Interface device for the entry of data into an instrument of small volume responsive to body movement

The invention concerns an interface device for the entry of data into an trument of small volume such as a timepiece and comprises a static touch responsive sensor formed by a plurality of juxtaposed electrodes. An electronic logic circuit on the one hand receives signals emitted by the sensor representing the position of a finger on the sensor and on the other hand provides output clock pulses to a first and a second counter. Switching means in the electronic logic circuit enable the switching of the clock pulses to the first counter if the finger is moved over the sensor at a speed v.sub.1 less than a threshold speed v.sub.2 or to the second counter if the finger is moved over the sensor at a speed v.sub.3 greater than the threshold speed v.sub.2. The invention may be employed in small timepieces for time setting for example.

BACKGROUND OF THE INVENTION 
This invention comprises an interface device for the entry of data into an 
instrument of small volume such as a timepiece comprising a static touch 
responsive sensor arranged to be manually actuable with each position of a 
finger corresponding to at least one symbol which may take the form of a 
number, letter or special sign, the sensor being formed by the 
juxtaposition of N electrodes adapted to provide at least N-1 coded 
information items representative of the position of the finger on the 
sensor. 
The invention relates to U.S. patent application No. 968,917 filed 13th 
December 1978 and allowed May 1980, and which concerns an interface device 
for data entry for small volume instruments and for which it comprises a 
particularly interesting improvement above all in view of its application 
to timepieces. The above cited application starts with the idea of a data 
entry device capable of being manually actuated and furnishing information 
representative of the position of a finger on a sensor, (resistive or 
capacitive), such information being independent of the width of the 
finger. To arrive at such a result use has been made of a sensor formed by 
the juxtaposition of a plurality of identical electrodes providing at 
their output information according to a binary code representing the 
position of the finger on the sensor. 
The cited system may already provide an important improvement in the case 
where it is employed for the correction of a timepiece having an 
electro-optic display. Taking as an example a situation where the 
timepiece includes in addition to the main display of the normal 
timekeeping operation which appears permanently, a secondary display which 
may sequentially display a count-down timer, an alarm timer, a diary, a 
time zone, a date, etc.. All these several functional modes may be 
indicated by permanent signs engraved or transfered onto the interior 
surface of the glass. The touch sensitive key described in the above cited 
application may initially be employed to select the functional mode or 
programme. By moving the finger on said key a symbol is also moved which 
for instance may frame one of the permanently fixed signs and thus one may 
select the programme chosen when the finger is removed from the key. 
Should one then actuate a pushbutton the display corresponding to the 
programme as chosen will be transfered into the correction mode, this 
being indicated by blinking of the display. The correction may again be 
made by the touch sensitive key. If the display chosen includes a digit or 
a group of two digits for instance such may be augmented by moving from 
left to right the finger over the sensor, or diminished by the same 
movement but from right to left. If the number to be corrected is that of 
the minutes of a main display for instance, it will be appreciated that 
relative to known systems of correction by means of a pushbutton the cited 
device may enable attainment much more rapidly of the required correction, 
in the case where the timepiece has advanced several minutes since it will 
be sufficient to move the finger from right to left over a short distance 
in order to diminish the number and thus to bring it to the value of the 
timing signal. 
The time indications displayed by a timepiece generally include not one but 
rather two groups of associated digits. The principal display of normal 
time of day comprises at least the indication of minutes and hours. On the 
auxiliary display there may appear an alarm time or a time zone of which 
the first digit group may indicate the hour and the second group the 
minute. Also there may appear on this display a diary for which the first 
group may indicate the month and the second the date. In a different mode 
or programme the same basis is provided for display of the day of the week 
and of the date which may appear simultaneously. If the watch is provided 
with a count-down timer, the latter may indicate a lapse of time 
comprising associated indications of hours and minutes which it must 
count-down. For each of the timing indications mentioned above it is 
necessary to foresee two correction modes, the first acting for instance 
on the group of numbers or digits to the left and the second on the group 
to the right. In known watches the changing of a correction mode from one 
timing to another and from one group to the other for each timing 
indication is accomplished by means of a pushbutton as has been mentioned 
in the preceding paragraph, this requiring thus a multiplicity of 
actuations of the pushbutton and thereby a succession of tedious 
operations. 
The present invention proposes to overcome the above mentioned difficulties 
through correcting as a group each timing indication and the digit groups 
of which it is composed without the necessity of changing the correction 
mode for each of the groups one after the other. It takes advantage of the 
fact that the finger may be displaced at different speeds on the sensor 
and that means may be employed in order that by slow motion one group may 
be corrected and that through a rapid motion another group may be 
corrected. 
