LABORATORY WORK №15

CAPACITIVE PROXIMITY SENSORS. OPTICAL PROXIMITY SENSORS

15.1. Objective

Analyze characteristics and applications of different position sensor types.

 

15.2. Task

Use experimental investigation to obtain sensibility characteristics of different position sensor types:

·          inductive proximity sensor;

·          capacitive proximity sensor;

·          optical proximity sensor;

·          ultrasonic proximity sensor.

Investigate the optical proximity sensor usage for rotation speed measurement.

 

15.3. Capacitive Proximity Sensors

Capacitive proximity switches detect conductive and non-conductive materials that can be in a solid or liquid state. They serve the purpose of monitoring product levels in containers, checking contents in filling and packaging systems as well as detecting, positioning, monitoring and counting objects, e.g. in sequence control systems, conveyor belts. Used for detecting media such as:

·          solid: wood, ceramic, glass, paper stacks, plastic, stone, rubber, ice, nonferrous metals, potatoes.

·          liquid: water, oil, beverages, adhesives, paints.

·          granular: plastic pellets, granulated products, grain, fodder, wood chip.

·          powder: dyes, detergents, sand, cement, fertilizer, salt, sugar, flour, coffee.

The function of the capacitive proximity switch is based on evaluating the influence exerted by an actuator on the electrical field at the active face of the switch. The approach of an influencing object increases the capacitance of the capacitor, which consists of a sensor electrode located behind the active face and the actuator connected to earth / mass. This increase in capacitance is dependent on the conductance and the dielectric constant of the actuator as well as its mass, surface area and its distance from the sensor electrode. The capacitive limit switch is equipped with an RC oscillator with a gain factor that increases as a result of the rise in capacitance of the previously described capacitor to such an extent that oscillation is induced. In limit switches, the capacitance required to induce oscillation can be determined by the built-in potentiometer intervening in the feedback of the oscillator. The block diagram of a capacitive proximity sensor is presented in Fig. 15.3.1.

The response sensitivity, i.e. the sensing distance with a given actuator can be adjusted in this way. The oscillator output signal is fed to an evaluation circuit that actuates the switching amplifier. In response to the approach of conductive material the actuating object and the active face of the sensor form the plates of a capacitor.

The change in capacitance and the consequently achievable sensing distance are large. In response to the approach of nonconductive material ε > 1 only the change in the dielectric constant is effective. The increase in capacitance is less than is the case for conductive materials. The resulting sensing distance is small. The maxi-mum obtainable switching distance of industrial capacitive sensors is approximately 60 mm. Reduction factors for some materials are presented in Table 15.1, the main parameters for the BERNSTEIN KCN–T12PS/004 M12 capacitive proximity sensor (used in laboratory equipment) – in Table 15.1.

 

 

Fig. 15.3.1. Block diagram of a capacitive proximity sensor

 

Table 15.1

Target material reduction factor of capacitive proximity sensor

 

Sensitivity of capacitive proximity sensor with regard to humidity is very high due to the high dielectric constant of water. They are very sensitive to the dirt also. On the other hand, they can be used for detection of objects through a non-metallic wall. The wall thickness in this case should be less than 4 mm and the dielectric constant of the material to be detected should be higher by a factor of 4 than that of the wall. Capacitive proximity sensors can be used for monitoring the winding of electrical wires and cables, because they react to copper containing electrical wires or cables of relatively small diameter, whereas inductive proximity sensors react at a smaller switching distance or not at all.

 

Optical Proximity Sensors

Optical proximity sensors use light beam for detection of objects. Red or infrared light is used for this purpose. Semiconductor light emitting diodes (LEDs) or lasers are a particularly reliable source of red and infrared light. They are small and robust, have a long service life and can be easily modulated. Photodiodes or pho-totransistors are used as receiver elements. When adjusting optical proximity sensors, red light has the advantage that it is visible in contrast to infrared light. Besides, polymer optic cables can easily be used in red wavelength range because of their reduced light attenuation. Infrared (non visible) light is used in instances, where increased light performance is required in order to span greater distances for example. Furthermore, infrared light is less susceptible to interference (ambient light). With both types of optical proximity sensor, additional suppression of external light influences is achieved by means of modulating the optical signal. The receiver (with the exception of through-beam sensors) is tuned to the pulse of the emitter. With through-beam sensors an electrical band-pass is used in the receiver. Particularly in the case of infrared light, the use of daylight filters further improves insensitivity to ambient light.

Usually the 660 nm wavelength emitters are used for visible red beam sensors with fibreoptic connection and 880 nm – for infrared sensors without fibreoptic.

Depending to the light beam path optical proximity sensors can be divided into barriers (through-beam and retro-reflective) and diffuse sensors (Fig. 15.3.2).

