Electromechanical Mechanisms

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ELECTROMECHANICAL MECHANISMS

17. ELECTROMECHANICAL MECHANISMS

Electromechanical mechanisms can be extremely complex assemblies. Consider an automobile, a clothes

washer, your computer printer, or the air conditioner, all are just big electromechanical components.

This chapter of the book is intended to expose the reader to a few miscellaneous electromechanical

components and assemblies that havent been reviewed in the previous chapters.

17.1. Solenoid Door Latch

Figure 17-1 shows a simple solenoid-activated door latch. The bolt is spring-loaded and interfaces with a

striker, so the system will automatically latch when the door is closed. To unlock the mechanism, the

solenoid is energized and the plunger toggles the link, which, in turn, pulls the bolt back.

Figure 17-1. Solenoid Latch

17.2. Hinge Cable

Electrically bridging a hinged assembly is a simple matter that seems to give a lot of people trouble. Simply

anchor a cable loop, as shown inFigure 17-2 between two screw-on blocks. It is important to allow enough

wire in the loop to accommodate the throw of the door.

17.3. Explosive Bolts

Explosive bolts are used in any application where an emergency or rapid release of a bolted component is

necessary. Military aircraft use explosive bolts to release the canopy as part of a controlled sequence just

prior to pilot ejection. Remote piloted deep submersibles use explosive bolts to attach their ballast. If the

control umbilical fails or is severed, the bolts fire and drop the ballast. The vehicle floats back to the surface

where it can be recovered and repaired.
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Figure 17-4. Basic Traction Elevator

17.5. Dash Pots

Dash pots are mechanical shock absorbers that are intended to smooth out the actions of a sensor or

drive.Figure 17-5shows two common dash potsa hydraulic unit and a pneumatic unit.

Pneumatic units are generally used for low-load applications, such as damping the motion of a turn table

tone arm or filtering out high-frequency signals on a vibration sensor. These units typically consist of a small

cylinder with a loosely fitting piston and rod, as shown in the upper illustration (A). Air is allowed to leak

between the gap formed around the outside of the piston and the inside diameter of the cylinder. At low

speeds the flow rate through this gap is sufficient to allow the piston and rod assembly to move unimpeded.

At higher speeds the gap restricts the flow and, in turn, places a load on the motion of the piston and rod.
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Figure 17-5. Dash Pot Shock Absorbers

Hydraulic dash pots operate in much the same manner as their pneumatic counterparts, except the flow is

controlled through a bypass loop, as shown in the lower illustration (B). The bypass loop can be set up with

a pair of needle valves and check valves, which allows the damping characteristics of both the extend and

the retract to be tuned independently.

Figure 17-6 shows a pneumatic dash pot used to dampen the motion of a pendulum accelerometer. The

dash pot will limit sudden impulse loads, while allowing long duration loads to be monitored.

Figure 17-7 shows a hydraulic dash pot used to limit the speed at which a solenoid-activated knife switch

throws.

Figure 17-6. Accelerometer Equipped with a Dash Pot
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Figure 17-7. Powered Knife Switch with Hydraulic Dash Pot

17.6. Spark Plugs

Spark plugs are simply a pressure feedthrough that is configured for a special purpose. These devices are

excellent high-pressure, high-voltage feedthroughs that can be used in all sorts of equipment. The electrical

terminal is simple, reliable, and can comfortably handle voltages as high as 40,000 volts. When using a

spark plug as a feedthrough, it is important to select a plug without an internal resistor, as shown in Figure

17-8. These units generally have an R in the code printed on the insulator.

17.7. Dynamic Braking

A permanent magnet or shunt wound DC motor can be used as a brake in certain applications. The idea is

that the motor is allowed to act as a generator and the power is dumped into a set of high-capacity resistors.

Figure 17-8. Spark Plug


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Figure 17-9. Dynamic Braking Schematic

Figure 17-9 shows a dynamic braking system with a shunt wound DC motor. The field current is controlled

by the Power/Brake control rheostat. The operation of the motor (run or brake) is controlled by the

Power/Brake switch. In the power mode, the motor is fed DC power and operated as a normal electric

motor. The speed of the motor is controlled by adjusting the field current. In the brake mode, the motor is

disconnected from the DC power and is connected to a resistive load dump. During this time, the spinning

motor acts as a generator and the rotational energy that is being introduced into the output shaft is removed

in the form of heat. The braking effect can be controlled by adjusting the field current. By integrating the

switch and rheostat into a common assembly, a single lever throttle/brake control can be configured.

17.8. Three Door Bell System

Figure 17-10 shows how to wire a three door bell system using a bell, buzzer, two single-pole buttons, and a

double-pole button. The bell is used for the primary door (front) and the buzzer is used for the secondary

door (back). The double-pole button is mounted on the third door (side) and is wired to operate both the bell

and the buzzer simultaneously. The power supply is a transformer with a 120-volt primary and an 18-volt

secondary.

