21-1 Thermistors 21-1 • Define thermistor and describe the classes of thermistors. • Explain how to test thermistors.
21-2 Photoconductive Cells 21-3 Photoconductive Diodes 21-4 Pressure Sensors
21-2 • Describe photoconductive cells (photocells) and give examples of how they are used. • Explain how to test photocells.
21-5 Flow Detection Sensors 21-6 Hall Effect Sensors
21-3
21-7 Proximity Sensors
• Define photoconductive diode (photodiode) and explain how it operates.
21-8 Ultrasonic Sensors
21-4 • Define and describe pressure sensors. • Explain how to test a pressure sensor.
21-5 • Define and describe flow detection sensors.
21-6 • • • •
Define Hall effect sensor and explain the Hall effect. Explain how Hall effect sensors operate. Explain how Hall effect sensors may be actuated. Describe Hall effect sensor applications.
21-7 • Define proximity sensor and explain how it operates. • Explain the difference between an inductive proximity sensor and a capacitive proximity sensor.
21-8 • Define ultrasonic sensor and explain how it operates. • Explain the difference between a direct mode ultrasonic sensor and a diffused mode ultrasonic sensor.
438
Semiconductor input devices are often referred to as transducers. Transducers provide a link in integrating industrial control systems because they convert mechanical, magnetic, thermal, electrical, optical, and chemical variations into electrical voltage and current signals. These voltage and current signals are used directly or indirectly to control devices within a system. Because of the variety of solid-state devices available, many types of electromechanical transducers are being replaced with solid-state transducers. Solid-state transducers include thermistors, photoconductive cells (photocells), photoconductive diodes (photodiodes), pressure sensors, flow detection sensors, Hall effect sensors, proximity sensors, and ultrasonic sensors.
21-1 Thermistors A thermistor is a temperature‑sensitive resistor whose resistance changes with a change in temperature. See Figure 21-1. Thermistors are popular because of their small size, which allows them to be mounted in places that are inaccessible to other temperature‑sensing devices. The operation of a thermistor is based on the electron‑hole theory. As the temperature of the semi‑ conductor increases, the generation of electron‑hole pairs increases due to thermal agitation. Increased electron‑hole pairs cause a drop in resistance. Thermistors may be directly heated or indirectly heated. Directly heated thermistors are used in voltage regulators, vacuum gauges, and electronic time-delay circuits. Indi‑ rectly heated thermistors are used for precision temperature measurement and temperature compensation. Each type of thermistor is represented by a separate schematic symbol.
Thermistors are commonly used in small appliances such as coffee makers to control temperature.
439
440 ELECTRICAL MOTOR CONTROLS for Integrated Systems
PTC and NTC Thermistors
THERMISTORS Device
Symbol THERMISTOR ELEMENT t°
t°
Thermistor
HEATER ELEMENT
DIRECTLY HEATED
INDIRECTLY HEATED
READING WITH HEAT
READING WITH NO HEAT
CURRENT FLOW INCREASES AS HEAT IS APPLIED TO THERMISTOR
THERMISTOR
FIRE
Two classes of thermistors are positive temperature coef‑ ficient (PTC) and negative temperature coefficient (NTC). Although the most commonly used thermistors are NTC thermistors, some applications require PTC thermis‑ tors. With a PTC thermistor, an increase in temperature causes the resistance of the thermistor to increase. With an NTC thermistor, an increase in temperature causes the resistance of the thermistor to decrease. The resistance of each thermistor returns to its original state (resistance value) when the heat is removed. Resistance refers to the operating resistance of a thermistor at extreme temperatures. Cold resistance is measured at 25°C (room temperature). However, some manufacturers specify lower temperatures. The speci‑ fication sheet should always be checked for the correct temperature specification. Hot resistance is the resis‑ tance of a heated thermistor. In a directly heated therm‑ istor, heat is generated from the ambient temperature, the current, and the heating element of the thermistor.
Thermistor Applications
VOLTAGE SOURCE
L1
L2 115 VAC
T
FAN MOTOR
SOLID-STATE RELAY
A fire alarm circuit is a common application of an NTC thermistor. See Figure 21-2. The purpose of this circuit is to detect a fire and activate an alarm. In normal operat‑ ing environments, the resistance of the thermistor is high because ambient temperatures are relatively low. The high resistance keeps the current to the control circuit low. The alarm remains OFF. However, in the presence of a fire, the increased ambient temperature lowers the resistance of the thermistor. The lower resistance allows current flow, activating the alarm.
THERMISTOR APPLICATIONS
FAN MOTOR TURNS ON AUTOMATICALLY AT HIGH TEMPERATURE ALARM
Figure 21-1. A thermistor is a temperature-sensitive resistor whose resistance changes with a change in temperature. FIRE -t°
A typical application of a thermistor is to control a fan motor. As the thermistor is heated, its resistance decreases and more current flows through the circuit. When enough current flows through the circuit, a solid‑state relay turns on. The solid‑state relay is used to switch on a fan motor at high temperatures. Such a circuit can be used to automatically reduce heat in attics or to circulate warm air.
NTC THERMISTOR
VOLTAGE SOURCE
Figure 21-2. In the presence of fire, the increase in temperature lowers the resistance of an NTC thermistor, which increases current and activates an alarm.
