GB2465372A

GB2465372A - Optical displacement transducer

Info

Publication number: GB2465372A
Application number: GB0820857A
Authority: GB
Inventors: Nicholas James Adkins
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-11-14
Filing date: 2008-11-14
Publication date: 2010-05-19
Anticipated expiration: 2028-11-14
Also published as: GB2465372B; GB0820857D0

Abstract

An optical displacement transducer comprises at least one light source 1, the light beam from which is divergent or convergent over all or part of its path to a detector 2, sensitive to light from the source. The transducer includes an enclosure 3 which admits no light other than that from the source and which has internal surfaces which absorb incident light from the source. The length of the light path from the source to the detector is arranged to vary in accordance with the displacement to be measured. Optical displacement transducer arrangements for measuring two or more displacements with a single light detector and in which the output produced by the detector is a sequence of electrical signals proportional to the individual displacements are also disclosed.

Description

I

Optical Displacement Transducer This invention relates to a displacement transducer.

The requirement to measure physical displacements in a range of situations and environments is well recognised and is currently met with a number of transducer types, principally-Inductive Capacitive Resistive Each one suffers from one or more of these three disadvantages Difficult and therefore expensive to manufacture Due to their very small output signal they require interface equipment that can be bulky and cumbersome They require additional equipment to enable their signal to be transmitted over any distance.

They require electrical connections that present a hazard when explosive or flammable vapours are present.

Optical displacement transducers are known, for example, WO 2006/070263 Al uses the intensity of reflected light to sense the position of elements within camera lens systems, JP56 130603 for the positioning of an object, U.S. Patent 4,521,683 uses a reflecting diaphragm that switches between a convex and concave shape. WO 2006/063379 Al describes an optical potentiometer using both variable and non-variable translucent screen elements, other existing devices depend on light reflective elements or light transmissive elements whose characteristics are non-uniform.

The behaviour of light, in particular divergent beams, convergent beams, reflected beams and scattered light from a surface, is well understood. The intensity of light from a light source as measured by a light detector varies in proportion to the distance between the light source and the light detector, such variation is predictable and therefore suitable to the purpose of displacement measurement.

To render this predictable behaviour of light more readily applicable to the practical measurement of displacement the present invention proposes at least one light source the light beam from which is divergent or convergent over all or part of its path to the detector, at least one light detector sensitive to the light from the light source, an enclosure which admits no light other than the light from the source and has internal surfaces which absorb that light from the source that is incident upon those internal surfaces, provision for varying the length of the light path between the source(s) and detector(s) according to the displacement to be measured, provision for attachment to the bodies between which the displacement is to be measured.

The present invention is capable of several embodiments which are capable of easy and economic manufacture. Different embodiments of the present invention will be described herein which measure linear displacements, angular displacements, operate without the need for physical connection to a power supply or monitoring equipment, can be connected via optical fibres thus avoiding the potential danger of electrical connections at the measuring location.

The light detector of the present invention might be a simple photodiode or a phototransistor however more technologically advanced light detectors can be used to considerable advantage.

For example Texas Advanced Optoelectronic Solutions (TAOS) provide a range of light detectors including those designated TSL25O, TSL235, TSL2550, with outputs in the form of voltage, frequency or serial data.

The light source of the present invention might be any device giving a reasonably stable intensity at a wavelength, or range of wavelengths, that sufficiently match the spectral response if the light detector. Light emitting diodes (LED's) are an obvious choice being inexpensive, robust, available in a range of optical characteristics and generally well suited to the present invention.

Laser diodes are suitable for embodiments of the present invention requiring a light source producing an essentially non-divergent beam.

The useful range of displacement that can be measured by a transducer depends on a significant change in output signal being caused by a displacement. The present invention can be implemented in ways that enable the measurement of a range of displacements starting at fractions of a millimetre.

