1

F2

A1

A2

For Force:

F1

A2

P = F1A1 = F2A2

-----

=

-----

F2

A1

For Motion:

∆ V1 = –∆ x1A1 = –∆ V2 = ∆ x2A2

∆

x2

A1

----------- =

-----

–∆

x1

A2

•The Hydrodynamic Effect - when fluid is moving quickly, it has high levels of kinetic energy. If the fluid impacts a surface, it transmits a high quantity of energy in a short period of time.

•Hydraulic Circuits typically contain,

1.Hydraulic Fluid

2.An Oil Reservoir

3.A Pump to Move Oil, and Apply Pressure

4.Pressure Lines

5.Control Valves - to regulate fluid flow

6.Piston and Cylinder - to actuate external mechanisms

•Oil Reservoir