Chapter 21 — Semiconductor Input Devices 441
Testing Thermistors A thermistor must be properly connected to an electronic circuit. Loose or corroded connections create a high resistance in series with the thermistor resistance. The control circuit may sense the additional resistance as a false temperature reading. The hot and cold resistance of a thermistor can be checked with a DMM. See Figure 21-3. To test the hot and cold resistance of a thermis‑ tor, the following procedure is applied: 1. Remove the thermistor from the circuit. 2. Connect the DMM leads to the thermistor leads and place the thermistor and a thermometer in ice water. Record the temperature and resistance readings. 3. Place the thermistor and thermometer in hot water (not boiling). Record the temperature and resistance readings. Compare the hot and cold readings with the manufacturer specification sheet or with a simi‑ lar thermistor that is known to be good.
TESTING THERMISTORS PC BOARD
1
REMOVE THERMISTOR FROM CIRCUIT
CONNECT DMM TO THERMISTOR AND PLACE IN ICE WATER; RECORD TEMPERATURE AND RESISTANCE READINGS
3
2
PLACE THERMISTOR IN HOT WATER; RECORD TEMPERATURE AND RESISTANCE READINGS
THERMOMETER
Tech Fact Thermistors have a base resistance rating such as 3 kΩ, 10 kΩ, 30 kΩ, 50 kΩ, and 1 MΩ. The base resistance rating is typically specified as the resistance at a set temperature, usually around room temperature (72°F to 77°F). When testing a thermistor, the resistance is measured after the thermistor has been at room temperature for several minutes to ensure a consistent measurement that can be compared to the rating. As a PTC thermistor is heated, its resistance will increase. As an NTC thermistor is heated, its resistance will decrease.
ICE WATER HOT WATER
Figure 21-3. The hot and cold resistance of a thermistor can be checked with a DMM.
21-1 CHECKPOINT 1. What happens to the resistance of a PTC thermistor when it is heated? 2. What happens to the resistance of an NTC thermistor when it is heated?
3. What is the most commonly used thermistor type?
21-2 Photoconductive Cells A photoconductive cell (photocell) is a device that conducts current when energized by light. Current increases with the intensity of light because resis‑ tance decreases. A photocell is, in effect, a variable resistor. A photocell is formed with a thin layer of semiconductor material, such as cadmium sulfide (CdS) or cadmium selenide (CdSe), deposited on a
suitable insulator. Leads are attached to the semicon‑ ductor material and the entire assembly is hermeti‑ cally sealed in glass. The transparency of the glass allows light to reach the semiconductor material. For maximum current‑carrying capacity, the photocell is manufactured with a short conduction path having a large cross‑sectional area.
442  ELECTRICAL MOTOR CONTROLS for Integrated Systems
Photocell Applications Photocells are used when time response is not critical. Photocells are not used when several thousand responses per second are needed to transmit accurate data. Applica‑ tions where photocells can be used efficiently include slow-responding electromechanical equipment such as pilot lights and street lights. A photocell can be used to control the pilot light in a gas furnace. See Figure 21-4. In this application, the light level determines if the pilot light (flame) in the furnace is ON or OFF. When a pilot light is present, the light from the flame reduces the resistance of the photocell. Current is allowed to pass through the cell and activate a control relay. The control relay allows the main gas valve to be energized when the thermostat calls for heat. The same procedure would be used for similar applications such as gas-powered water heaters, clothes dryers, and ovens (commercial or residential), as well as similar electrical applications.
in the relay causes the normally closed (NC) contacts to open, and the light turns off. The resistance increases with darkness, causing the NC contacts to return to their original position and turning on the light.
PHOTOCELL APPLICATIONS PHOTOCELL
PHOTOCELL SOLID-STATE RELAY
L1
L2 115 VAC
PHOTOCELLS Device
Photocell
L
Symbol CONTROL CIRCUIT TO MAIN BURNER GAS VALVE
PHOTOCELL
NORMALLY CLOSED CONTACTS LAMP TURNS ON IN ABSENCE OF LIGHT
Figure 21-5. A photocell can be used to determine when a streetlight should turn on or off.
Testing Photocells PILOT FLAME
PILOT BURNER NOZZLE
Figure 21-4. A photocell can be used to determine if the pilot light on a gas furnace is ON or OFF.
A photocell can also be used in a streetlight circuit. See Figure 21-5. In this circuit, an increase of light at the photocell results in a decrease in resistance and current flow through the solid-state relay. The increased current
Humidity and contamination are the primary causes of photocell failure. See Figure 21-6. The use of quality components that are hermetically sealed is essential for long life and proper operation. Some plastic units are less rugged and more susceptible to temperature changes than glass units. To test the resistance of a photocell, the following procedure is applied: 1. Disconnect the photocell from the circuit. 2. Connect the DMM leads to the photocell. 3. Cover the photocell and record dark resistance. 4. Shine a light on the photocell and record light resistance. 5. Compare the resistance readings with manufacturer specification sheets. Use a similar photocell that is known to be good when specification sheets are not available. All connections should be tight and corrosion free.
Chapter 21 — Semiconductor Input Devices 443
TESTING PHOTOCELLS 1
HIGH RESISTANCE
DISCONNECT PHOTOCELL FROM CIRCUIT
2
CONNECT DMM LEADS TO PHOTOCELL
3
COVER PHOTOCELL AND RECORD RESISTANCE
LOW RESISTANCE
4
SHINE LIGHT ON PHOTOCELL AND RECORD RESISTANCE
Figure 21-6. Humidity and contamination are the primary causes of photocell failure.
21-2 CHECKPOINT 1. What happens to the resistance of a photoconductive cell as light on it increases?
2. What happens to the current flowing through a photoconductive cell as light on it increases?
21-3 Photoconductive Diodes A photoconductive diode (photodiode) is a diode that is switched on and off by light. A photodiode is similar internally to a regular diode. The primary difference is that a photodiode has a lens in its housing for focusing light on the PN junction. See Figure 21-7.
