Survey on the techniques of speed measurement by Muhammad Azri bin Musman(CD150009) Muhammad Fadhilrulrahman bin Abd Razak (CD150175) Aiman Nur Hakim bin Ahmad Rizal ( AD150186) Farah Nur Athirah bt Janudin (AD150204) Nurul Amalia Nabila bt Abdullah (CD150190) A report submitted in partial fulfilment of the requirements for the course of Control Engineering (BDA30703) Faculty of Mechanical and Manufacturing Engineering Universiti Tun Hussein Onn Malaysia Semester 1, 2017/18 Assessment for the assignment (Rank: 1(very poor) to 5(excellence)) Sections Weight Rank Marks = weight x rank Remarks Introduction 5 Techniques 5 Discussion 5 Conclusion 3 Reference 2 Total 20 /100
1.0 INTRODUCTION A physical quantity such as time, energy, mass, speed, volume, area, length and others should be measureable. All these physical quantity is measured using its own measurement device. Every measurement device has its own working principle. The measurement process requires invariably a detector that responds to be measured quantity by producing a measurable change in some property of the detector. In this paper, we focused our research on the speed measurement devices. There are number of speed measurement devices and working principles. Some of the speed measurement devices are tachometer, airspeed indicator, speedometer and tachymeter. This device is important in engineering for some reasons. Firstly, the right measurement is used to generate data for design. Any designs that created by engineers should be compatible with the right measurement. Or else, the designs would be invalid and will not be a success. Secondly, measurement is important because it is used to generate data to validate or propose a theory. A theory that is proposed needs to be validated using the right measurement. If the measurement of the physical quantity cannot be done, the theory cannot be approved. Hence, this paper is done to differentiate the working principle of the speed measurement devices and also to describe the techniques to use the devices in the real world applications. 1
2.0 DIFFERENT TECHNIQUES USED IN SPEED MEASUREMENT a) Tachometer Figure 1 : Measuring Speed by Using Tachometer Figure 2 : Car Speed by Using Tachometer Figure 1 show a man used tachometer for measuring the rotation speed of fan. It can measuring instrument for use in maintenance, and production. In Figure 2, a tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. Tachometer is an instrument used for measuring the rotation or revolution speed of objects, such as an engine or a shaft. In machinery and devices, the speed of rotting shafts or motors needs to be measured. For example, car drivers must know the exact rotation speed of the shaft in order to select the suitable gear and throttle settings for particular driving conditions. If using a car at unsuitable gear or prolonged use at high speeds, there will be an insufficient lubrication, which impacts the engine. The working principle of an electronic tachometer is quite simple. The ignition system triggers a voltage pulse at the output of the tachometer electromechanical part whenever the sparks plug fires. The electromechanical part responds to the average voltage of the series of pulses. It shows that the average voltage of the pulse train is proportional to engine speed. The signal from the perception head is transmitted by standard twin screened cable to the indicator. The tachometers are temperature compensated to be able to handle operations over an ambient temperature range of 20 to +70 C (-4 to +158 F). 2
The tachometer measures revolutions per minute (RPMs) of engines and is widely used in automobiles, airplanes, marine engineering field, laser instruments, to estimate traffic speed and volume, and medical applications. b) Optical type Photoelectric (optical type) sensors usually sense a reflective target on the rotating shaft. Here rotation is transmitted to the measuring instrument via an infrared (IR) light beam/laser beam coming from the instrument, which is then reflected by a reflective tape on the object. As explained, traditional tachometers require physical contact between the instruments and the objects being monitored. Laser tachometer is a powerful choice where this type of direct-contact type measurement is not workable for technical or safety reasons. Laser tachometers work by pulsing a laser beam against the rotating element. The rotating element will have one reflective spot, and the tachometer measures the rate at which the light beam is reflected back. Figure 3: Block diagrams of the KMI 15/x and KMI 16/x rotational speed sensors with integrated signal conditioning circuit, from Philips Semiconductors 3
In Figure 3 show the KMI 16/x sensor modules provide an open collector output, while KMI 15/x sensor modules have a current interface that requires only two wires to connect them in the application. Photoelectric sensors are readily present in everyday life. It help safely control the opening and closing of garage doors, turn on sink faucets with the wave of a hand, control elevators, open the doors at the grocery store, detect the winning car at racing events, and so much more c) Magnetic type Magnetic rotational speed sensors use various magnetic proximity measuring principles to monitor the speeds of machine components in a range between 0 and 30,000 rpm. Rotational speed measurement using a magneto resistive (MR) sensor is achieved by counting ferromagnetic marks, such as teeth of a passive gear wheel or the number of magnetic elements of a magnetize ring. Magneto resistive sensors make use of the magneto resistive effect the property of a current-carrying magnetic material to change its resistivity in the presence of an external magnetic field. Although inductive sensors can be used for this task, magneto resistive sensors have an advantage that the output signal level does not vary with rotation speed, as in the case of inductive sensors. MR sensors are static and the output signal is generated by the bending of magnetic field lines according to the position of the target wheel. As bending of the magnetic field lines also occurs when the target is not moving, these sensors can measure very slow rotations, even down to zero hertz (0 Hz). However, note that MR sensors cannot directly measure rotational speed, but are sensitive to the motion of a toothed wheel made from ferrous material (passive target, a sensor fitted with a permanent magnet) or a rotating wheel having alternating magnetic poles (active target) 4