Since the present invention depends for its accuracy on a known level of light intensity from the light source it is preferable to have available means for determining this intensity and holding it constant or compensating for changes. If the light source is a light emitting diode (LED) then supplying current to it from a constant current source will bold the LED's light intensity substantially constant over the short term. The temperature of the LED might be monitored and the known effect of temperature on the light output of the LED compensated for in the supply current. Long term ageing of LED' s over thousands of hours of operation causes a reduction of their light intensity for a given input current. One method for compensating is to monitor the output light intensity of the light source with a separate detector and apply correction to the drive current. A more detailed description of such a preferred solution will be given later with reference to figures 9 and 10, figure 24 shows how the light intensity at the transducer might be monitored without the need for an additional light detector. An alternative method is to monitor the detector output at one or more known displacements and by means of external electronic circuitry or data processing to apply a correction to the light source current or otherwise compensate for changes in light source intensity. Such a system might also be used to compensate for manufacturing tolerances involved in the production of the light sources and the detectors.

In many embodiments of the present invention herein described the reflective elements and translucent elements do not rely for their operation on any variation in optical properties over their surface making their manufacture simple and economical.

The input to and output from the transducer might be in the form of light for instance via optical fibres thus obviating the need for any electrical connections and allowing the transducer to be used in contact with explosive or flammable vapours.

Other embodiments of the present invention enable one or several displacements to be measured and those measurements transmitted by means of a radio frequency signal or an optical signal to a remote receiver. By the well known means of reduced duty cycle working the batteries powering such embodiments of the present invention might be made to last for long periods of time.

Further embodiments of the present invention might be installed in their measuring locations without batteries or any other supply of power being permanently installed, their power being supplied only when measurements are required.

Yet further embodiments of the present invention are able to measure more than one displacement with a single light detector and present those measurements as a multiplexed output in the form of data, frequency, or analogue voltage.

The drawings of the embodiments of the present invention do not show specific means of attachment to the bodies between which the displacement is to be measured, such means will be well understood by those practiced in the mechanical arts. For the sake of clarity the enclosure or its component parts have been omitted from some of the drawings. A general principle to be regarded is that of the exclusion of stray light from the inside of the transducer enclosure be that from outside the enclosure or light generated inside the enclosure but not required for measurement. Methods of design and manufacture for the exclusion of and absorption of light are well known by those practiced in the relevant art and such methods may include surface contours, surface texture, surface coatings and chemical surface treatments.

Drawings showing electronic schematics show principle components only, those practiced in the electronic arts will be able to implement the functions depicted therein and described in the accompanying text. The present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a preferred embodiment of the present invention in which the light path is varied by displacing the light source relative to the light detector.

Figure 2 shows a preferred embodiment in which the light path is varied by means of a reflective element.

Figure 3 shows a preferred embodiment in which a TAO TSL25O light detector is superimposed upon the light source and also shows a concave reflective element.

Figure 4 shows a preferred embodiment in which variation in the length of the light path is opposed by a spring.

FigureS shows a preferred embodiment in which part of the enclosure is in the form of flexible bellows.

Figure 6 shows a preferred embodiment in which the light signals are taken to and from the transducer by means of optical fibres.

Figure 7 shows a preferred embodiment in which the reflective element moves about a hinge.

Figure 8 shows a method of measuring angular displacement.

Figure 9 shows a light detector encapsulated with the light source and an additional monitoring detector with feedback cotttrol of the light source output.

Figure 10 is a schematic of a system for electronically stabilising the output of the light source.

Figure 11 is a schematic showing an inductively coupled power supply and an optically coupled output signal.

Figure 12 is a schematic showing a displacement transducer powered by a battery and transmitting its output via a radio link.

Figure 13 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure 1 using a narrow angle light source.

Figure 14 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure lusing a wide angle light source.

Figure 15 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure 2 using a narrow angle light source reflecting from a cavity in the reflective element.

Figure 16 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure 2 using a narrow angle light source reflecting from a matte white reflective element.

Figure 17 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure 2 using a wide angle light source reflecting from a flat mirrored reflective element, line 1, and a matte white reflective element, line 2.

Figure 18 shows a graph of output voltage against linear displacement for a transducer of the general arrangement of Figure 3 using a wide angle light source and a variety of reflective elements.

Figure 19 shows the arrangement of components in a preferred embodiment for measuring three displacements with a single light detector.

Figure 20 shows a timing diagram related to Figure 19 Figure 21 shows the arrangement of components in a preferred embodiment for measuring three displacements with a single light detector and translucent elements.

Figure 22 shows a method by which a coarse and a fine measurement of a displacement might be made.