17.9. Utility Transformer

In many situations it is advantageous to have a 120-VAC receptacle adjacent to a major equipment

installation. A 120-volt utility receptacle allows maintenance equipment and powered hand tools to be used

without the hassle of running several hundred feet of extension cord. However, capital equipment is

generally wired with a service that does not offer this utility voltage (240 volt, delta three phase or 480-

volt three phase). In these cases, a simple utility transformer can be configured, as shown inFigure 17-11. A

suitable control transformer is selected and mounted in a NEMA (National Electric Manufacturers

Association) cabinet along with a 120-VAC receptacle. Control transformers are readily available with dual-

voltage inputs and integral fuse sets. The cabinet can be mounted directly onto or adjacent to the power

disconnect that services the equipment.
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Figure 17-10. Three Door Bell System

Figure 17-11. 120-VAC Utility Transformer


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Figure 17-12. 120-VAC Utility Transformer Schematic

Figure 17-12 shows a schematic representation of the utility control transformer. It is important to use both

input and output fuses , as shown.

17.10. String Drives

Figure 17-13 shows a typical arrangement used in radio receivers to adjust the frequency with a variable

capacitor. A string is wrapped around a small capstan mounted on the back of a knob. The string is routed

around an idler and the large tuner pulley. Over the length of the string, a pointer is mounted to indicate the

relative frequency on the scale. Although these types of drives are most commonly found on radios, they are

applicable to a variety of other applications.

Figure 17-13. String Tuner Drive

When the variable capacitor is replaced with a potentiometer, the scale can indicate voltage, resistance,

volume, balance, and the like.

17.11. Motorized Locking Systems

For high security systems, large pins or bolts are commonly used to lock a heavy door in place. Figure 17-

14 shows a worm drive locking system with four bolts. When the door is closed, the motor is activated and

the driven gear forces the bolts out into a corresponding frame. When the motor polarity is reversed, the

bolts are retracted back into the door. In this manner a relatively small gear motor can be used to lock a

rather substantial door.
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Figure 17-14. Motorized Locking Pins

17.12.Air Compressor Control

A typical reciprocating air compressor provides an excellent example of how simple it is to control high-

horsepower motors with relatively low power, and therefore low cost components. Figure 17-15 shows a

typical commercial reciprocating air compressor. These units are normally supplied in the 7.5- through 30-

horsepower range. They turn on when the air pressure in the receiver is below a preset lower limit and turn

off when the receiver pressure reaches a preset upper limit.

Figure 17-15. Packaged Air Compressor

Figure 17-16 shows the electrical schematic for the compressor. The motor is connected to the power

source through a motor controller with a set of overload heaters. The coil is controlled with an upper/lower

limit pressure switch. The control circuit is normally operated from a 120-VAC control transformer, as shown.


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Figure 17-16. Packaged Air Compressor Schematic

Figure 17-17. Pneumatic Control Station

17.13. Pneumatic Control Stations


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Figure 17-17 shows a pneumatic control station configured to control the positions of two air cylinders on a

piece of nearby equipment. A pair of four-way, venting solenoid valves is mounted to the output of a

pressure regulator. The solenoids receive their signals from a plant-wide control loop.

17.14. Fuel Injector Nozzles

Virtually all modern automobiles use electronically timed fuel injection. Any other fuel induction method

simply wont meet the stringent pollution standards that are called for by our government. The modern fuel

injection system centers around a set of valved injector nozzles, as shown in Figure 17-18. A nozzle is

mounted into each intake port on an engine. The valves are opened and closed via a signal provided from a

central computerized controller.

The nozzle itself consists of a poppet valve that is controlled by an electrical pulse. The fuel flows through

the center of the poppet and is stopped at the valve seat. When the coil receives a pulse, the poppet raises

and the fuel is allowed to spray into the port. The amount of fuel that flows is controlled by the duration of

time that the valve is energized.

17.15. Spot Welders

Spot welders join metals by introducing a high-energy electrical pulse into a confined area. The amount of

energy is high enough to melt and fuse the base metals, forming a single piece. Figure 17-19 shows a

typical spot welding circuit. To accomplish a weld, two pieces of sheet metal are pinched between a pair of

tips. When the tips are closed they form the secondary winding of a transformer. The primary winding is

connected to a bank of storage capacitors. The capacitors are slowly charged with a small power supply.

When the capacitors reach full charge, they are switched into the primary coil circuit via an ignitron and they

dump their entire power into the transformer and, consequently, into the weld site. For more information on

transformers, seeChapter 5. For more information on ignitrons, seeChapter 14.

Figure 17-18. Electronic Fuel Injection Nozzle
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Figure 17-19. Spot Welder Circuit

17.16. Toasters

One electromechnical device that we have all experienced is the ordinary bread toaster. These are clever

devices that will perfectly toast a slice of bread every time. Figure 17-20 shows a schematic representation

of a typical bread toaster. The bread is placed into the slot and rests on a bread tray. When the tray is

lowered, it closes a limit switch and is latched into place. As the heaters cook the bread, the coiled bimetal

strip heats up and eventually pulls the latch open, allowing the bread tray to pop up. By adjusting the preload

on the coiled bimetal strip, the down time can be adjusted and the brownness of the toast can be controlled.

Figure 17-20. Bread Toaster

Citation

Brian S. Elliott: Electromechanical Devices & Components Illustrated Sourcebook. ELECTROMECHANICAL

MECHANISMS, Chapter (McGraw-Hill Professional, 2007), AccessEngineering
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PageContents

Solenoid Door Latch

Hinge Cable

Explosive Bolts Traction Elevator

Dash Pots

Spark Plugs

Dynamic Braking

Three Door Bell System

Utility Transformer

String Drives

Motorized Locking Systems

Air Compressor Control

Pneumatic Control Stations

Fuel Injector Nozzles

Spot Welders

Toasters
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