Calibration of Tachometers C.S. Veldman Acoustics and Vibration Electrical Metrology National Metrology Laboratory CSIR, PO Box 395, Pretoria, 0001 Tel: (012) 841-4008 Fax: (012) 841-4458 e-mail: email@example.com WWW: http://nml.csir.co.za/ Abstract: This paper is aimed at providing a that is turning, or more accurately the number of look at tachometers, the operating principles, turns in a known period. the use of tachometers and the calibration of tachometers. For the purpose of this paper we assume that we are only interested in the number of complete I. INTRODUCTION. turns or revolutions. This paper is aimed at demonstrating some So a tachometer is a device that ascertains how calibration techniques for the calibration of many times an object has turned in a certain tachometers. Unlike the inventor of the period of time. Our job as metrologists is to tachometer who had to solve the problem of how provide the standard so to speak against which to measure the speed at which an object is such devices can be measured. spinning . We as metrologists don’t have to re- invent the tachometer, we have to calibrate it. If we were to look at the problem mathematically the following equation can be used to calculate Tachometers have been in use for many years, rotational speed: one can almost say from as early as the invention of the wheel. A tachometer’s sole ' R (1) purpose is to measure the speed at which an t object is rotating. Simply put, to measure an object’s rotational speed. Where v = Rotational speed in RPM So how does a tachometer work and how was it R = Revolutions invented? The answer is I don’t know. But as with t = period in minutes any measurement and especially the first time one wants to measure an unknown and lets From equation one it is evident that in order to presume a newly defined quantity, in this case obtain the result v we need to be able to the rotational speed, it poses some problems. If measure two components, R and t. you were to ask any inventor he or she will tell you that one cannot invent something or find a Each component on its own is by today’s solution if there is no problem! So the first step to standards easy to measure. For R you need a finding a solution is to define the problem. counter and for t you need to measure time. II. WHAT DOES A TACHOMETER III. HOW TO CALIBRATE TACHOMETERS MEASURE? Now that we know what the inventor of the To answer the question of what does a tachometer knew when he developed the first tachometer measure we need to define some tachometer, by defining exactly what it was he properties: wanted to measure, one can start to solve the problem of how to calibrate a tachometer. Measure: ‘ascertainment of extent by comparison with a standard. If we were to calibrate the first type of tachometer we could most probably go right Rotational Speed: Speed is defined as the ahead and start calibrating. But with technology, distance travelled in a came all kinds of tachometers using different known period. kinds of technologies to sense the rate at which an object is turning. The device that is used to Therefore, rotational speed can be defined as the sense, also called the pick-up of the tachometer distance travelled in a known period by an object provides us with some means on grouping tachometers. explain the different options it would be better to consider different types of tachometers on their Before taking a look at calibrating tachometers own. let’s consider some types of tachometers: 1) Contact Tachometers A. Types of Tachometers These types of tachometers are surely the most Apart from the pick-up type, tachometers are difficult to calibrate and with the largest grouped in two categories, contact and non- uncertainty involved. contact tachometers. The difficult part lies in that one needs to provide Certainly the most well known type of tachometer a spinning object to mechanically connect the is the mechanical pick-up; a contact tachometer. UUT to. This does not have to be a disk but can An example of such a tachometer is a rev- be a rotating shaft. This set up normally takes counter found in motor cars. Although contact the form of an electrical motor with a variable tachometers are still widely used, mechanical speed control. tachometers are slowly dying out. They are being replaced by electronic tachometers with a Let us consider only the tachometer generator for mechanical pick-up. the moment. With an excellent mechanical design one can obtain a nearly frictionless Non-contact tachometers use a variety of pick-up spinning disk (we use a disk as it extends the use methods, normally limited to one pick-up method of the generator to calibrate other types of per tachometer. These pick-up methods include tachometers as well). By using a “heavy” disk we sound, light (visible and infra red), magnetic, add some inertia and the disk acts as a flywheel. induction and electro-magnetic pick-ups. Basically anything that can provide a signal or Now let us consider the system with the UUT indication that the object has completed a connected to the generator. Do we use a fixed revolution can be used as a pick-up. Some mechanical coupling, eliminating any possibility tachometers even have more than one type of of slippage or do we use the rubber coupling pick-up or can be adapted to be used with supplied with some of the tachometers. The fixed different pick-ups. coupling would certainly be preferred by the metrologist as it would provide a smaller uncertainty. However the calibration should B. Calibration of Tachometers closely simulate the conditions used by the client and hence the same coupling method would be The calibration of tachometers follows the same preferred. principle as most calibration work in that the unit under test is compared to a known value. If we What happens when the UUT is connected to the refer back to equation one, our definition of generator? At first the generator slows down as RPM, we can see that the standard needed for the UUT acts as a load on the generator. The the calibration of tachometers is 1/t, which is speed control has to compensate for this load. equal to frequency. So in effect we need to The metrologist must thus ensure that the UUT calibrate a frequency counter that counts pulses does not overload the system. This might not over a sixty second period and not over a one happen with new generation tachometers, but second period. some older mechanical designs are really difficult to turn and will definitely load any By considering the different types of tachometers system. or rather the different pick-up methods used by tachometers, it is clear that the calibration This points out a consideration in the design of method will be determined by the type of pick-up the generator. How to obtain traceability for the utilised by the tachometer, as all tachometers generator and at the same time detect and use the same measuring principle. correct for any loading or speed varying effects. By measuring the RPM of the disk directly. This The most simple consideration for the calibration can be done using an optical encoder attached to of a tachometer would be to generate a rotating the shaft, or by using a sensor attached to the signal preferably more accurate than can be disk. Using a frequency counter to count the measured