Figure 23 shows the output signals related to Figure 22.

Figure 24 shows a preferred embodiment for measuring two displacements with a single light detector the light signals being conducted by optical fibres.

Tests were conducted and the data for the graphs, FIGURES 13 to 18, obtained using these devices A narrow angle visible red LED with a beam angle of 10 degrees or A wide angle visible red LED with a beam angle of 60 degrees and A TSL25O light to voltage optical sensor.

A TSL25O light to voltage converter was used as the light detector in every test.

The output voltage of the TSL25O is directly proportional to the light intensity.

FIGURE 1 shows the basic embodiment of the present invention in which the light source 1 emits a divergent beam of light, the light detector 2 is sensitive to the light from source 1.

The light source 1 is fixed into the enclosure 3 and the light detector 2 is fixed into enclosure 4.

Enclosure 3 is a close, free sliding fit inside enclosure part 4 and thus permits the distance between source I and detector 2 to be varied. The total enclosure consisting of enclosure part 3 and enclosure part 4 is required to prevent light entering the cavity between the two parts from any source other than the light source 1 and to absorb as much as is practicable of the light that reaches its internal surfaces from light source part 1. The light detector 2 will thus receive only direct light from the divergent beam from the light source 1 the intensity of the received light and therefore the electrical output signal of detector 2 will vary in proportion to the distance between source 1 and light detector 2. Methods for excluding light and creating light absorbing surfaces by means of mechanical surface preparation and surface treatments and coatings are well known by those skilled in the relevant arts.

For the transducer output to be proportional to the effective length of the light path between the emitter and detector the radiation pattern of the light sources should preferably be uniform although as long as the radiation pattern is constant any non uniforniity might be accommodated in the calibration of the transducer. The effect of an uneven pattern is shown by Line 1 of FIGURE 13. This effect can be substantially removed by interposing a translucent screen with the light scattering characteristics of ground glass. A screen of plastic drafting film was used to produce the graph lines 2,3, and 4 of FIGURE 13 which shows the effectiveness of the screen in producing a proportional output. Line 1 shows the output voltage without a screen. Line 2 shows the output voltage with the screen placed 1mm in front of the detector. Line 3 shows the output voltage with the screen placed 4mm in front of the detector. Line 4 is the same as Line 3 but scaled to show a maximum voltage of 2.5 volts.

It will be appreciated that the effect of the screen is to scatter the light from the source and thus prevent any coherent patterns being passed to the detector, such patterns are particularly evident in the beams of narrow angle LEDs. The screen is effectively an area of multiple surface features providing multiple light sources The light scattering features of the screen's surface might be regular or irregular or of a larger or smaller scale.

It is an advantage if a displacement transducer requires external signals to be connected to one of its parts only, for example the non-moving part, leaving the moving part free of signal connections and more easily joined or coupled to the measured object. Also, signal connections made to the non-moving part suffer less fatigue. FIGURE 2 shows an embodiment of the present invention that achieves this by mounting the light source 1 and the light detector 2 in the same enclosure 3, fixed relative to each other and shielded so as to prevent, to a sufficient degree, light passing directly from the source I to the detector 2. The enclosure 3 is extended to form a cylinder in which the piston-like reflective element S is free to move along the longitudinal axis towards and away from the light source 1 and detector 2. Variation of light intensity at the detector is achieved by movement of the reflective element 5 on whose end, facing the emitter and the detector, is formed a reflective surface. l'his reflective element 5 can conveniently be fixed to or coupled to a moving body and the displacement of that body relative to the fixed part of the transducer enclosure 3 can be measured. The reflective surface can be formed on or applied to the reflective element 5 in a variety of ways well known to those skilled in the art of surface preparation and coating to give the required reflective characteristics and to ensure the stability of those characteristics.

The reflective surface can take many forms including, but not limited to; a mirror surface, a light scattering surface and may in addition be of flat, concave, or convex form.

FIGURES 15 to 18 show the characteristics of various reflector types. It will be evident from the Figures that the present invention is capable of implementation in a variety of ways to enable a wide range of displacements to be measured.