The device of this invention and the means employed for the realisation 
thereof are defined in the claims hereto attached.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a top view of the timepiece in a first functional mode according 
to the invention. It comprises a case 1 in which are mounted an electronic 
module for which one will note the display 2, a plurality 3, 4, 5, 6 and 7 
of pushbuttons and a sensor 100 formed from four electrodes 101 which are 
identical and juxtaposed side by side. The display 2, which may be formed 
from liquid crystal means comprises in addition to the permanent display 8 
of the normal time of day comprising three groups of two digits displaying 
hours 9, minutes 10 and seconds 11, an auxiliary display 12 which may 
display for instance the day of the week 13, the month 14 and the date 15. 
Signs 16 permanently fixed for instance by transfer under the watch glass 
may signal the functional modes which are possible, and to which one may 
switch the secondary display. For instance the sign A G 1 signifies diary 
1 and the sign D the date. A frame 17 formed from liquid crystals may 
signal in which mode the arrangement is operating. It will now be 
described how the time setting is effected for the main display, how the 
functional mode is selected and how new data may be introduced into the 
secondary display. 
Time setting 
The time may be corrected if for one reason or another the main display 8 
has advanced or is running slow relative to the standard time signal and 
also at the time of changing of the energy source of the watch. With the 
point of a ball-point pen a short duration pressure will be applied to 
pushbutton 7. This actuation causes the seconds indicating group of digits 
11 to blink, thus signalling that the correction mode has been entered for 
this group. At this moment a single application of the finger to any 
portion of the sensor 100 will reset the seconds to 0. Such application 
should coincide with the return to 0 to the seconds of the time signal. 
Following thereafter the pushbutton 6 will be actuated which has as its 
effect initially to stop the blinking of the group 11 then to cause the 
groups 9 and 10 (hours and minutes respectively) to blink together. These 
two groups will then be found in the correction mode. 
In order to correct the group 10 alone and in accordance with the 
invention, the finger will be moved over the sensor 100 at a first speed 
v.sub.1 less than a threshold speed v.sub.2. It is necessary to observe 
that from the moment that the finger comes in contact with the sensor the 
display will cease blinking thus facilitating the correction to be 
effected. The digits will be augmented if the finger is moved from left to 
right over the sensor and diminished if the finger is moved in the reverse 
sense. Since the sensor has a limited number N of electrodes (four in the 
example) it will not be possible to add or to remove in a single motion of 
the finger over the sensor more than 2 N-1 units form the displayed number 
(seven in the example). As may be seen from the cited patent application 
if it is necessary to modify the displayed number by more than 2 N-1 units 
it will be sufficient to lift the finger when it attains the end of the 
sensor and to repeat the operation starting from the other end and so on. 
As soon as the desired number has been attained the finger will be removed 
from the sensor, this having as consequence to freeze the last digit 
attained. 
The correct only group 9 and in accordance with the invention one will move 
the finger over the sensor 100 as a speed v.sub.3 greater than the 
threshold speed v.sub.2. As in the case of the preceding group the figures 
will be augmented if the finger is moved from left to right and diminished 
if the opposite sense is employed. Although it is theoretically possible 
at speed v.sub.3 as in the case of speed v.sub.1 as previously examined to 
modify the number displayed by 2 N-1 digits through running over the 
entire sensor, numerous trials have shown that a discrimination based 
entirely on the difference in speed was not sufficiently certain and that 
it was necessary to take into account differences in speed which occur 
along the route while the finger is moving over the sensor. For this 
reason in order to correct group 9 and in accordance with the preferred 
variant of the invention it is necessary not only to traverse the sensor 
at speed v.sub.3 but initially to touch the first electrode and to leave 
the last at a speed substantially equal to said speed v.sub.3 in order to 
add or subtract a single unit from the number of the hours. 
The means employed in the electronic module to correct the one or the other 
group are explained further on with reference to FIG. 3. 
As soon as the main display has been locked on the time signal by means of 
the measures which have just been explained, the correction mode will be 
terminated by pressing again on pushbutton 6. 
