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Calibration of Tachometers

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					                                  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: csveldma@csir.co.za 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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