Photoconductive Diode Operation In a photodiode, the conductive properties change when light strikes the surface of the PN junction. Without light, the resistance of the photodiode is high. The resistance is reduced proportionately as the photodiode is exposed to light. The current flowing through the photodiode increases as the resistance decreases.
Photoconductive Diode Applications Photodiodes respond much faster than photocells. Also, they are usually more rugged than photocells. Photodi‑ odes are found in movie equipment, conveyor systems, and other equipment requiring a rapid response time. Photodiodes can be used for positioning an object and turning functions on and off, such as in a filling machine.
Tech Fact When a photoconductive diode is used in certain applications, its lens of may become covered with debris and must be cleaned. For example, the sensors used to detect an object under a closing garage door are susceptible to dirt accumulation that may prevent them from operating properly.
444  ELECTRICAL MOTOR CONTROLS for Integrated Systems
PHOTODIODE APPLICATIONS
PHOTODIODES Device
Device
AMPLIFIER
PLC
Photodiode
LENS
ELECTRON FLOW PHOTODIODE
LIGHT CATHODE
ANODE
P-TYPE MATERIAL
N-TYPE MATERIAL
LIGHT SOURCE
Figure 21-7. A photodiode is a diode that is switched on and off by a light.
In a filling machine, a constant light source is placed across the conveyor from the photodiode so that cartons can move between the light source and the photodiode. See Figure 21-8. The photodiode is energized as long as there are no cartons in the way to prevent light from passing. The photodiode de-energizes when a carton passes between the light source and the photodiode. The programmable logic controller (PLC) records the re‑ sponse and stops the conveyor, placing the carton in the correct position and filling the carton. This arrangement eliminates the need for slow mechanical equipment.
Figure 21-8. Photodiodes are used to position objects and turn machine functions on and off.
21-3 CHECKPOINT 1. What happens to the resistance of a photodiode when light is shining on it? 2. What happens to the current flowing through a photodiode when light is shining on it?
21-4 Pressure Sensors A pressure sensor is a transducer that outputs a voltage or current with a corresponding change in pressure. See Figure 21-9. Pressure sensors come in a wide range of pressure ranges. A voltage-output pressure sensor typically outputs 0 VDC to 10 VDC proportional to the rating of the pressure sensor. A current-output pressure sensor typically outputs 4 mA to 20 mA proportional to the rating of the pressure sensor. A pressure sensor can include a switching output (contact or solid-state) that is designed to activate or deactivate when the sensor reaches a predetermined value.
PRESSURE SENSORS Device
Symbol
Pressure Sensor
PRESSURESENSING ELEMENT
Figure 21-9. A pressure sensor is a transducer that changes resistance with a corresponding change in pressure.
Chapter 21 — Semiconductor Input Devices 445
A pressure sensor is used for high‑ or low‑pressure control, depending on the switching circuit design. Pres‑ sure sensors are suited for a wide variety of pressure measurements on compressors, pumps, and other similar equipment. A pressure sensor can detect low pressure or high pressure, or it can trigger a relief valve. Pressure sensors are also used to measure compression in various types of engines because they are extremely rugged.
TESTING PRESSURE SENSORS 2
CONNECT DMM LEADS TO PRESSURE SENSOR
3
ACTIVATE DEVICE BEING MONITORED; RECORD VOLTAGE/ CURRENT AT HIGH PRESSURE (LOWER VOLTAGE/ CURRENT)
4
OPEN RELIEF VALVE TO REDUCE PRESSURE; RECORD VOLTAGE/CURRENT AT LOW PRESSURE (HIGHER VOLTAGE/ CURRENT)
Testing Pressure Sensors Pressure sensors are tested by checking the voltage or cur‑ rent output and then comparing the value to manufacturer specification sheets. See Figure 21-10. To test a pressure sensor, the following procedure is applied: 1. Disconnect the pressure sensor from the circuit. 2. Connect the DMM leads to the pressure sensor. 3. Activate the device being monitored (compressor, air tank, etc.) until pressure builds up. Record the voltage or current (depending on output type) of the pressure sensor at the high‑pressure setting. 4. Open the relief or exhaust valve and reduce the pres‑ sure on the sensor. Record the voltage or current of the pressure sensor at the low‑pressure setting. 5. Compare the high and low voltage or current readings with manufacturer specification sheets. Use a replace‑ ment pressure sensor that is known to be good when manufacturer specification sheets are not available.
DISCONNECT PRESSURE SENSOR FROM CIRCUIT
1
Figure 21-10. Pressure sensors are tested by checking the voltage or current output and then comparing the value to the manufacturer specification sheets.
21-4 CHECKPOINT 1. If a pressure sensor that is rated to output 0 VDC to 10 VDC with a specified pressure operating range of 0 psi to 500 psi outputs 2 VDC, is it working properly according to manufacturer specifications?
2. If a pressure sensor that is rated to output 4 mA to 20 mA DC with a specified pressure operating range of 0 psi to 500 psi outputs 2.5 mA, is the pressure sensor working properly according to manufacturer specifications?
21-5 Flow Detection Sensors A flow detection sensor is a sensor that detects the move‑ ment (flow) of liquid or gas using a solid-state device. Because a flow detection sensor is a solid-state device, there are no moving mechanical parts that can become damaged due to corrosion or product deposits. A flow detection sensor operates on the principle of thermal conductivity. The flow detection sensor head, which is in contact with the medium (liquid or air) to be detected, is heated to a temperature that is a few degrees higher than the medium to be detected.