Selection of the functional mode 
Field 16 of FIG. 1 shows on two superposed lines the various functions 
possible and which may be adjusted in the timepiece. In the timepiece 
considered as example will be found in the upper line and from left to 
right symbols indicating successively the following functions: 
chronograph, count-down timer, alarm 1 and alarm 2; in the lower line and 
from left to right are to be found an acoustic time signal, diaries 1 and 
2, the time zone and the date. FIG. 1 illustrates the watch presently in 
date mode and the sign D is surrounded by a frame 17. On the auxiliary 
display appear the day of the week at 13 (FR=Friday), the month at 14 
(1=January) and the date at 15 (the 25th). Should one wish to have appear 
on the auxiliary display another function, for example that of diary 1 
shown in FIG. 2, the finger will be moved at speed v.sub.1 from right to 
left over the sensor 100 until frame 17 surrounds the sign A G 1. At this 
instant upon removing the finger from the sensor the frame will begin to 
blink. During the blinking time which might be set to be between 3 and 4 
seconds for example, one may actuate pushbutton 6 which will cause the 
desired function to appear on the auxiliary or secondary display. It is to 
be noted that if pushbutton 6 is not actuated during the above-mentioned 
time lapse the frame will return to its original position (to that shown 
in FIG. 1). Set to its new function the display indicates at 20 the sign A 
1 signifying "diary 1", at 21 "month" and at 22 the "date". In the watch 
considered as an example this diary indication will automatically appear 
on the auxiliary or secondary display whatever may be the date otherwise 
displayed by this display, during twenty-four hours, at the moment when 
the calendar date coincides with the diary date. It should be remembered 
here that the choice of the functional mode is obtained by moving the 
finger at the speed v.sub.1 over the sensor, the rapid speed v.sub.3 
having no influence on such choice. 
Correction of the auxiliary display 
To enter the correction mode for the auxiliary or secondary display, 
actuation of pushbutton 6 is required. 
In the case as shown in FIG. 1 and following a first actuation of 
pushbutton 6, groups 14 and 15 blink together. The timepiece is then set 
to the date (group 15) by moving the finger at speed v.sub.1 over the 
sensor and to the month (group 14) by moving at speed v.sub.3 as has been 
explained in the case of correction of hours and minutes. A second 
actuation of pushbutton 6 causes the group 13 representing the day of the 
week to blink which may then be changed by moving the finger at speed 
v.sub.1 over the sensor. A third actuation of pushbutton 6 will remove the 
entire display 12 from the correction mode and it will even erase the 
month indication 14 which is no longer desired. 
In the case of FIG. 2 one may act in the same manner on sensor 100 and on 
pushbutton 6 in order to correct groups 21 (month) and 22 (date) of the 
diary. Actuation of pushbutton 6 will bring the diary function into the 
correction mode, this effecting blinking of groups 21 and 22 together, 
while a further actuation will remove the display from this correction 
mode. 
The various manipulations necessary to correct the indications given by the 
timepiece have been explained above. The means employed according to the 
invention to correct one group of data rather than another based on the 
different speeds of movement of the finger over the sensor are now to be 
described having reference to FIGS. 3, 4 and 5. 
FIG. 3 shows the basic block diagram of the electronic circuit associated 
with the timepiece shown in FIGS. 1 and 2. 
According to the patent application cited hereinabove, this circuit 
comprises a digital sensor 100 provided with N juxtaposed electrodes 101. 
Outputs 103 are associated with each of electrodes 101. The electrodes 101 
of sensor 100 provide binary coded information to the outputs 103 
representing the position of the finger on the sensor. The N electrodes 
101 of such a sensor permit defining of 2 N-1 positions of the finger, 
i.e. 2 N-1 coded output information items, independently of the width of 
the finger. There are thus N positions corresponding to the N electrodes 
to which may be added N-1 positions taken respectively between each two 
adjacent electrodes. The 2 N-1 coded information items, provided by N bits 
may be transformed to M bits with M&lt;N in order to simplify the electronic 
circuit associated therewith and to eliminate the effect of the finger 
width. A transformation circuit is represented at 104 and receives at its 
input 103 the N bits provided by the sensor and provides at its output a 
code reduced to three bits 107. 
In accordance with the invention the outputs 107 of the transformation 
circuit (transcoder) are coupled to the input of a logic circuit 113, 
certain elements of which are arranged according to the cited patent 
application and which furnishes at its output clock pulses 124a, 125a 
respectively to the input of a first (115a) or to a second (115b) counter 
according to the speed of movement of the finger, and a control signal 
109a controlling the counting sense of said counters. Outputs 116a and 
116b of each of the counters feed utilization circuits 111a, 111b and the 
displays 110a, 110b relating thereto respectively. 
