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1. WO1993018370 - DISPOSITIF DE NIVELLEMENT

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

LEVELLING DEVICE

This invention relates to a levelling device, particularly in the form of a spirit level, and has as its object the provision thereof in a convenient and effective form.

According to the invention a levelling device comprises a casing having at least one exterior reference surface, and at least one light emitting element visible externally of the casing, the arrangement being such that, in use, if the reference surface or one of the reference surfaces is placed against a surface of an object, said at least one light emitting element provides a visual indication as to whether or not said reference surface is at a predetermined angle to the vertical or horizontal.

The invention will now be described, by way of example, in which:

Figure 1 is a schematic, longitudinal inside view of a casing of a levelling device of the invention;

Figure 2 is a schematic side view showing the arrangement of a bubble vial together with a pair of emitters and detectors forming part of a levelling device of the invention;

Figure 3 is an end view of the arrangement of Figure 2;

Figure 4 is an electronic circuit diagram used in one embodiment of a levelling device of the invention; and

Figure 5 is a further electronic circuit diagram used in another embodiment of a levelling device of the invention.

Figure 1 shows a levelling device of the invention which is in the form of, and is intended to be used in exactly the same ways as, a conventional spirit level. The main difference is that instead of, or in addition to external indication by the normal indicating device (the bubble in a conventional spirit level vial) the device of the invention has an electronic display consisting of an array of light emitting elements, preferably light emitting diodes.

An electronic spirit level according to one embodiment of the invention is of the general form shown in Figure 1 having a generally hollow rectangular casing 10 made up of two halves, the casing being divided longitudinally. Preferably a rubber seal would be provided between the halves to make the level waterproof. As with a conventional level, the casing has a flat, longer reference surface 11 which is intended to be placed against any surface whose inclination it is wished to check. The other longer surface 12 of the casing, which constitutes the front of the level, has a central recess 13 in which are received an array of light
emitting diodes 14, there being, in this example, five diodes. This front surface 12 contains a push button 15 for switching the level on. As will later be described, the level incorporates automatic switch-off means.

Within the casing, there is schematically shown a nine volt battery 16 having a pair of connections to a printed circuit board 17 which carries various electronic components to be described hereinafter, including the light emitting diodes 14. One of the connections between the battery and the printed circuit board 17 is via the push-button 15.

Also contained in the casing is a further printed circuit board 18 which carries a conventional spirit level bubble vial as well as a pair of sensors and a corresponding pair of emitters for detecting the position of the bubble, and thus the orientation of the level, as will be described. This board 18 is connected to the board 17 by wires 19 as shown. Instead of using two printed circuit boards, a single board or more than two boards could be used.

As mentioned, the level includes a standard spirit level bubble vial 20 containing a transparent coloured or colourless liquid 21 and an air bubble 22 of well defined size. Figures 2 and 3 show the arrangement whereby the bubble position is detected using two pairs of infra-red or visible light emitters 23a, 24a and sensors 23b, 24b. The emitters could instead emit electromagnetic, waves of longer or shorter wavelengths, with alternative suitable sensors being used. As shown in Figure 3, an emitter and an associated sensor are preferably diametrically opposed at opposite sides of the cylindrical vial 20, so that in operation light passes from the emitters to the sensors through the diameter of the vial. When the bubble 22 is not in the direct light path, as is shown in Figure 2 for the emitter 24a and sensor 24b, the liquid in the tube acts as a one-dimensional convex lens, concentrating the emitted light from emitter 24a onto the sensor 24b which generates a strong electrical output signal. In the case of emitter 23a and associated sensor 23b, the bubble 22 is directly in the light path and thus changes the shape of the liquid to form a concave lens which diverges the emitter light so that a smaller amount falls onto the sensor 23b therefore generating a weak electrical output signal. The reduction of the output signal is
progressive in that sensor output begins to fall as an edge of the bubble comes into view, and continues to fall to a minimum as the bubble lies centrally between an emitter and its associated sensor.

Although it is preferred to use a pair of emitters and associated sensors, it is possible to use only one sensor and one associated emitter and to detect the edge of the bubble by measuring the electric signal level detected by the sensor and comparing it with a fixed reference value. This method however has drawbacks, notably that the response would be similar for ends of the bubble so that a false, inverse reading would be produced as the bubble passed the sensor and approached an end of the vial. Also such a 'single ended' method would bring into play the effects of thermal and other sources of electrical drift, giving inaccurate results, and demanding more complicated circuitry with stabilised power supplies etc.