When the medium is flowing, the heat produced at the sensor head is conducted away from the sensor, cooling the sensor head. When the medium stops flowing, the heat produced by the sensor head is not conducted away from the sensor head. A thermistor in the sensor head converts the heat not conducted away into a stronger electrical signal than what is produced when the heat is conducted away. This electrical signal is used to operate the sensor’s output (contacts, transistor, etc.). See Figure 21-11.
446 ELECTRICAL MOTOR CONTROLS for Integrated Systems
FLOW DETECTION SENSORS
LIQUID FLOW DETECTION SENSORS
HEATING ELEMENT WITH THERMISTOR THAT CONVERTS TEMPERATURE DIFFERENCE INTO AN ELECTRICAL SIGNAL HEATED SENSOR HEAD
SENSOR HEAD HEAT CONDUCTED AWAY BY LIQUID OR GAS FLOW SENSOR HEAD HEAT NOT CONDUCTED AWAY FROM SENSOR HEAD
LIQUID OR GAS FLOW
FLOW DETECTION SENSOR
L1
STOP
NO LIQUID OR GAS FLOW
OL
1
M
2
PUMP MAIN STARTER
M
3
M
(2, 3)
ALARM
Figure 21-11. A solid-state flow detection sensor operates on the principle of thermal conductivity.
Because there is a small delay (usually less than 30 sec) before the sensor heats enough to signal no flow, the flow detection sensor acts like a motor starter overload, which allows for a short time delay to pass before signaling a problem. This short time delay helps prevent false alarms. The sensor also includes adjustments for setting the sensor to detect different product types and flow rates (speed past the sensor). Different color light-emitting diodes (LEDs) are usually included on the sensor for indicating different operating conditions. When used to monitor the flow of a liquid, a flow detection sensor is mounted within a pipe in which there should be flow during normal operation. The sensor can be mounted in a vertical or horizontal pipe and the product can flow in either direction. See Figure 21-12. In this circuit, the motor starter contacts in line 3 close each time the pump motor is operating. If there is product flow, an NC (held open during flow) switch is used to prevent the alarm from sounding. The alarm sounds if the flow stops and the motor starter is still ON. When used to monitor the flow of a gas (usually air), a flow detection sensor can be mounted within a duct in which there should be flow during normal operation. However, if the flow is critical for a safe work environment, several flow sensors can be used to ensure the flow is mov‑ ing in all parts of the exhaust system. See Figure 21-13.
L2
START
FLOW SENSOR (SWITCH) OPEN WHEN FLOW DETECTED
Figure 21-12. A flow detection sensor can be used to monitor product flow in a pipe.
GAS FLOW DETECTION SENSORS EXHAUST SYSTEM
L1
SYSTEM ON
FLOW DETECTION SENSOR
L2
OFF
1
CR
(2)
FLOW DETECTION SENSOR (SWITCH) CLOSED WHEN FLOW DETECTED 2
(3)
TR CR
3 TR
ALARM
ON-DELAY TIMER TO ALLOW STARTUP TIME
Figure 21-13. A flow detection sensor can be used to monitor airflow in painting or welding exhaust system applications.
Chapter 21 — Semiconductor Input Devices 447
In this circuit, an NO (held closed during flow) switch is used to operate a control relay. The control relay operates an on‑delay timer. The on‑delay timer sounds an alarm if flow stops for longer than the timer’s set time period. The time delay prevents the alarm from sounding during the first few seconds of system startup. To eliminate nuisance sounding of the alarm, the time delay can be set for 10 seconds, or longer for less critical applications.
21-5 CHECKPOINT 1. In a thermal-type flow detection sensor, does the sensor’s thermistor produce higher or lower electrical signal when there is no flow? 2. Is a thermal-type flow detection sensor a fastacting or slow-acting change detection type?
21-6 Hall Effect Sensors Sensor Packaging
Theory of Operation A Hall generator is a thin strip of semiconductor mate‑ rial through which a constant control current is passed. See Figure 21-14. When a magnet is brought near the Hall generator with its field directed at right angles to the face of the semiconductor, a small voltage (Hall voltage) appears at the contacts placed across the narrow dimension of the Hall generator. The voltage varies depending on how close the magnet is to the Hall generator, which acts as an analog signal. When the magnet is removed, the Hall voltage drops to zero. The Hall voltage is dependent on the pres‑ ence of the magnetic field and on the current flowing in the Hall generator. The output of the Hall generator is zero if either the current or the magnetic field is re‑ moved. In most Hall effect devices, the control current is held constant and the magnetic induction is changed by movement of a permanent magnet. Note: The Hall generator must be combined with associated electronics to form a Hall effect sensor.
Hall effect sensors are sometimes used in printers to detect open covers or missing paper.
p
edia Cli
To meet many different application requirements, Hall effect sensors are packaged in a number of different configurations. Typical configurations include cylin‑ der, proximity, vane, and plunger. See Figure 21-15. Cylinder and proximity Hall effect sensors are used to detect the presence of a magnet. Vane Hall effect sensors include a sensor on one side and a magnet on the other, and are used to detect an object passing through the opening. Plunger Hall effect sensors include a magnet that is moved by an external force acting against a lever.
M
A Hall effect sensor is a sensor that detects the proxim‑ ity of a magnetic field. The Hall effect principle was discovered in 1879 by Edward H. Hall at Johns Hopkins University. Hall found that when a magnet is placed in a position where its field is perpendicular to one face of a thin rectangle of gold through which current was flowing, a difference of potential appeared at the oppo‑ site edges. He found that this voltage is proportional to the current flowing through the conductor and that the magnetic induction is perpendicular to the conductor. Today, semiconductors are used for the sensing ele‑ ment (Hall generator) in Hall effect sensors. Hall volt‑ ages obtained with semiconductors are much higher than those obtained with gold and are also less expensive.
