LABORATORY WORK №14

INVESTIGATION OF POSITION SENSOR CHARACTERISTICS

14.1. Objective

Analyze characteristics and applications of different position sensor types.

 

14.2. 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.

 

 

 

14.3. Classification of position sensors

A sensor is a technical converter, which converts a physical variable (e.g. temperature, distance, pressure) into a different, more easily evaluated variable (usually an electrical signal). Additional terms for sensors are: encoders, effectors, converters, detectors, transducers. Sensors are devices which can operate both by means of contact, e.g. limit switches, force sensors, or without contact, e.g. light barriers, air barriers, infrared detectors, ultrasonic reflective sensors, magnetic sensors, etc. Even a simple limit switch can be interpreted as a sensor.

According to the output signal type sensors can be divided into 5 types.

Type A – sensors with switching signal output (binary signal output). Examples – proximity sensors, pressure sensors, filling level sensor, bimetal temperature sensor. These sensors can be connected directly to programmable logical controllers (PLC).

Type B – sensors with pulse rate output. Examples – incremental length and rotary angle sensors, quadrature encoders. Generally, PLC-compatible interfaces are available.

Type C – sensors with analogue output and without integrated amplifier and conversion electronics usually provide very small analogue output signal (e.g. in the millivolt range), which is to be evaluated only by using additional circuity. Examples – piezoresistive or piezoelectric sensor components, Pt-100 or thermoelectric cells, magnetoresistor and Hall sensor components, pH and conductivity measuring probes, potentiometers.

Type D – sensors with standardized analogue output and integrated amplifier or conversion electronics. Typical output signals are: 0 to 10 V; 1 to 5 V; –5 to +5 V; 0 to 20 mA; 4 to 20 mA; –10 to +10 mA.

Type E – sensors and sensor systems with standardized digital signal output interface, e.g. RS-232-C, RS-422-A, RS-485, CAN, Profibus, etc.

Sensors which detect whether or not an object is located at a certain position are known as proximity sensors. Sensors of this type provide a «Yes» or «No» statement depending on whether or not the position, to be defined, has been taken up by the object. Usually these sensors have binary output (Type A).With many production systems, mechanical position switches are used to acknowledge movements which have been executed. Additional terms used are micro-switches, limit switches or limit valves. Because movements are detected by means of contact sensing, relevant constructive requirements must be fulfilled. Also, these components are subject to wear. In contrast, proximity sensors operate electronically and without contact.

The advantages of contactless proximity sensors are:

·          Precise and automatic sensing of geometric positions.

·          Contactless sensing of objects and processes; no contact between sensor and workpiece is required with electronic proximity sensors.

·          Fast switching characteristics; as the output signals are generated electronically, the sensors are bounce-free and do not create error pulses.

·          Wear-resistant function; electronic sensors do not include moving parts which can wear out.

·          Unlimited number of switching cycles.

·          Suitable versions are also available for use in hazardous conditions (e.g. areas with explosion hazard).

Today, proximity sensors are used in many areas of industry for the reasons mentioned above. They are used for sequence control in technical installations and for monitoring and safeguarding processes. In this context sensors are used for early, quick and safe detection of faults in the production process. The prevention of damage to man and machine is another important factor to be considered. A reduction in downtime of machinery can also be achieved by means of sensors, because failure is quickly detected and signalled.

 

Inductive proximity sensors

Inductive proximity sensors detect the presence of electrically conductive (metal) objects that come within range of their oscillating field. Most of inductive proximity sensors are operated using Eddy Current Killed Oscillator (ECKO) principle. This type of sensor consists of few elements: coil, oscillator, demodulator, trigger circuit, and an output (Fig. 14.3.1). The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going. When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor eddy currents increase, increasing the load on the oscillator and further decreasing the amplitude of the field. Demodulator removes the oscillator’s carrier and leaves only amplitude value. Schmitt trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition. As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition.

 

 

Fig. 14.3.1. Magnetic fields in inductive proximity sensors

The ferrite core concentrates the radiated field in the direction of use. A shielded proximity sensor has a metal ring placed around the core to restrict the lateral radiation of the field. Shielded proximity sensors can be flush mounted in metal. A metal-free space is recommended above and around the sensor’s sensing surface. If there is a metal surface opposite to the proximity sensor it must be at least three times the rated sensing distance of the sensor from the sensing surface.