Using two pairs of emitters and associated sensors overcomes these drawbacks by allowing inherently stable 'null' methods to be used. Figure 4 shows an electronic circuit carried by the boards 17 and 18 and which incorporates these methods. The level can incorporate a single vial 20 preferably arranged so that it indicates when the surface 11 is horizontal (or alternatively vertical), but the circuit of Figure 4 does in fact show the use of two vials at right angles to each other, whereby the level can indicate when the surface 11 is vertical or horizontal. Of course suitable orientations of one or more vials would allow the disposition of the surface 11 at other predetermined angles to the vertical or horizontal to be indicated.

As shown in Figure 4, the vertical vial 25 is illuminated by emitters 26a, 27a which are connected in series and therefore carry identical currents. Any changes in current due to circuit elements or battery voltage drop will effect the two emitters equally and so the balance between them will be preserved. Ageing effects should also balance as the emitters have
identical conditions of service. Sensors 26b, 27b are connected back-to-back so that all common-mode background illumination is cancelled, leaving only difference signals. The cathode of sensor 27b is connected to 0 volts and the difference signal output is taken from the cathode of sensor 26b. When the bubble is centrally positioned between the two emitter/sensor pairs, the outputs from the two sensors will be identical and will be totally cancelled giving an output of 0 volts.
Movement of the bubble in one direction will cause a positive output as one sensor is illuminated more
strongly, and movement in the other direction will give a negative output. Load resistors 26c, 27c are connected across the sensors 26b, 27b respectively, the load resistors themselves being connected in series. Off-set voltages caused by differences in the emitters, sensors or load resistors can be compensated by a slight movement of the bubble. This may be provided by an acceptably small tilt within the working tolerance of the level, not noticeable by a user, or can be provided during
calibration by moving the vial slightly along its axis.

The sensing of the bubble works equally well with the emitters above the vial and the sensors below it. It is also possible to operate the level with the sensors in a horizontal plane but the sensitivity is reduced as the bubble occupies only half of the gap between the emitters and the sensors .

The signal from the cathode of sensor 26b is centred upon the circuit negative rail (0V) and swings positive or negative as the bubble moves . Typically the output signal is 50 mV maximum, and has a source
impedance of 4.4 K ohms. This must be amplified
sufficiently to drive two similar bipolar operational amplifiers. These must be of the type which can operate with their inputs down to the negative supply voltage. One amplifier is used in a standard non-inverting
configuration to handle only the positive going input signals, whilst the other is used in the standard
inverting mode to handle just the negative going signals. Only one amplifier is active at any time depending upon the polarity of the signal from the sensors.

Interaction between the amplifier input circuits is negligible because the feedback loop for whichever one is unused is broken by its output's inability to swing below 0 V.

Amplification for positive going signals is provided by an operational amplifier 28 which has its gain set, for example, to just over 100 by a resistor 30 connected in parallel, and a resistor 30a connected between its negative input and earth. Amplification for negative going signals is provided by an operational amplifier 31 which has its gain set to a similar level as amplifier 28 by a series resistor 32 and parallel
resistor 33, and the source impedance of the sensors which is determined by the series combination of load resistors 26c, 27c.

The amplifier outputs each drive two light
emitting diodes. As shown in Figure 4, this embodiment employs an array of five diodes 34-38 respectively, the centre one 36 of which, lights green whereas the others light red.

Diode 35 is driven via a resistor 39 and is the first to light for negative going signals, followed by diode 34 which lights in addition to diode 35 at a higher voltage determined by a Zener diode 40. Thus as the level is tilted further away from the horizontal or vertical, depending upon in which state it is being used, the diode at one side of the centre diode 36 initially lights and as the tilt angle increases the outermost diode also lights. For positive going signals diode 37 lights first, followed by diode 38. It is possible to make the end diodes light brighter than the inner ones adjacent thereto by changing resistor values, and
additionally it is of course possible to have only an end diode 34 or 38 light at increased tilt with the inner diode 35 or 37 switching off.