448 ELECTRICAL MOTOR CONTROLS for Integrated Systems
HALL GENERATORS PERMANENT MAGNET HALL GENERATOR
N
THIN STRIP OF SEMICONDUCTOR MATERIAL
S
HALL VOLTAGE
CONSTANT CURRENT SOURCE NO MAGNETISM
INCREASING MAGNETISM
MAXIMUM MAGNETISM (VOLTAGE CHANGE DUE TO INCREASED MAGNETISM)
Figure 21-14. A Hall generator is a thin strip of semiconductor material through which a constant control current is passed.
Hall Effect Sensor Actuation
HALL EFFECT SENSORS
Hall effect sensors may be activated by head-on, slideby, pendulum, rotary, vane, ferrous proximity shunt, and electromagnetic actuation. The actuation method depends on the application. Head-on Actuation. Head-on actuation is an active method of Hall effect sensor activation in which a magnet is oriented perpendicular to the surface of the sensor and is usually centered over the point of maximum sensitivity. See Figure 21-16. The direction of movement is directly toward and away from the Hall effect sensor. The actuator and Hall effect sensor are positioned so the south (S) pole of the magnet approaches the sensitive face of the sensor.
Honeywell
Figure 21-15. Hall effect sensors are available in a variety of packages for different applications.
Slide-by Actuation. Slide-by actuation is an active method of Hall effect sensor activation in which a mag‑ net is moved across the face of a sensor at a constant distance (gap). See Figure 21-17. The primary advan‑ tage of slide-by actuation over head-on actuation is that less actuator travel is needed to produce a signal large enough to cycle the device between operate and release.
Chapter 21 — Semiconductor Input Devices 449
PENDULUM ACTUATION
HEAD-ON ACTUATION DIRECTION OF MAGNET MOVEMENT
N MAGNET PERPENDICULAR TO SENSOR
START
N N
S
S
MAGNET DIRECTION OF TRAVEL
FINISH
0°
30
°
0°
S
S 20°
N GAP
°
30
S
GAP
HALL EFFECT SENSOR
HALL EFFECT SENSOR
SINGLE-POLE
MULTIPLE-POLE
Figure 21-18. Pendulum actuation is a combination of the head-on and slide-by actuation methods.
Figure 21-16. In head-on actuation, a magnet is oriented perpendicular to the surface of the sensor and is usually centered over the point of maximum sensitivity.
SLIDE-BY ACTUATION
DIRECTION OF MAGNET TRAVEL
N
Vane Actuation. Vane actuation is a passive method of Hall effect sensor activation in which an iron vane shunts or redirects the magnetic field in the air gap away from the sensor. See Figure 21-19. When the iron vane is moved through the air gap between the Hall effect sensor and the magnet, the sensor is turned on and off sequentially at any speed due to the shunting effect. The same effect is achieved with a rotary-operated vane.
MAGNET
Hall Effect Sensor Applications
S
GAP
HALL EFFECT SENSOR
Figure 21-17. In slide-by actuation, a magnet is moved across the face of a Hall effect sensor at a constant distance (gap).
Pendulum Actuation. Pendulum actuation is a method of Hall effect sensor activation that is a combination of the head-on and slide-by actuation methods. See Figure 21-18. The two methods of pendulum actuation are single-pole and multiple-pole. Single or multiple signals are generated by one actuator.
Hall effect sensors are used in a wide range of applica‑ tions requiring the detection of the presence (proximity) of objects. Hall effect sensors are used in slow-moving and fast-moving applications to detect movement. They are also used to replace mechanical limit switches in applications that require the detection of an object’s position. Hall effect sensors are also used to provide a solid-state output in applications that normally use a mechanical output, such as a standard pushbutton on a level switch. Conveyor Belts. A Hall effect sensor may be used for monitoring a remote conveyor operation. See Figure 21-20. In this application, a cylindrical Hall ef‑ fect sensor is mounted to the frame of the conveyor. A magnet mounted on the tail pulley revolves past the sensor to cause an intermittent visual or audible signal at a remote location to ensure that the conveyor is run‑ ning. Any shutdown of the conveyor interferes with the normal signal and alerts the operator. Maintenance is minimal because the sensor makes no physical contact and has no levers or linkages to break.
450 ELECTRICAL MOTOR CONTROLS for Integrated Systems
Sensing the speed of a shaft is one of the most com‑ mon applications of a Hall effect sensor. The magnetic field required to operate the sensor may be furnished by individual magnets mounted on the shaft or hub or by a ring magnet. Each change in polarity results in an output signal. See Figure 21-21.
VANE ACTUATION IRON VANE SHUNT WIDTH DIRECTION OF TRAVEL
SHAFT SPEED SENSING SHAFT SPACE WIDTH
MAGNET
HALL EFFECT SENSOR
LINEAR VANE SHUNTING IRON VANE DIRECTION OF VANE TRAVEL
S
HALL EFFECT SENSOR SHAFT
5
4
S 3
HALL EFFECT SENSOR
2
RING MAGNET MAGNET
1
GAP HALL EFFECT SENSOR
ROTARY VANE SHUNTING N
S
SHAFT
N
Figure 21-19. In vane actuation, an iron vane shunts or redirects the magnetic field in the air gap away from the Hall effect sensor.
HALL EFFECT SENSOR
S
N
S N N
S
REMOTE CONVEYOR MONITORING
RING MAGNET
Figure 21-21. Each change in polarity results in an output from a Hall effect sensor used in a shaft speed sensor application.