An unshielded proximity sensor does not have a metal ring around the core to restrict lateral radiation of the field. Unshielded sensors cannot be flush mounted in metal. There must be an area around the sensing surface that is metal free. An area of at least three times the diameter of the sensing surface must be cleared around the sensing surface of the sensor. In addition, the sensor must be mounted so that metal surface of the mounting area is at least two times the sensing distance from the sensing face. If there is a metal surface opposite to the proximity sensor it must be at least three times the rated sensing distance of the sensor from the sensing surface.

For sensing distance normalization a standard target is defined. A standard target is defined as having a flat, smooth surface, made of mild steel that was 1 mm thick. Steel is available in various grades. Mild steel is composed of a higher content of iron and carbon. The standard target used with shielded sensors has sides equal to the diameter of the sensing face. The standard target used with unshielded sensors has sides equal to the diameter of the sensing face or three times the rated operating range, whichever is greater. If the target is larger than the standard target, the sensing range does not change. However, if the target is smaller or irregular shaped the sensing distance (S_n) decreases. The smaller the area of the target the closer it must be to the sensing face to be detected.

Target Size Correction Factor. A correction factor can be applied when targets are smaller than the standard target. To determine the sensing distance for a target that is smaller than the standard target (S_new), multiply the rated sensing distance (S_rated) by the correction factor (T):

 

                                                (14.1)

 

Table 14.1

Target size correction values

 

Thickness of the target is another factor that should be considered. The sensing distance is constant for the standard target. However, for nonferrous targets such as brass, aluminium, and copper a phenomenon known as «skin effect» occurs. Sensing distance decreases as the target thickness increases. If the target is other than the standard target a correction factor must be applied for the thickness of the target.

The target material also has an effect on the sensing distance. When the material is other than mild steel correction factors need to be applied. The reduction factors for FESTO SIEH M12 sensor (used in laboratory equipment) are presented in Table 14.2.

 

Table 14.2

Target material reduction factors for FESTO SIEH M12 inductive

proximity sensor

 

Rated operating Distances. The rated sensing distance (S_n) is a theoretical value which does not take into account such things as manufacturing tolerances, operating temperature, and supply voltage. In some applications the sensor may recognize a target that is outside of the rated sensing distance. In other applications the target may not be recognized until it is closer than the rated sensing distance. Several other terms must be considered when evaluating an application. The effective operating distance (S_r) is measured at nominal supply voltage at an ambient temperature of 23 °C ± 0.5 °. It takes into account manufacturing tolerances. The effective operating distance is ±10 % of the rated operating distance. This means the target will be sensed between 0 and 90 % of the rated sensing distance. Depending on the device, however, the effective sensing distance can be as far out as 110 % of the rated sensing distance. The useful switching distance (S_u) is the switching distance measured under specified temperature and voltage conditions. The useful switching distance is ±10 % of the effective operating distance.

The assured (guaranteed) operating distance (S_a) is any switching distance for which an operation of the proximity switch within specific permissible operating conditions is guaranteed. The assured operating distance is between 0 and 81 % of the rated operating distance.

The main parameters for FESTO SIEH M12 inductive proximity sensor (used in laboratory equipment) are presented in Table 14.3.

 

Table 14.3

The main parameters for the FESTO SIEH M12 inductive

proximity sensor

 

14.4. 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. 6.10 and adjust the zero position.

 

 

Fig. 14.4.1. Sensor and a measurement scale installation

 

2.       Connect the sensor to a power supply and a load according to the scheme at Fig. 14.4.2, using relay and indication units.

 

 

Fig. 14.4.2. 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 14.4.

 

Table 14.4.

Sensor testing results

Material
Iron
Aluminium
Copper
Plastic

 

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

Fig. 14.4.3. Ultrasonic sensor connection diagram

 

5.   Perform the ultrasonic sensor test: connect it according to the schematic at Fig. 14.4.3, 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. 14.4.4, 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 14.5. 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. 14.4.4. Motor shaft rotation speed measurement equipment: a – counter

unit; b – motor unit with installed optical sensor

 

Table 14.5

Rotation speed measurement results

Control voltage, V	Counter indication,	Rotation speed, rpm

 

 

14.5. Content of Report

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

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

3.       Conclusions.

 

 

14.6. 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.