Figure 4: AM4096 magnetic encoder chip from Renishaw In Figure 4, it show magnetic encoder chip provide linear and rotary magnetic encoding and signal conditioning. Differential sensing reduces displacements and magnetic field errors. All of the sensor and processing electronics have been placed within the compact silicon design. The rotation of a simple north/south magnet is picked up by the AM4096 s sensor and provides absolute positional information output to an accuracy of better than 0.1 degree. Rotational speed measurement is an important operation for applications in industry, automation, industrial trucks, mobile working machines, large diesel engines, hydro motors, electric motors, gears, rail vehicles, dumper trucks, linear and angular encoders, position measurement systems, robotics motion and vehicle control. Especially in electrical drive systems, rotational speed sensors are essential for a proper functionality of the whole machine. 5
3.0 DISCUSSION a) Tachometer The advantages of the technique with respect to performances and costs are the meter is the direction of rotation is indicated by the polarity of the output voltage. It is because in the operation of the tachometer is the polarity of output voltage indicates the direction of rotation. The output voltage is measured with the help of moving coil voltmeter calibrated in terms of speed. A linear relationship can be derived between output voltage and speed by carrier frequency excitation. In respects to costs, it is rugged and inexpensive. Moreover, it need less maintenance and ripple free output. The disadvantages of the technique with respect to performances are the calibration is difficult as the input voltage should be maintained absolutely constant. Besides, at high speed there is a non-linear relationship between output voltage and input speed. Hence we need the reference winding with higher frequency. b) Optical Type The advantages of the technique with respect to performances and costs are the variable reluctance sensors are passive sensors as they does not require external power supply. In respect to costs, the wire and magnets used are pretty inexpensive, so the optical type does not have high cost. Besides they are light weight, robust and can withstand in high temperature and high vibration environment. The disadvantages of the technique with respect to performances are the target materials must be ferrous only. Besides, when the magnitude of the induced voltage in these sensors is proportional to target speed, it becomes difficult to design circuitry appropriate for very low speed signals. Thus Variable Reluctance sensors have a definite lower limit on the speed of the target. Moreover, even the sensors are cheap, the cost advantage is offset by the additional electronic circuitry required to process the low amplitude induced voltages. 6
c) Magnetic Type The advantages of the technique in particular in respect to performances and costs are it is quick and relatively uncomplicated and gives immediate indications of defects. Other than that, It shows surface and near surface defects, and these are the most serious ones as they concentrate stresses. The method can be adapted for site or workshop use but it is inexpensive compared to radiography. While, large or small objects can be examined and elaborate pre- cleaning is not necessary. The disadvantages for this type are restricted to ferromagnetic materials - usually iron and steel, and cannot be used on austenitic stainless steel. It is messy and most methods need a supply of electricity. Sometimes unclear whether the magnetic field is sufficiently strong to give good indications and the method cannot be used if a thick paint coating is present. Next, spurious, or non-relevant indications are probable, and thus interpretation is a skilled task. Some of the paints and particle suspension fluids can give a fume or fire problem, particularly in a confined space. 7
4.0 CONCLUSION Basically, we know that there are many types of speed measurement such as Tachometer, Optical Type and Magnetic Type. For Tachometer, it requires a permanently mounted speed sensors that observe a target on the machines rotating shaft. As we know that, a tachometer is a sensor device for measuring the rotation speed of an object such as the engine shaft in a car. This device indicates the revolutions per minute (RPM) performed by the object. The device comprises of a dial, a needle to indicate the current reading, and markings to indicate safe and dangerous levels. Applications of this device rarely in automobiles, airplanes, trucks, tractors, trains and light rail vehicles, numerous types of machinery and prime movers and to estimate traffic speed and volume. Several types of sensors are available including Proximity such as Optical Type and Magnetic type. Permanently mounted systems usually use a Proximity or Hall Effect type sensor. Laser and Optical Sensors are more suited for portable equipment and special applications. Optical Sensors work well with Reflective Tape as a target on the shaft but are not recommended for permanent installation as the sensor may become dirty over time or the tape may come loose. 8
5.0 REFERENCES S.P. Venkateshan.(2008). Mechanical Measurement. Ane Books India : Taylor& Francis Group. Shen, C. (2011). Tachometer/Speed Switch Building Block Applications. Hong Kong : Shimmer Books. 43(1), 1-55. Lui, C.& Qui J.(2007). Rotational Speed Direction Based on the Output Duty Cycle of a Differential Hall Effect Gear Tooth Sensor. The International Journal, 10(5), 780-785. E. (2017, March 21). Speed Sensor. Retrieved September 26, 2017, from https://www.engineersgarage.com/articles/speed-sensor-types Md Abu Jauad Khan Aliv, Researcher and Freelance IT Consultant Follow. (2014, February 10). Tachometer. Retrieved September 26, 2017, from https://www.slideshare.net/abujauad/tachometer-31064948 9