CONVEYOR BELT
Liquid Level Monitoring. Another application of a Hall
TAIL PULLEY MAGNET
HALL EFFECT SENSOR
Figure 21-20. A Hall effect sensor may be used for monitoring a remote conveyor operation.
effect sensor is as a low-liquid warning sensor. A lowliquid warning sensor measures and responds to the level of a liquid in a tank. One method used to determine the liquid level in a tank uses a notched tube with a cork floater inserted into the tank. The magnet is mounted in the cork floater assembly, which is forced to move in one plane (vertically). As the liquid level goes down, the magnet passes the digital output sensor Hall effect sensor. The liquid level is indicated when the sensor is actuated. See Figure 21-22.
Chapter 21 — Semiconductor Input Devices 451
SECURITY SYSTEMS
LIQUID LEVEL MONITORING
HALL EFFECT SENSOR
NOTCHED TUBE
CORK FLOATER ASSOCIATED ELECTRONIC CIRCUITRY
N
DIGITAL OUTPUT HALL EFFECT SENSOR
DOOR LATCH RELAY
MAGNET
MAGNETIC CARD SECURITY DOOR
LINEAR OUTPUT HALL EFFECT SENSOR
SN
FLOAT MAGNET
TANK
LIQUID LEVEL
Figure 21-22. A Hall effect sensor can be used to monitor the level of liquid in a tank.
A linear output Hall effect sensor may also be used to indicate the liquid level in a tank. As the liquid level falls, the magnet moves closer to the sensor, causing an increase in output voltage. This method allows measurement of liquid levels without any electrical connections in the interior of the tank. Common ap‑ plications include fuel tanks, transmission fluid reser‑ voirs, and stationary tanks used in food and chemical processing plants. Security Systems. A door-interlock security system
can be designed using a Hall effect sensor, a magnetic card, and associated electronic circuitry. See Figure 21-23. In this circuit, the magnetic card slides by the sensor and produces an analog output signal. This analog signal is converted to a digital signal to provide a crisp signal to energize the door latch relay. When the solenoid of the relay pulls in, the door is unlocked. For systems that require additional security measures, a series of magnets may be molded into the card.
Figure 21-23. A door-interlock system can be designed using a Hall effect sensor, a magnetic card, and associated electronic circuitry.
Beverage Guns. Hall effect sensors are used in bever‑ age gun applications because of their small size, sealed construction, and reliability. See Figure 21-24. The small size of the Hall effect sensor allows seven sensors to be installed in a hand-held device. Hall effect sensors cannot be contaminated by syrups, liquids, or foodstuffs because they are completely enclosed in the beverage gun. The beverage gun is also completely submersible in water for easy cleaning and only requires low maintenance.
BEVERAGE GUNS BEVERAGE GUN
PRINTED CIRCUIT BOARD
MAGNET MOLDED INTO BUTTON
HALL EFFECT SENSOR
Figure 21-24. Hall effect sensors are used in beverage gun applications because of their small size, sealed construction, and reliability.
452 ELECTRICAL MOTOR CONTROLS for Integrated Systems
Length Measurement. Length measurement can be accomplished by mounting a disk with two notches on the extension of a motor drive shaft. See Figure 21-25. In this circuit, a vane Hall effect sensor is mounted so that the disk passes through the gap. Each notch represents a fixed length of material and can be used to measure tape, fabric, wire, rope, thread, aluminum foil, plastic bags, etc.
LENGTH MEASUREMENT PRODUCT
HALL EFFECT VANE SENSOR
NOTCHED DISK
MOTOR DRIVE SHAFT
MOTOR
Figure 21-25. Length measurement can be accomplished by mounting a disk with two notches on the extension of a motor drive shaft.
Level/Degree of Tilt. Hall effect sensors may be installed in the base of a machine to indicate the level or degree of tilt. See Figure 21-26. Magnets are installed above the Hall effect sensors in a pendulum fashion. The machine is level as long as the magnet remains directly over the sensor. A change in state of output (when a magnet swings away from a sensor) is indication that the machine is not level. The sensor and magnet combination may also be installed in such a manner as to indicate the degree of tilt.
Tech Fact Hall effect sensors are used in many applications because of their small size, long operating life, and energy efficiency. They are available with an operating supply voltage of just a few volts and an operating current of a few microamperes, making them ideal for battery-powered devices. Hall effect sensors are used in portable computer keypads to operate each key and save battery life. Because of their reliability, Hall effect sensors are also used as automobile switches for the shift lever, power windows, seats, mirrors, and traction control systems.
Joysticks. Hall effect sensors may be used in a joystick application. See Figure 21-27. In this application, the Hall effect sensors inside the joystick housing are actuated by a magnet on the joystick. The proximity of the magnet to the sensor controls the activation of different outputs used to control cranes, operators, mo‑ tor control circuits, wheelchairs, etc. Use of an analog device also achieves degree of movement measure‑ ments such as speed.
JOYSTICKS JOYSTICK MAGNET
LEVEL/DEGREE OF TILT HALL EFFECT SENSORS MAGNET
Figure 21-27. Hall effect sensors may be used in a joystick application.
21-6 CHECKPOINT
HALL EFFECT SENSOR
Figure 21-26. Hall effect sensors may be installed in the base of a machine to indicate the level or degree of tilt.
1. Is the output of a Hall effect sensor of the digital (ON/OFF) type or analog (varying) type as a magnet moves closer to the sensor? 2. Can a magnet actuate a Hall effect sensor by moving in a sideways or straight manner?