The logic circuit 113 includes as in the case of the previously cited 
patent application a comparator circuit 109 which may be of the type 74 C 
181 for example, to the inputs A entry of which are directly coupled 
outputs 107 of the transcoder 104 and to the inputs B entry of which via 
three D flip-flops 108 are coupled delayed outputs 107. The three outputs 
109a, 109b and 109c of the comparator assume the logic state 1 when the 
conditions at the input are respectively A&gt;B, A.noteq.0 and A.noteq.B. It 
is noted that the input A of circuit 109 represents the actual position of 
the finger on the sensor and the input B the immediately preceding 
position of the finger on the sensor. From this fact: 
(a) when condition A.noteq.0 is obtained the finger is in contact with the 
sensor, 
(b) when the condition A&gt;B is obtained the finger has been displaced 
towards the left. When not obtained it has been displaced towards the 
right or not changed position. This therefore defines the sense of 
counting of the reversible counters 115a and 115b, 
(c) when condition A.noteq.B is obtained there has been a movement of the 
finger over the sensor. 
The logic circuit 113 furthermore and according to the invention comprises 
three delay timing circuits 120, 121 and 122 having respective delays of 
.tau..sub.1, .tau..sub.3 and .tau..sub.2 ; AND gate 124 which is enabled 
if signals to the two inhibit inputs are at the 0 state with the direct 
input at the 1 state; AND gate 125 which is enabled if its three inputs 
are at the 1 state and a D flip-flop of the type D 123. All these elements 
are arranged according to the invention as shown in the block diagram of 
FIG. 3. It should be noted that the AND gates are well-known from the 
state of the art and that the timing circuits may be formed for instance 
each one by two integrated circuits of the type 555 coupled in series. 
To consolidate the ideas already presented it will be supposed that the 
timepiece which has been earlier described above shows on its auxiliary 
display, the function diary 1 for which the month and the date are to be 
entered (see FIG. 2). To the group 21 (month) corresponds reversible 
counter 115a to the group 22 (date) corresponds the reversible counter 
115b. The month is displayed at 110a and stored at 111a; the date is 
displayed at 110b and stored at 111b. Two cases may occur: 
1. Date correction 
According to the invention the finger should be moved at a speed v.sub.1 
less than the threshold speed v.sub.2 over the sensor in order to correct 
the group of digits representing the date (display 110b). 
When the finger enters into contact with one of the electrodes 101 of 
sensor 100 it will introduce at the input of comparator 109 a new coded 
value. As long as the D flip-flop 108 has not received the clock pulse it 
will maintain at outputs Q, corresponding to the coded value B at the 
second input of the comparator 109 the present value of A present before 
the contact of the finger on the sensor. The instant of contact is shown 
by t.sub.a on FIG. 4 which represents the operation of the arrangement 
during movement at slow speed v.sub.1. At this moment A has changed its 
value which is arbitrarily symbolised in the timing diagram by a line 
passing from a low state to a high state. At this same moment t.sub.a the 
condition A.noteq.B is realised and the output 109c goes from the 0 state 
to the 1 state. From this moment on the timing circuits 120, 121 and 122 
may generate a short pulse (for instance 2 ms) at the end of each of their 
respective delays .tau..sub.1, .tau..sub.3 and .tau..sub.2. A short pulse 
is emitted by the timing circuit 122 following a time .tau..sub.2, which 
is fixed for instance at 30 ms. This pulse via line 122a is conducted to 
the clock inputs of D flip-flop 108 thereby bringing the Q outputs of said 
flip-flop to the same state as that at its inputs D, and since the value 
of A has not changed it will be found that A=B and that the output 109c 
goes to 0. This switching is shown by arrow 130 on FIG. 4. It is necessary 
to note here that in the cited patent application the clock pulses for 
flip-flop 108 were supplied by an exterior generator at 100 Hz. The system 
according to this invention presents an interesting improvement through 
the fact that the clock is internal to the system shown by circuit 113 
thereby achieving complete self-synchronisation. The timing circuit 121 is 
of the type known as retriggerable. It exhibits the particularity of not 
providing an output so long as its input is at the 1 state. However if its 
input goes to the 0 state it may provide an output pulse at the end of its 
delay .tau..sub.3, for instance 25 ms, and which begins to be counted at 
the moment of the changeover to the 0 state. Such situation is shown on 
the timing diagram of FIG. 4 where it will be noted that a short pulse is 
emitted on line 121a following a delay .tau..sub.3, which runs from the 
moment when the line 109c has changed to the 0 state. On line 121a of the 
diagram small crosses are used to symbolize the fact that the timing 
circuit 121 may not furnish a pulse so long as line 109c is in the state 
1. A short pulse emitted on line 121a by the timing circuit 121 is 
transmitted to the clock input of the D flip-flop 123. The input D of this 
flip-flop is connected to the output 109b (A.noteq.B) of comparator 109. 