Current through either diode 35 or 37 also passes through the base/emitter junction of a transistor 41, turning it on. Connected to the collector of the
transistor 41 is a NAND gate 42 in the form of a Schmidt trigger device which drives the centre balance light emitting diode 36. An identical NAND gate 43 inverts the l.e.d. drive signal and passes a gating signal via a capacitor 44 to a standard audio oscillator circuit formed by a resistor 45, a capacitor 46 and a NAND gate 47, identical to NAND gates 42 and 43. As the bubble reaches its centre point in its vial 25 and diode 36 lights, the output of NAND gate 43 switches to positive. Capacitor 44 couples the positive change to one input of gate 47, and enables the oscillator which drives a piezo transducer (sounder) 48. Capacitor 44 charges via a resistor 49 and after a time the gating input of gate 47 falls far enough to switch off the oscillator. The circuit thus produces a timed 'beep' each time the balance condition is reached. A short discharge path is provided for capacitor 44 by the input protection circuit of gate 47. This is acceptable as the capacitor is a low value, and the current is limited by the output
capability of gate 43.

The central diode 36 is lit only when all of the other light emitter diodes are off and indicates that the vial is in a level condition. A slight tilt either way results in current flowing via either diode 35 or diode 37, turning on transistor 41 and turning off the central diode 36. The sensitivity of transistor 41 is determined by a resistor 50 which sets the minimum current to turn off diode 36. Reducing its value allows a larger 'blend' area where diode 35 or 37 begins to light before diode 36 is turned off. As inverter 43 is a Schmidt trigger inverter, the switching of diode 36 is abrupt. The values of series resistors 51, 52 for the diodes 34, 38 respectively are chosen to give these diodes the same brightness as the diodes 35, 37, but as mentioned above these values could be chosen to make these outer diodes light brighter or dimmer. In an alternative embodiment the centre diode 36 could be permanently lit, being the sole diode lit when the level is in balance, but with at least one other diode being lit when it is out of balance.

To allow the level to be used in vertical and horizontal orientations, a horizontal vial 53 is provided in addition to the vertical vial 25. The vial 53 is illuminated by emitters 54a, 55a and has associated sensors 54b, 55b respectively. In the circuit shown in Figure 4, both the vials 25, 53 are illuminated all the time. The vial which is not active, i.e. is not
currently in the horizontal plane, is of such a length that its bubble is off to one end or the other of its vial and is completely clear of the sensing area. The result of this is that the sensor outputs for that vial are balanced and make no contribution to the signal that is being provided from the active vial. Any unbalance detected is thus due solely to the active vial and is amplified and displayed as explained.

For correct operation, the outputs from the inactive vial must be taken into account. To this end, the corresponding pairs of diodes (26b and 55b, and 27b and 54b) are connected in inverse parallel. This
connection maintains a low impedance state for the sensors as both are forward biased. Without this
connection, the sensors on the inactive vial reverse bias the sensors on the active vial and the output signal is much reduced.

Power to the circuit is from a small battery, such as that shown at 16 in Figure 1. In order to conserve power, and to simplify the switching and operation, a simple time delayed switch-off circuit is used. This, however, could be omitted if required.

To switch the level on, the push button 15 is pressed for a brief time. Pressing push button 15 charges a capacitor 56 and sets the inputs of a NAND gate 57, identical to NAND gates 42, 43 and 47, positive. The output of gate 57 switches to 0 volts and turns on a PNP transistor 58 via a resistor 59, the transistor applying power to the circuit. The capacitor discharges via a resistor 60 and the input voltage to the gate 57
gradually falls to 0 volts . When it has fallen
sufficiently, the output of the gate switches to
positive, turning off the transistor 58 and removing power from the circuit. To function correctly in this circuit, the gate must be connected directly to the battery at all times. A diode 61 has been added to ensure that the gate is not damaged by accidental supply reversals during battery changes. A resistor 62 is a pull-up, provided to ensure that the transistor is turned off. This is needed because the output of the gate is prevented from swinging to the full positive supply rail by the forward voltage drop of the diode 61. The time delay is set by the values of capacitor 56 and resistor 60 to approximately 100 seconds, in this embodiment.

This part of the circuit has been designed
carefully to leave all of the inputs and outputs of the inverter in well defined conditions during power-down. This is essential because it is necessary for the
inverter to be powered at all times to keep the automatic switch off circuit as simple as possible.