Chapter 21 — Semiconductor Input Devices 453
21-7 PROXIMITY SENSORS A proximity sensor (proximity switch) is a solid-state sensor that detects the presence of an object by means of an electronic sensing field. A proximity sensor does not come into physical contact with the object. Proxim‑ ity sensors can detect the presence or absence of almost any solid or liquid. Proximity sensors are extremely versatile, safe, and reliable. Proximity sensors may be used in applications where limit switches and mechani‑ cal level switches cannot be used. Proximity sensors can detect very small objects, such as microchips, and very large objects, such as automo‑ bile bodies. All proximity sensors have encapsulated solid-state circuits that may be used in high-vibration areas, wet locations, and fast-switching applications. Proximity sensors are available in an assortment of sizes and shapes to meet as many application requirements as possible. See Figure 21-28.
field that varies in strength depending on whether or not a metallic target is present. The generated alternating field operates at a radio frequency (RF). See Figure 21-29.
INDUCTIVE PROXIMITY SENSORS EXTERNAL WIRES
TARGET
EXTERNAL CONTROL CIRCUIT
MAGNETIC LINES OF FORCE
OSCILLATOR CIRCUIT
INDUCTIVE PROXIMITY SENSOR OUTPUT SWITCH (THYRISTOR OR TRANSISTOR)
PROXIMITY SENSORS SWITCH NOT ACTIVATED
MAGNETIC FIELD PRODUCED BY OSCILLATOR CIRCUIT USED TO DETECT TARGET NONMETALLIC TARGET
INDUCTIVE PROXIMITY SENSOR SWITCH ACTIVATED
METALLIC TARGET
Banner Engineering Corp.
Figure 21-28. Proximity sensors are available in an assortment of sizes and shapes to meet as many application requirements as possible.
The two basic proximity sensors are the inductive proximity sensor and the capacitive proximity sensor. An inductive proximity sensor is a sensor that detects only conductive substances. Inductive proximity sensors detect only metallic targets. A capacitive proximity sensor is a sensor that detects either conductive or noncon‑ ductive substances. Capacitive proximity sensors detect solid, fluid, or granulated targets, whether conductive or nonconductive. The proximity sensor used depends on the type and material of the target.
Inductive Proximity Sensors Inductive proximity sensors operate on the eddy current killed oscillator (ECKO) principle. The ECKO principle states that an oscillator produces an alternating magnetic
INDUCTIVE PROXIMITY SENSOR
Figure 21-29. Inductive proximity sensors use a magnetic field to detect the presence of a metallic target.
When a metallic target is in front of an inductive proximity sensor, the RF field causes eddy currents to be set up on the surface of the target material. These eddy currents upset the AC inductance of the sensor oscillator circuit, causing the oscillations to be reduced. When the oscillations are reduced to a certain level, the sensor triggers, which indicates the presence of a metallic object. Inductive proximity sensors detect fer‑ rous materials (containing iron, nickel, or cobalt) more readily than nonferrous materials (all other metals, such as aluminum, brass, etc.).
454 ELECTRICAL MOTOR CONTROLS for Integrated Systems
Nominal sensing distances range from 0.5 mm to about 40 mm. Sensitivity varies depending on the size of the object and the type of metal. Iron may be sensed at about 40 mm. Aluminum may be sensed at approxi‑ mately 20 mm. Applications of inductive proximity sensors include positioning of tools and parts, metal detection, drill bit breakage detection, and solid-state replacement of mechanical limit switches.
Capacitive Proximity Sensors A capacitive proximity sensor measures a change in ca‑ pacitance that is caused by the approach of an object to the electrical field of a capacitor. A capacitive proximity sensor detects all materials that are good conductors in addition to insulators that have a relatively high dielectric constant. A dielectric is a nonconductor of direct electric current. Capacitive sensors can detect materials such as plastic, glass, water, moist wood, etc. See Figure 21-30.
Two small plates that form a capacitor are located directly behind the front of the sensor. When an object approaches the sensor, the dielectric constant of the ca‑ pacitor changes, thus changing the oscillator frequency, which activates the sensor output. Nominal sensing dis‑ tances range from 3 mm to about 15 mm. The maximum sensing distance depends on the physical and electrical characteristics (dielectric) of the object to be detected. The higher the dielectric constant, the easier it is for a capacitive sensor to detect the material. Generally, any material with a dielectric constant greater than 1.2 may be detected. See Figure 21-31.
DIELECTRIC CONSTANT Material Acetone Ammonia (liquid)
CAPACITIVE PROXIMITY SENSORS TARGET
EXTERNAL SENSOR
1.000985
Epoxy resin
3.3 to 3.7 3.7 to 10
Lime marble
2.2 to 2.5
Paper (dry)
8.5
Porcelain Powdered milk Rubber
CAPACITIVE FIELD
CAPACITIVE PROXIMITY SENSOR
SWITCH NOT ACTIVATED
Salt Sugar Turpentine
OUTPUT SWITCH (THYRISTOR OR TRANSISTOR)
Water
TARGET WITH A DIELECTRIC CONSTANT LESS THAN 1.2
2.0
Glass
Petroleum jelly CAPACITOR CIRCUIT
20.7 15 to 24
Carbon dioxide Gasoline
EXTERNAL CONTROL CIRCUIT
Number
2.0 2.2 to 2.9 6 to 8 3.0 3 to 15 3 2.2 80 to 88
Wood, dry
2 to 6
Wood, wet
10 to 30
Figure 21-31. Capacitive sensors work based on the dielectric constant of the material to be sensed.
CAPACITIVE PROXIMITY SENSOR SWITCH ACTIVATED
TARGET WITH A DIELECTRIC CONSTANT GREATER THAN 1.2
CAPACITIVE PROXIMITY SENSOR
Figure 21-30. Capacitive proximity sensors use a capacitive field to detect the presence of a target.