Figures 13 to 18 show the output, in volts, of a TSL25O light to voltage converter plotted against displacement measured in mm. FIGURES 13 to 18 also have their Y axes in "output frequency equivalent kHz". This indicates the frequency output of a TSL235 light to frequency converter used as the light detector 2.

In the case of a transducer of the general arrangement of FIGURE 1 the displacement is the distance between the light source I and the light detector 2. In the case of a transducer of the general arrangement of FIGURE 2 the displacement is the distance between the closest points of light detector 2 and the reflective element 5.

FIGURES 13 and 14 show the results obtained from transducers of the general arrangement of FIGURE 1. FIGURE 13 has already been described and line 4 shows this transducer to be able to give good resolution for displacements from 0 to 50mm. FIGURE 14 shows the results obtained using a wide angle LED as the light source 1 and no translucent screen in front of the light detector 2. It shows good resolution for displacement of 0 to 15 mm.

FIGURES 15 to 17 show the results obtained from transducers of the general arrangement of FIGURE 2. FIGURE 15 shows the results obtained with a narrow angle LED as the light source 1. The reflective region of the reflective element 5 was in the form of a circular hole of 6.4mm diameter and 3.6mm depth formed by drilling with a standard twist drill into the aluminium body of the reflective element 5. The reflector thus formed was a bright aluminium cavity, the remaining face of the reflective element 5 being blacked and substantially non-reflective.

FIGURE 15 shows good resolution for a displacement between 10 to 40 mm.

FIGURE 16 shows the results obtained with a narrow angle LED as the light source 1. The reflective region of the reflective element 5 was a white paper disk of 12mm diameter and shows good resolution for a displacement between 12 to 40 mm.

FIGURE 17 shows the results obtained with a wide angle LED as the light source 1. Line 1 was obtained using a reflective element 5 with a 12mm diameter flat, mirrored aluminium surface.

Line 2 was obtained using a reflective elementS with a 12mm diameter flat, white paper surface.

The lines show good resolutions for displacements of 8 to 25 mm and 5 to 20 mm respectively.

FIGURE 18 shows the results obtained for a transducer of the general arrangement of FIGURE 3 using a wide angle LED as the light source 1 and a TSL25O light to voltage converter as the light detector 2. The transparent package of the light detector 2 allows it to be placed in front of the light source 1 thus permitting a more compact implementation of the present invention. Veiy little light passes directly from source 1 to the light sensitive part of detector 2 as can be seen from lines 1, 2 and 3 of FIGURE 18 which show the output voltage falling to below 0.2 volts.

Five different reflective elements 5 were used each being 8mm diameter aluminium formed to a mirror finish, line 1 was a flat reflector, line 2 a concave form of 5mm radius of curvature, line 3 a concave form of 6 mm radius, line 4 a concave form of 7mm radius, line 5 a concave form of 8mm radius.

FIGURE 4 shows a sectioned view of an embodiment of the present invention in which displacement of the reflective element 5 towards the light detector 2 is resisted by the spring 6 the measured displacement being proportional to the force required to compress the spring.

FIGURE 5 shows a sectioned view of an embodiment of the present invention in which the housing 3 is connected to the reflective elementS by a concertina form 7. The concertina form 7 might perform the functions of excluding, for example, light, dirt, dust, fluids and be made from flexible rubber, plastic, metal or similar material and presenting the minimum of resistance to the displacement of the reflective element 5. If the concertina form is made of a springy material then the measured displacement will be proportional to the force require to compress it thus it will perform the function of spring 6 of FIGURE 4.

FIGURE 6 shows a sectioned view of an implementation of the present invention in which the light is conducted to the transducer housing 3 through one or more optical fibres or light guides 9 and light is conducted from the transducer through one or more optical fibres or light guides 8 the light source and the light detector being at remote point some distance from the transducer.

By this means the transducer is rendered entirely free of electrical connection and their attendant ignition hazard allowing the transducer to be safely operated in inflammable or explosive environments such as fuel tanks. FIGURE 6 shows the optical fibre or light guide 9 terminating in the housing 3 at 10 where optical devises might be fitted to change the characteristics of the light beam and the characteristics of the transducer. For example a translucent screen might be fitted to disperse the light beam or a lens or lenses might be fitted to produce a narrow beam.