by the tachometer. This is a principle revolutions provides one with an accurate used in nearly all fields of metrology. But what traceable readout of the speed at which the disk immediately springs to mind is how do you then is spinning . calibrate the “tachometer generator”? In order to If a rubber coupling or similar coupling method is rotational speed of the disk by the number of used, care must be taken on the contact angle slots. and the pressure applied to minimise any slippage and not to place any on load the This has the advantage that the generator is generator. utilised more effectively. The UUT is then also calibrated in the same fashion as it is to be used. What about the ranges one can perform this But is this really necessary? calibration? It is not too difficult to have a generator with a direct drive operating at 1 RPM A Pulsating Light Source: stability over the range 80 RPM to 3 000 RPM. Below 80 RPM, close to 1 Hz, the feedback Photo tachometers can also be calibrated using control becomes critical and frictional forces a light source that is emitting a pulsed light. The starts to influence how smoothly the generator is light source should ideally have the same turning. Above 3 000 RPM the disk starts to spin wavelength as the light source of the tachometer. at what seems dangerously high speeds, as one A lot of people use a strobe-scope light for its listens to the whining noise and I normally don’t sharp turn on time and high intensity. Both of feel comfortable performing calibrations at 5 000 these qualities can have a negative effect on the RPM. calibration. The intensity of the strobe-scope light is normally so high that one might overlook a 2) Non-Contact Tachometers poor performing device, thinking that it is operating properly. In effect when the unit is These tachometers are easy to use and normally used using its own light source it might not require little or no installation. They can be used function at all. to measure rotational speed at a distance away from the object making them safe to use. The sharp turn on and turn of time of the strobe- scope is also not a good representation of the 3) Photo Tachometers form of the reflected light when the unit is operated. Although divided into two categories, namely those tachometers operational in the visible What is then the best method to calibrate these region of the light spectrum and those operating devices? I want to answer the question as in the infra-red region, their operating principle is follows: Let us consider the operating principle of the same. these devices. They use a photo detector to detect a pulsating light. Here the presence of The tachometer pick-up consists of a light source light indicates that the object has completed one that is pointed at the rotating object. As these are revolution. This signal is then fed into a counter normally battery operated devices, this light has that counts the number of pulses in a 60 second very low output power or beam strength. period. Surely one can simulate the infalling light onto the detector using a similar light source, The light is reflected back at the tachometer driven by a square wave. All that needs to be either by some reflective tape attached to the calibrated is the counting ability of the object or by its surface. The reflecting surface tachometer.5 provides a pulsating light to a light sensitive detector, normally situated next to the As a good metrologist your duty does not end tachometers light source. there. One will have to verify that the tachometer as a system is operational and this procedure For the calibration of these types of tachometers can only be done using a tachometer generator. one can use two methods. What is gained by using a pulsed light source? Rotating Disk: By using the same tachometer The range that can be generated using a light generator as described for the contact source is much larger than with the generator. tachometers, one can attach a piece of reflected One can simulate a tachometer range from 10 tape onto the disk. This would limit one to the RPM to 100 000 RPM using only a frequency same calibration range as for the contact generator, a frequency counter and an LED tachometer. (Light Emitting Diode). Attaching the reflective surface behind the disk 4) Sound Pick-up Tachometers allows one to generate different RPM ranges by either using a single slot disk or a disk with As with photo tachometers, the easiest method multiple slots, effectively multiplying the of calibrating these types of tachometers, is by emulating the conditions under which the unit is This is not necessary with digital tachometers. used. The main application of these types of Digital tachometers divides the input signal in tachometers is as vehicle tachometers. The order to extend i’s operating range, making it sound pick-up picks up the sound of the engine’s unnecessary to calibrate all of its ranges. spark plug firing. These tachometers normally have an adjustment corresponding to the number V. SUMMARY. of cylinders in the engine. A tachometer performs the same function as a Using a spark plug connected to an ignition coil, frequency counter, with the only difference being one can reproduce the sound necessary for the it counts over a period of a minute. tachometer’s operation. The ignition coil is then driven by a power amplifier and a signal Don’t go overboard and establish an elaborate generator connected to its input. measurement setup to perform a simple measurement, but be cautious and understand To complete the measurement setup it is exactly what needs to be measured. Always advisable to measure the sound pulses using a study the operating principle of the UUT and use suitable microphone system and to monitor the an appropriate calibration setup for the type of performance of the measurement setup using an tachometer. oscilloscope. Verify the calibration procedure before 3) Magnetic Tachometers implementing the procedure to calibrate instruments. For the calibration of tachometers using a magnetic pick-up one needs to generate a magnetic field. This can be done by winding a coil on an air former. The coil is then coupled to a signal generator to produce the required magnetic field. The tachometer is then placed close to the coil. IV. CONCLUSION When one wants to ascertain if the calibration method used to calibrate a tachometer is good enough, one should consider the following points; 1) Is the calibration procedure representative of the operating conditions of the UUT? This excludes the environmental conditions. This is certainly an important criteria. Not only does this verify the operating condition of the UUT, ie: is it working as it should, but it also forms the foundation to be used in providing traceability. 2) Can the measurement results be verified. For good laboratory practice one should consider ISO Guide 25. It states that all procedures used have to be verified and the laboratory must be able to prove its ability to implement these procedures. A. Ranges As with most instruments, tachometers have a selection of ranges and it is impossible to calibrate all the ranges completely. Older mechanical types of tachometers should be calibrated preferably over all of its ranges.
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