21-7 CHECKPOINT 1. What type of proximity switch detects metallic objects? 2. What type of proximity switch detects any object that has a high dielectric constant?
Chapter 21 — Semiconductor Input Devices 455
21-8 ULTRASONIC SENSORS An ultrasonic sensor is a solid-state sensor that can detect the presence of an object by emitting and receiv‑ ing high‑frequency sound waves. Ultrasonic sensors can provide an analog voltage output or a digital voltage out‑ put (switched output). The high‑frequency sound waves are typically in the 200 kHz range. Ultrasonic sensors are used to detect solid and liquid targets (objects) at a distance of up to approximately 1 m (3.3′). Ultrasonic sensors can be used to monitor the level in a tank, detect metallic and nonmetallic objects, and detect other objects that easily reflect sound waves. Soft materials such as foam, fabric, and rubber are difficult for ultrasonic sensors to detect and are better detected by photoelectric or proximity sensors. Ultrasonic sensors are used to detect clear objects (glass and plastic), which are difficult to detect with photoelectric sensors. For this reason, ultrasonic sen‑ sors are ideal for applications in the food and beverage industry, or for any application that uses clear glass or plastic containers.
Ultrasonic Sensor Operating Modes The two basic operating modes of ultrasonic sensors are the direct mode and the diffused mode. In the direct mode, an ultrasonic sensor operates like a direct-scan photoelectric sensor. In the diffused mode, an ultrasonic sensor operates like a scan diffuse photoelectric sensor. See Figure 21-32.
Direct mode is a method of ultrasonic sensor op‑ eration in which the emitter and receiver are placed opposite each other so that the sound waves from the emitter are received directly by the receiver. Ultra‑ sonic sensors used in the direct mode usually include an output that is activated when a target is detected. The output is normally a transistor (PNP or NPN) that can be used to switch a DC circuit. Outputs are available in NO and NC switching modes. Ultrasonic sensors include an adjustment for adjusting (tuning) the sensor sensing distance. Tuning the receiver to the emitter minimizes interference from ambient noise sources that may be present in the area. Diffused mode is a method of ultrasonic sen‑ sor operation in which the emitter and receiver are housed in the same enclosure. In the diffused mode, the emitter sends out a sound wave and the receiver listens for the sound wave echo bouncing back off an object. Ultrasonic sensors used in the diffused mode may include a digital output or an analog output. The analog output provides an output voltage that varies linearly with the target’s distance from the sensor. See Figure 21-33. The sensor typically includes a light-emitting diode (LED) that glows with intensity proportional to the strength of the echo. The analog output sensor includes an adjustable background sup‑ pression feature that allows the sensor to better detect only the intended target and not background objects.
ULTRASONIC SENSORS
HIGH FREQUENCY SOUND WAVES
ULTRASONIC SENSOR
EMITTER RECEIVER
DIRECT MODE EMITTER
E R
OBJECT TO BE DETECTED
RECEIVER EMITTED WAVES
ECHO WAVES
DIFFUSED MODE Figure 21-32. Ultrasonic sensors detect objects by bouncing high-frequency sound waves off the objects.
456 ELECTRICAL MOTOR CONTROLS for Integrated Systems
ULTRASONIC SENSOR ANALOG OUTPUT 10
OUTPUT VOLTAGE (in VDC)
9
BACKGROUND SUPPRESSION SETPOINT (ADJUSTABLE)
8 7 6 5 4 3
OUTPUT VOLTAGE VARIES LINEARLY WITH TARGET DISTANCE
2 1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
TARGET DISTANCE (in m)
Figure 21-33. An ultrasonic sensor used in the diffused mode can provide an analog output that varies linearly with the target’s distance from the sensor.
An outdoor air temperature (OAT) sensor is a semiconductor output device used in vehicle and aircraft applications.
21-8 CheCKPoiNT 1. What is the operating mode of an ultrasonic sensor called when the emitter sound waves travel in only one direction to the receiver?
2. What is the operating mode of an ultrasonic sensor called when the emitter sound waves travel in one direction to the detected object and bounces back in the opposite direction to the receiver?
Additional resources Review and Resources
Access Chapter 21 Review and Resources through the Electrical Motor Controls for Integrated Systems DVD or by scanning the above QR code with your mobile device.
Applying Your Knowledge
Refer to the Electrical Motor Controls for Integrated Systems DVD for interactive Applying Your Knowledge questions.
Workbook and Applications Manual
Refer to Chapter 21 in the Electrical Motor Controls for Integrated Systems Workbook and the Applications Manual for additional exercises.
Chapter 21 — Semiconductor Input Devices 457
ENERGY EFFICIENCY PRACTICES Energy-Efficient Lighting Controls After motors, lighting is the next highest consumer of energy in buildings, accounting for approximately 20% of the total energy consumed worldwide. Proper building lighting improves the productivity and safety of building occupants. Lighting-system control saves energy and can substantially reduce the operating costs of a building. For example, lighting in an area of a facility can be turned on automatically when an individual enters the area. After a set time period of not sensing an individual, the lights can be set to turn off automatically. Automated lighting control allows a system to operate without input and saves time and energy. The flexibility of automated lighting systems also accommodates changes to lighting requirements. For example, changes to schedules, tasks, or workplace configurations can be easily implemented while maintaining lighting control goals. An automated lighting control system may include ultrasonic sensors that activate when sensing changes in the reflected sound waves caused by an individual moving within its detection area. The sensor emits low-power, high-frequency sound waves and monitors for a phase shift in the reflected sound returning to the sensor, which indicates a moving object. The reliance on motion for sensor activation can also cause false deactivations when occupants are still for long periods of time.