Provision might also be made within the housing 3 for the light intensity to be sampled and the sample conducted through one or more optical fibres or light guides to the remote point and used to monitor and maintain, by control of the light source, a constant light level at 10.

FIGURE 7 shows an embodiment of the present invention in which the light source 1 and the light detector 2 are fixed in a housing 12 and the reflective element 13 moves about the hinge 14.

In a further embodiment of the present invention the light source 1 might be fixed so as to move, relative to the light detector 2, about the hinge. The casing to exclude external light has been omitted from the drawing for clarity.

FIGURE 8 shows an embodiment of the present invention for the purpose of measuring angular displacement in which the reflective element 17 is formed on or attached to a shaft 16. The reflective element 17 is of a spiral form or, if a particular transducer response is required, a modified spiral form. The housing for the light source 1, light detector 2 and the casing required to exclude external light from the transducer has been omitted from the drawing for clarity. As the shaft 16 revolves about its centre the distance from the surface of the reflective element 17 to the light source 1 and light detector 2 changes and the electrical output of the light detector 2 changes in proportion and indicates the angular position of the shaft 16. Embodiments of the present invention might vary from that shown in FIGURE 8 by, for instance, forming the reflective element on or attaching it to the circumference of the shaft.

The simple configuration shown in FIGURE 8 is capable of measuring a rotation of a little under 360 degrees at which point the light detector 2 will detect the rapid change in light intensity at the transition from the highest to lowest point, or lowest to highest point, of the reflective element a mechanical stop might be used to prevent the light detector 2 from encountering this transition. Alternative embodiments of the present invention allow the displacement range of the transducer to be extended by detecting coarse positional information. As the shaft 16 and the reflective element 17 complete one rotation the electrical output of light sensor 2 changes from lowest to highest or highest to lowest level at the disjuncture of the surface of the reflective element 17. Clearly it is easily possible by electronic means to register these relatively rapid transitions and store them, or their algebraic sum, as a count of the nwnber of complete turns of the shaft. Thus, in the case of a lead screw for example, the number of complete turns will provide coarse positional information while the light detector 2 will provide fine positional information. This can be accomplished with no additional electrical connections to the transducer. Alternatively a separate and independent means of determining the direction and number of complete revolutions of the shaft may be used, for example a pair of opto switches.

A further embodiment of the present invention is shown in FIGURE 22 by which fine and coarse measurements of angular displacement might be made. Shaft 1 6a has fixed to it the reflective element I 7a and the gear 18 which drives gear 19 the two gears providing a reduction of angular displacement of the ratio, for example, 4 to 1. Gear 19 is fixed to shaft I 6b which is fixed to reflective element 17b. The light sources Ia and lb are energised in sequence and a multiplexed output obtained for light detector 2 in the manner described later in connection with FIGURE 19.

Figure 23 shows the relationship between the fine and coarse measurements, the effects of multiplexing are not shown.

FIGURE 9 shows an enclosure 20 in which two principle components of the present invention, the light source 1 and the light detector 2 encapsulated together with a photodiode 18 and an electronic control circuit 19. The drawing is partially sectioned to reveal the photodiode 18 and the electronic current control circuit 19. Figure 10 is schematic illustrating the operation of the circuit. Photodiode 18 receives a sample of light from the light source 1 and sends a signal, proportional to the light intensity, to the input 22 of the control circuit 19. An external signal is applied to input 21 of control circuit 19 by means of which the light intensity from light source 1 is set and maintained constant.

FIGURE 11 is a schematic showing an embodiment of the present invention in which a displacement transducer module 35 is used in combination with an energising and reading module 34. The un-powered transducer module 35 is installed in its measuring position. When a measurement reading is required the reading module 34 is brought close to the transducer module 35 so that the coil 28 is magnetically coupled with coil 29. The oscillator 27 energises coil 28 which in turn energises coil 29. The rectifier 30 supplies direct current to the transducer module 35 circuitry. Direct current smoothing circuitry has been omitted from the drawing. The constant current generator 25 supplies current to the light source 1. The voltage stabiliser 26 supplies voltage to the light detector 2 which, in this example, is a TSL235 light to frequency converter. The output signal of the light detector 2 is in the form of a train of pulses whose frequency is proportional to the measured displacement. This pulse train energises the light emitting diode 31 thus converting the electrical signal to a train of light pulses to be received by the photodiode 32 in the reading module 34. The counter/display unit 33 receives the electrical signal from photodiode 34 and provides a display of frequency, period or a direct reading of, for example, displacement or pressure. Data can also be sent from the counter/display module 34 to, for example, a data logger or computer.

