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APPARATUS FOR DETERMINING THE
POSITION OF A MOVABLE OBJECT
BACKGROUND OF THE INVENTION 5
Many peripheral working aids are already available on the computer market to help the user control the cursor on the screen. The most popular of these is the so-called "mouse" whose movement controls the movement of the cursor. Most of the mice hitherto available 10 operated by optical or mechanical processes and the positions were determined by means of a special indicator board with special support for the mice or with wires stretched across the indicator board. Since this requires relatively elaborate mechanical work or the 15 formation of special surfaces, which increase enormously in cost with the increasing requirements for accuracy and power of resolution, there has for some time now been a quest for different systems for determining position.
The present invention relates to an apparatus for determining the position of a movable object within a given boundary and relatively to fixed reference points by means of acoustic signals, preferably ultrasonic pulses, comprising transmitter and receptor means for 25 the ultrasonic pulses and means for determining their travelling time between the movable object and the fixed reference points.
An apparatus of this kind is described in the article "Zweiohrige Maus" (two-eared mouse) by P. Wendt in 30 "mc Die Mikrocomputer Zeitschrift", No. 10/1984. In this apparatus, the receiver means are microphones and to measure the travelling time, counters are started in response to a trigger signal when sound pulses are transmitted by the transmitter and stopped on reception of a 35 signal by the microphones. Care must be taken to ensure that the outlet opening of the transmitter is directed towards the microphones because otherwise the travelling time may be determined by reflections. The position determination is not very accurate, partly for this 40 reason and partly owing to the relatively large size of the microphones. This known apparatus therefore cannot be used as so-called digitizer, i.e., as peripheral instruments for feeding the coordinates of a drawing into a computer system for geometrical data processing. 45
It is an object of the present invention to provide an apparatus for position determination distinguished by very precise determination of the position and a high power of resolution.
SUMMARY OF THE INVENTION 50
The solution to this problem provided according to the invention is characterized in that the transmitter and receiver means are formed by cylindrical ultrasound transducers. 55
The use of cylindrical ultrasound transducers enables the coordinates of the mouse to be calculated very simply and accurately, and does not require the transmitter and receiver means to be aligned with each other. The precision of position determination and the power of 60 resolution provided by the invention are greater by about one order of magnitude than in the cited "TwoEared Mouse."
The apparatus according to the invention may be used both as wired and as wireless cursor, that is to say 65 that the movable object may be connected to the control unit of the apparatus through a lead, which enables the travelling time to be determined by a relatively
simple procedure, or it may have no physical connection to the control unit, so that the mobility of the object is not restricted by a lead.
According to a first, preferred further development of the apparatus of the invention, means are provided for the transmission of pulses in the region of visible or invisible light for the purpose of wireless indication of the point in time of the transmission or reception of the ultrasound pulses and means are provided for the detection of these light pulses.
This results in maximum freedom of mobility of the movable object and the wireless indication according to the invention of the point in time of transmission or reception of the ultrasound pulse by means of optical pulses and renders the measurement of travelling time substantially in sensitive to electrical disturbances.
According to a second, preferred further development of the apparatus according to the invention, the cylindrical ultrasound transducers are so-called^ell transducers the surface of which is covered with a plastic foil electret which is metallized on one side and forms the movable electrode of a condenser which has an air gap.
This arrangement enables extremely short pulses, so-called acoustic shock waves to be transmitted to the surrounding air, where the precision of calculation of the mouse coordinates is further improved.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained in more detail with reference to an exemplary embodiment illustrated in the drawings, in which:
FIG. 1 is a diagrammatic sketch of the apparatus according to the invention shown in plan view;
FIG. 2 is a block circuit diagram indicating the mode of operation of the apparatus according to the invention;
FIG. 3 is a set of time graphs explaining the mode of operation;
FIG. 4 is a top plan view of an ultrasound transducer;
FIG. 5 is a section taken on the Line V—V of FIG. 4;
FIG. 6 is a sketch representing a variation of the apparatus of FIG. 1; and
FIG. 7 is a diagrammatic sketch of a personal computer with an apparatus according to the invention.