The electronic circuit of a further embodiment of a level of the invention is shown in Figure 5. This has been designed to allow the level to operate with one or both vials visible to the user. In these circumstances the exclusion of external light effects must be achieved by electronic means. In this embodiment only the vial 25 is shown, together with this associated emitters, sensors and load resistors. The same numbering has been used for these components as in Figure 4. Other identical or similar components are also given the same reference numerals as in that figure. The circuit for the pushbutton 15 would be as in Figure 4.

To allow the circuit to reject ambient light, the vial light emitting diodes 26a, 27a are pulsed on and off at a high frequency, and the sensor output is A.C.
coupled to the amplifier 31 so that only the pulse response is amplified and any continuous or low frequency illumination is ignored.

The circuit of Figure 5 shows a NAND gate in the form of a Schmidt trigger oscillator 64 with a parallel resistor 65 and a capacitor 66 determining the operating frequency. The output from this oscillator is a square- wave which is buffered by a NAND gate 67 and used to drive the light emitting diodes 26a, 27a which illuminate the vial 25. Output from the sensors 26b, 27b is in the form of positive or negative pulses as the emitters are pulsed on and off, mixed with a steady D.C. offset due to daylight falling unevenly on the sensors, and low frequency interference from 50 Hz. lighting. A capacitor 68 passes the pulses easily but completely blocks the D.C. offset and greatly reduces the low frequency
interference. The D.C. stripping effect of the capacitor

68 means that the D.C. content of the desired pulses is lost, so that negative going and positive going pulses all appear the same. To restore the polarity information it is necessary to clamp the signal from capacitor 66 to

0 Volts between pulses. This is achieved by a transistor

69 which is turned on and off in synchronism with the pulses clamping the signal from capacitor 68 to 0 Volts whenever it is turned on. To minimise any D.C. offset due to the transistor 69, it is operated in the inverse mode, with its emitter and collector interchanged. In this mode the current gain is much lower, but the
transistor is more effective as a switch, having a lower D.C. offset voltage.

Once the pulses have their D.C. levels restored, they are amplified in the same way as in the circuit of Figure 4. The outputs from the amplifiers are in the form of pulses, but as the frequency is high, appear to the eye to be on continuously. The centre diode 36, is controlled as in Figure 4 by the output from transistor 41 which is turned off only when diode 35 or diode 37 is off. However in this circuit, even while diode 35 or diode 37 appear lit, they are switched off between pulses, so that diode 36 would light during these
intervals and appear to be on all of the time. To overcome this, a NAND gate 70 is provided. One input is driven by the pulse source and the other is driven from the collector of transistor 41. This arrangement ensures that diode 36 cannot be lit during the intervals between pulses, but allows normal operation the rest of the time. A capacitor 71 prevents the centre diode 36 from lighting very dimly by spikes between the pulses caused by the slight phase shifts introduced by the amplifiers 28 and 31. A NAND gate 72 between the gate 70 and the diode 36 inverts the pulses from the gate 70 so that the correct drive polarity is provided for the diode 36.

A refinement introduced into this circuit is the phasing of the pulse supplied to transistor 69. Instead of switching it on between pulses, it is switched on during pulses. This simply inverts the phase of the pulses from capacitor 68 so that the display light emitting diodes 34 to 38 respectively are driven during the times that the emitters 26a, 27a are off. The benefit of this is that it tends to even out the current drain from the battery and so give at least marginally extended battery life.

Further improvements and variations to the
embodiments disclosed are as follows.

A single light source could be used to illuminate each vial, or both vials simultaneously. Similarly a single sensor could be used with one or more sources of illumination pulsed or continuously illuminated. A variation is possible using two emitters switched on and off in anti-phase, with a single detector, followed by capacitive coupling and D.C. restoration as in Figure 5.

As well as, or instead of the 'beep' referred to when balance of the level is reached, an audible output of fixed or variable pitch could be provided to indicate when the device is tilted. Rising or falling amplitude or pitch could indicate an approaching balance, and also possibly indicate the direction of the error. It is however felt that the simple system with a single tone that sounds for a short period each time the centre light emitting diode turns is advantageous in overcoming the possible irritation of a continuous output tone. The audio indication of balance (or off-balance) can be omitted if only the visual indication is required.

As far as the casing is concerned, the push button could be provided in a recess thereof in order to avoid accidental operation.

Although described herein specifically in relation to a levelling device, any novel and inventive circuits or parts thereof disclosed in the description and
drawings may well be suitable for use in other
applications as well as with the embodiment disclosed herein.