FIGURE 12 is a schematic showing an embodiment of the present invention in which a self-provided and energised alone in sequence so as to produce a reference signal from light detector 2.

FIGURE 21 shows the arrangement of components in a preferred embodiment of the present invention for measuring two or more displacements with a single light detector and providing what is commonly termed a "multiplexed" output signal. FIGURE 21 shows the arrangement for measuring three angular displacements but the present invention might be implemented so as to measure any practical number of angular or linear displacements using any practical number of light sources and light detectors. The enclosure to exclude external light has been omitted from the drawing for clarity. Each light source la, lb. ic, is energised in sequence and illuminates its respective translucent element 50a, 50b, SOc. FIGURE 20 is an example of a timing diagram for such a method showing the relationship between the sequentially energised light sources and the position data output from light detector 2, the form of the data depending on the type of device used as light detector 2. Light source la is hidden in FIGURE 21. By way of example and considering the operation, of the embodiment shown in FIGURE 21, along one axis only, the translucent element 50b is in the form of a spiral and fixed to the shaft Sib whose angular displacement is to be measured. The optical characteristics of the translucent element 50b are uniform throughout its rotation. Light source lb emits a parallel beam illuminating a small area of the facing side of translucent element SOb rotation of which displaces its surface relative to light source lb and light detector 2. The light beam from light source lb. being parallel, will illuminate a constant area of the translucent element 50b regardless of its distance from the light source lb thus providing, as seen from the position of light detector 2, a source of scattered light at a distance that varies with the rotation of the translucent element SOb the intensity of light reaching light detector 2 thus being proportional to the angular displacement of shaft 51b. It should be noted that a parallel beam is specified in the foregoing explanation purely for ease of description, the present invention might be implemented with light sources, la,lb,lc which emit beams that are not parallel. Rotation might be limited to less than 360 degrees by means of a mechanical stop or the rapid transition at the end of the spiral might be used to count the number of complete turns as described in the explanation of FIGURE 8.

The present invention as shown in FIGURES 19 to 24 might be embodied using reflective elements and translucent elements with non-linear reflective and translucent characteristics, techniques for producing such characteristics are well known and include; varying grey shades, an increasing proportion of dark or opaque lines or areas, tapering apertures or reflective areas. A single transducer might use one or a number of techniques to obtain light levels that vary in the required proportion to the displacements being measured.

FIGURE 22 shows the general arrangement of components in a preferred embodiment of the present invention in which light detector 2 receives light from source la via reflective element 1 7a and thus measures the angular displacement of shaft 1 6a, this measurement signal is represented in FIGURE 23. The 4:1 reduction gears 18 and 19 transfer the angular displacement to the shaft 16b and its reflective element 17b. When light source lb is energised in its turn light detector 2 will generate a measurement signal according to the position of reflective element 17b as represented in Figure 23. Thus the sequential outputs of light detector 2 indicate the number of turns of shaft 16a and the precise angular displacement of shaft 16a. The ratio of the gears 18 and 19 can be selected to suite the measurement requirement, higher ratios might be achieved with a gear train. Other means of implementing a reduction drive, such as toothed pulleys and belts, are well known.

FIGURE 24 is a part-sectioned view showing the general arrangement of components in a preferred embodiment of the present invention in which the output of light detector 2 is a sequence of electrical signals that are proportional to the individual displacements, relative to the enclosure 3, of reflective element 5a, and reflective element 5b. Also included in the sequence is a reference signal provided by the reference reflective surface 11. Except for the reference reflective surface the internal surfaces of enclosure 3 are non-reflecting, enclosure 3 also excludes unwanted light. Light sources la, ib, ic, are energised sequentially and their light directed to their respective reflective elements through optical fibres 9a, 9b, 9c. The sequence of reflected light is received at one end of optical fibre S and conducted to the light detector 2 the signal from which is either analogue voltage or frequency or binary data depending on the device used.