DETAILED DESCRIPTION OF THE
FIG. 1 shows two ultrasound transmitters 1 and 2 for measuring the position of a movable object or mouse (M) which is manually displaceable within a surface area 3. The ultrasound transmitters 1 and 2 form parts of the base station (B) of the apparatus which in addition has a calculator 4 and an infrared receiver 5. The calculator 4 activates the ultrasound transmitters 1 and 2 to transmit ultrasonic pulses I and is connected to the infrared receiver 5.
The mouse M substantially comprises an ultrasound receiver 6 and an infrared transmitter 7. The ultrasound transmitters 1 and 2 are spaced apart by the distance D in one coordinate direction, the x-direction in the drawing.
The position of the mouse M in relation to the fixed coordinate system having x and y axes is determined by successively measuring the travelling times of the ultrasonic pulses 1 from the ultrasound transmitters 1 and 2 of the base station B to the ultrasound receiver 6 of the
mouse M. A short, accurately defined ultrasonic pulse I is released by application of a starting pulse S to the ultrasound transmitter 1 or 2 and a counter begins counting simultaneously with the starting pulse.
The ultrasonic pulses I are received by the ultrasound 5 receiver 6 of the mouse M, the time intervals between release and reception of each ultrasonic pulses I depending on the distances in space di and d2 of the ultrasound receiver 6 of the mouse M from the ultrasound transmitter 2 and 1, respectively, of the base station B. The 10 ultrasonic pulses I received by receiver 6 are converted into electrical signals in the mouse M and the first maximum of the required signal is detected. By means of this detection, the infrared transmitter 7 of the mouse returns an infrared pulse sequence IR to the base station I5 B. This pulse sequence IR is received by the infrared receiver 5 of the base station B and stops the counter.
The state of the counter is thus characteristic of the travelling time of the corresponding ultrasonic pulse over the distance di or 62 and hence, the velocity of 20 sound being known, it is also characteristic of the distance di or d2 of the mouse M from the transmitter 2 or 1. From the values di and 62 and the known distance D between the two ultrasound transmitter 1 and 2, the calculator 4 is able to determine the x and y coordinates 25 of the mouse M according to the following formulas.
x = 75- (P - d\)(p - d2)
D + d\ + d2 where: p = j
(Assuming that the zero point of
coordinate system lies at the
center of the ultrasound
FIG. 2 is a block circuit diagram of the apparatus according to the invention with base station B and mouse M illustrating the function of the apparatus. 45
The mouse M consists, as illustrated, of three functional blocks, namely the ultrasound receiver or receiver transducer 6, a pre-amplifier 8 with maximum detector 9 and a digital logical part 10 with infrared transmitter 7. 50
The ultrasound receiver 6 has the form of a short, hollow cylinder containing in its bore a plexiglass disc with crosswires 11 so that the mouse M can be accurately positioned on a particular point. The crosswires 11 may be combined with a magnifying lens as a posi- 55 tioning aid. The ultrasound receiver 6 is externally completely grounded and protected against mechanical influences and very powerful electrical disturbances by a grounded metal grid 12.
In order that an ultrasound pulse received by the 60 ultrasound receiver 6 may be optimally detected by means of its maximum or its first maximum, a preceding undershoot should reach its maximum as rapidly as possible and die down equally rapidly. The forms of sound pulse which fulfill this condition are those which 65 can be approximated by a sine wave of a half or whole period. Both forms are permissible since the first maximum of the ultrasound pulse is detected.
In conformity with these permissible forms of sound pulse, the ultrasound transmitters 1 and 2 (FIG. 1) transmit sound pulses I which have approximately the form of a sine (180° segment) half wave or a (360° segment) sine wave. The ultrasound receiver 6 converts this acoustic signal into a corresponding electrical signal which also has the form of a sine half wave or sine wave. This electrical signal is selectively amplified by the preamplifier 8 and the first maximum of this amplified signal is detected by the maximum detector 9. On detection of the first maximum of the amplified electrical signal corresponding to the ultrasound pulse, the maximum detector 9 transmits a corresponding signal to the digital logical part 10, by which the transmission of an infrared pulse sequence is started by the infrared transmitter 7, the start bit of which sequence stops the counter of the base station. The data format of the infrared pulse sequence is so chosen that in addition to the start bit, the state of three operating keys Ti and T3 and the operating state of the mouse M are transmitted serially to the base station B. The pulse width of a pulse of the infrared pulse sequence is preferably 10 microseconds, the distance between two successive pulses is preferably 64 microseconds, and the key ratio of the infrared pulse sequence is therefore approximately 1:6.