 

 

Fig. 15.3.2. Optical proximity sensors: a – through-beam; b – retro-reflective;

c – diffuse; where E – emitter; R – receiver; O – object

 

 

Through-beam sensors consist of separately assembled emitter and receiver components whereby wide sensing ranges can be achieved. For the interruption of the light beam to be evaluated, the cross-section of the active beam must be covered. The object should permit only minimum penetration of light, but may reflect any amount of light. These sensors have enhanced reliability because of permanent light during non-operation, wide range, can detect small objects at large distances, are suitable for aggressive environment, good positioning accuracy, but two separate proximity sensor modules (emitter and receiver) and separate electrical connections are required, failure of the emitter is evaluated as «object present» (important with accident prevention applications).

Retro-reflective sensors have light emitter and light receiver installed in one single housing. An additional reflector is required. Interruption of the light beam is evaluated. Interruption of the light beam must not be compensated by direct or diffuse reflection of an object. Transparent, bright or shiny objects may in some cases remain undetected. Mirroring objects must be positioned in such a manner that the reflecting beam does not impinge on the receiver. Failure of the emitter is evaluated as «object present» too.

Diffuse sensors have the emitter and receiver fitted in the same housing too. The object diffusely reflects a percentage of the emitted light thereby activating the receiver. The switching distance largely depends on the reflectivity of the object. The size, surface, shape, density and color of the object as well as the angle of impact determine the intensity of the diffused light so that as a rule only small distances within a range of a few decimeters can be scanned. The background must absorb or deflect the light emission, i.e. when an object is not present, the reflected light beam must be clearly below the response threshold of the receiving circuit. The right sensivity adjustment is very important for these sensors. Failure of the emitter is evaluated as «no object present». Transparent and non-reflective (black) object recognition is complicated. Different material sensivity correction factors are presented in Table 15.2.

Usually the emitter and receiver pulse sinchronization is implemented for better influence cancelling. Typical optical sensor block diagram is presented in Fig. 15.3.3.

 

Table 15.2

Target material correction factors for diffuse optical proximity sensor

 

 

 

Fig. 15.3.3. Block diagram of optical proximity sensors

 

The main parameters for FESTO SOE – L series optical proximity sensor with fiber optic (used in laboratory equipment) are presented in Table 15.2. Sensors without fiber optic have very similar characteristics, too.

 

15.4. Method of testing

1.       Install the capacitive proximity sensor (looks like blue plastic screw with wiring) on a testbench to the measuring scale as shown in Fig. 14.4.1 and adjust the zero position.

2.       Connect the sensor to a power supply and a load according to the scheme at Fig. 15.4.1, using relay and indication units.

 

 

Fig. 15.4.1. Sensor connection diagram

 

3.       Perform the sensor switching distance measurements: for each of four provided material plates (check the material!) find distances x₁ – switch-off point moving from sensor; x₂ – switch-on point moving to the sensor;  – hysteresis. Write the obtained results write in Table 15.3.

 

Table 15.3

Sensor testing results

Material
Iron
Aluminium
Copper
Plastic

 

4.       Perform steps 1 to 3 for inductive and optical sensor.

 

Fig. 15.4.2. Ultrasonic sensor connection diagram

 

5.       Perform the ultrasonic sensor test: connect it according to the schematic at Fig. 15.4.2, and measure the output voltage moving the obstacle (perpendicular book) in front of the sensor every 5 cm till 1 m. Make a reliance graph.

6.       Perform a motor shaft rotation speed measurement using an optical retro-reflective proximity sensor. Take a look at Fig. 15.4.3, and make connections: connect power to the counter, motor and sensor; connect voltmeter to the motor unit control output; connect sensor output to «CLOCK» input of counter unit. Put the counter unit switch to the «imp. / s» position. Take a counter marks every 1V motor control voltage increase and put them into Table 15.4. Calculate the rotation speed for each value using formula:

 

[rpm]

 

where: N – counter indication; n – pulse number per rotation (count the holes on the disc). Make rotation speed ω dependence to the control voltage chart.

 

 

Fig. 15.4.3. Motor shaft rotation speed measurement equipment:

a – counter unit; b – motor unit with installed optical sensor

Table 15.4

Rotation speed measurement results

Control voltage, V	Counter indication,	Rotation speed, rpm

 

15.5. Content of Report

1.       Objective, task and model of the laboratory work.

2.       Work results (connection diagrams, tables and charts).

3.       Conclusions.

 

15.6. Control questions

1.       What does «proximity sensor» mean?

2.       Explain operation principle of inductive proximity sensor.

3.       What are the main characteristics of inductive proximity sensor?

4.       Explain operation principle of capacitive proximity sensor.

5.       What are the main characteristics of capacitive proximity sensor?

6.       Explain operation principles of optical proximity sensor.

7.       What are the main characteristics of optical proximity sensor?

8.       Explain operation principles of ultrasonic proximity sensor.

9.       What are the main characteristics of ultrasonic proximity sensor?

10.  Find three application examples for each mentioned sensor type.