The mouse M is supplied with current from a feed element 14 which in turn is supplied by batteries 13. The mouse M should be switched off as often as possible owing to the relatively high current consumption of the analog circuit part composed of the pre-amplifier 8 and the maximum detector 9. A special on/off switching logic 15 with contact sensor 16 is provided for this purpose.
When initially the mouse M is switched off, the first contact with the mouse causes the feed voltage for the electronic circuit to be switched on (feed element 14) and thereafter the infrared transmitter 7 transmits a sequence of infrared pulses to the base station B, indicating to the station B that the mouse M is in operation and that position determination should be carried out. If no further contact is then made with the mouse M within a certain time amounting to a few seconds, say four seconds, then a switching off bit is inserted into the infrared pulse sequence, whereby the mouse M is dismissed from the base station B and the feed element 14 is switched over to stand by.
The base station B consists, as illustrated, of the ultrasound transmitters or transmitter transducers 1 and 2, the calculator 4, the infrared receiver 5, two electric pulse transmitters 17 and 18, a digital logical part 19 and two counters 20 and 21. In addition, there are provided two connections 22 for the current supply, a connection 23 for grounding the base station B and a connection 23' for a calculator.
Each time a measurement is to be carried out, the electrical pulse transmitters 17, 18 receive a trigger signal S which starts the corresponding counter 20 or 21. The electrical pulse transmitter 17 or 18 produces a suitably shaped pulse which is transmitted to the associated ultrasound transmitter 1 or 2 and causes an ultrasound pulse I to be sent out. The base station B then receives an infrared signal from the infrared transmitter 7 in the mouse M, and this signal is converted into an electrical signal in the infrared receiver 5. This electrical signal reaches the corresponding counter 20 or 21 to stop this counter and in addition is transmitted to the digital logical circuit 19 which decodes the information
of the infrared pulse sequence and reads it into a register from which it can be read out by the calculator 4.
As already mentioned, the processes described above take place successively for the two ultrasound transmitters 1 and 2, i.e., the time of travel of sound from the 5 ultrasound transmitter 1 to the mouse M is first noted and thereafter the time of travel from the ultrasound transmitter 2 to the mouse M. Again, as already mentioned, the sound pulses transmitted by the ultrasound transmitters 1 and 2 have approximately the form of a 10 sine wave of one half or one whole period. These sound pulses should, of course, be as inaudible as possible.
It has been found that this requirement is very satisfactorily fulfilled by a sound pulse having the form of a sine wave of exactly one whole period. The loudness of 15 the "bang" released by the ultrasound transmitters 1, 2 can be reduced to such an extent by using a sound pulse form of this kind that it is masked by the normal sound level in an office and is hardly noticed.
This voltage pulse is produced by the electric pulse 20 transmitter 17, 18 which thus has the function of producing an electrical signal of a suitable form for the ultrasound transmitter 1 or 2 after a trigger pulse has been received by the calculator 4. This electrical signal is a rectangular voltage pulse having an amplitude of 25 about 200 V and a duration of about 1.5 microseconds. This amplitude is necessary to ensure that the sound pulse transmitted by the ultrasound transmitter 1, 2 will be sufficiently powerful, and the form of the sound pulse arises from the properties of the ultrasound trans- 30 mitters 1, 2.
The ultrasound transmitters 1, 2 as well as the ultrasound receiver 6 of the mouse M basically consist each of a condenser with an air gap and one solid and one movable electrode. The movable electrode consists of a 35 plastic foil which is metallized on one side and is stretched by its other side over the fixed metallic electrode. The conductive surface of the plastics foil and the solid electrode thus form a condenser. When a voltage is applied to the electrodes of this condenser, the plas- 40 tics foil forming the movable electrode is stretched and thereby sends out an acoustic wave front. Conversely, the acoustic wave front received by the ultrasound receiver 6 of the mouse M produces a movement in the plastics foil which gives rise to a corresponding change 45 in voltage between the electrodes of the condenser. The theory and properties of ultrasound transducers will not be further discussed here but reference may be made to U.S. Pat. No. 4,317,005 to de Bruyne and to the publication, "An Ultrasonic Radar Graphic Input Table" by P. 50 - de Bruyne in SCIENTIA ELECTRICA, Vol. 30, Fasc. 1 (1984), pages 1 to 26, and the literature references given in this publication, all of which are incorporated herein by reference.
For satisfactory resolution, the two ultrasound trans- 55 mitters 1 and 2 should be placed at a certain distance D apart (FIG. 2). In one embodiment which has been tried out in practice, D=40 cm. For further details of the ultrasound transmitters 1 and 2, see FIG. 4 and the corresponding part of the description. 60
The infrared receiver 5 has the function of transmitting an electrical signal for each infrared signal received. This electrical signal is used to stop the corresponding counter 20 or 21 and to determine which items of information are transmitted from the mouse M and 65 hence which of the keys Ti and T3 is depressed.
Infrared receivers of this kind are currently manufactured in integrated form for entertainment electronics.
They are suitable for data transmission and substantially insensitive to disturbances. Even better results are obtained by using a maximum detector (similar to that used for the detection in the mouse M of an electrical signal corresponding to an ultrasound pulse I) for the reception of the infrared signal.
The digital logical part 19 serves mainly for decoding, i.e., for serial/parallel conversion of the received infrared data and their transmission to the calculator 4. The calculator 4 serves, inter alia, to conduct the trigger pulses for the ultrasound transmitters 1, 2 to the pulse transmitters 17 and 18 and to start the counters 20,21. In addition, the calculator 4 supplies the pulse repetition frequency, i.e., the frequency with which the individual sound pulses are transmitted. The two counters 20 and 21 preferably count with a frequency of 10 MHz and therefore enable time to be measured to an accuracy of 0.1 microseconds.
The measurement of sound travelling time from the ultrasound receivers 1, 2 in the base station B to the mouse M is illustrated by a set of time graphs in FIG. 3 in which the time axis (abscissa), however, is not true to scale.
FIG. 3 shows in line a the trigger pulse S (FIG. 1) transmitted from the pulse transmitter 17 or 18 to the ultrasound transmitter 1 or 2 after receiving a starting command from the calculator 4. This trigger pulse is rectangular in form with an amplitude of 200 V and a length of 1.5 microseconds. The starting command of the calculator 4 at the same time starts the corresponding counter 20 or 21 (line g).
The trigger pulse S of the pulse transmitter 17 or 18 causes a sound pulse I in the form of a sine wave segment illustrated in line b to be transmitted from the ultrasound transmitter 1 or 2. The length of the sound pulse I is 6.5 microseconds. The sound pulse I triggers a corresponding electrical signal in the ultrasound receiver 6 of the mouse M and this electrical signal is transmitted to the maximum detector 9 after amplification in the pre-amplifier 8. The output signal of the pre-amplifier 8 is indicated by I' in line c.
The maximum detector 9 detects the first maximum of the signal I' with the aid of the positive voltage change in this signal immediately following the maximum. As shown in line d, this voltage change produces in the maximum detector 9 an approximately trapezoidal signal P with a steep ascending flank. According to line e, this steep ascending flank releases the starting bit of the infrared pulse sequence IR of the infrared transmitter 7. The individual pulses of the infrared pulse sequence IR have a length of 10 microseconds and the distance between their successive centers is 64 microseconds.
The infrared receiver 5 converts the received infrared pulse sequence IR into a TTL-signal R (line f) whose positive flank stops the counter 20 or 21 (line g).
FIGS. 4 and 5 show an ultrasound transducer UW, which with slight modifications may be used either as ultrasound transmitter 1, 2 or as ultrasound receiver 6 (FIG. 2). FIG. 4 is a top plan view (with the upper part removed) and FIG. 5 is a view in section, both on a scale of about 2:1.
The ultrasound transducer is a cylindrical, so-called Sell transducer and basically it consists of a condenser with an air gap, one electrode of this condenser being fixed whereas the other electrode is movable. The cylindrical form on this transducer increases the accuracy of measurement of the travelling time of the ultrasound