Since the finger is in contact with the sensor the condition A.noteq.0 is 
realised and the output 109b is in the state 1. This same state 1 will be 
found at the Q output of the D flip-flop 123 as soon as the leading edge 
of the short pulse emitted by the timing circuit 121 has appeared. This 
switching is shown by the arrow 131 on FIG. 4. 
It is also seen that to output A.noteq.B is connected a timing circuit 120 
which emits at its output 120a a short pulse at the end of its delay 
.tau..sub.1, which may for instance be of 25 ms. It may equally be seen 
that during the period separating t.sub.a from time t.sub.b, i.e. as soon 
as the finger has touched the sensor and rests stationary thereon, gate 
125 is never enabled since there is never a coincidence of 1 states on 
the three inputs via lines 109b, 120a and 123a as shown by the timing 
diagram of FIG. 4. Line 125a remains at the 0 state: no clock signal 
arrives at counter 115b and thus no new value appears on the corresponding 
display. The arrangement is desirable in practice since it enables 
avoiding untimely changes which are difficult to control. 
As has been mentioned above and in the cited patent application if the 
sensor is provided with N electrodes it will be possible to obtain a 
maximum of 2 N-1 coded information items at the output of the transcoder 
104 if the finger is moved from one end to the other of the sensor. As has 
just been explained in the preceding paragraph, it is desired that the 
first contact of the finger on the sensor will not result in the output of 
information. From the preferred arrangement according to this invention 
there will thus only be a maximum of 2 N-2 information items available at 
the input of counter 115b. For instance if the sensor is comprised of four 
electrodes a movement of the finger from one end to the other thereof will 
result only in the output of six information items. It is also possible to 
foresee an arrangement based on like principles whereby information is 
provided only by the transition between adjacent electrodes, in which case 
the maximum information items available will be limited to N-1. 
If now the finger starts to move on the sensor the moment will arrive when 
the value of A is about to change state. This moment is indicated on FIG. 
4 by the time t.sub.b. From this moment the fact that A is different from 
B will result in a state 1 on line 109c. As the lines 109b (the condition 
A.noteq.B is still present) and 123a are at the state 1 gate 125 will be 
enabled as soon as its input to which is connected line 120a goes to state 
1. This arrives after the delay .tau..sub.1. This situation is illustrated 
on FIG. 4 at time t.sub.x. The pulse appearing on line 125a may then add 
or subtract a single unit from the date counter 115b (according to whether 
A&gt;B or that A&lt;B). Several other clock pulses may thus arrive at counter 
115b if the finger continues to move over the sensor. 
At the moment when the desired symbol appears on the display, such symbol 
possibly being a digit, a letter or any special sign, the finger must 
leave the sensor in order that the data be definitely fixed either in the 
utilization circuit 111b or in the corresponding display 110b. This 
instant is indicated by time t.sub.c on the timing diagram of FIG. 4. 
Since the finger is no longer in contact with the sensor the condition 
A.noteq.0 is no longer realised and line 109b passes to the state 0. The 
short pulse appearing on line 120a at the end of delay .tau..sub.1 cannot 
traverse gate 125 since input 109b is at the state 0. A short pulse 
appearing on line 121a places the Q output (123a) of D flip-flop 123 in 
the same state as its D input i.e. at the state 0 since once again the 
condition A.noteq.0 is no longer realised (arrow 132). Thus in lifting the 
finger from the sensor new information is neither added nor subtracted in 
the counter 115b which is an advantage evidently desirable in practice. 
The timing diagram of FIG. 4 shows also that counter 115a to which 
corresponds the display of months never receives clock pulses. In order 
that the contrary should happen it would be necessary that at a certain 
moment line 120a would be at state 1 while at the same time lines 123a and 
109b were at the state 0. This situation however never arrives in the slow 
cycle which has just been described and as shown by the timing diagram. 
2. Month correction 
According to the invention the finger must move at a speed v.sub.3 greater 
than the threshold speed v.sub.2 over the sensor in order to correct the 
group of digits representing the month (display 110a). 
Reference should be made to the timing diagram of FIG. 5 in order to 
understand the operation of the arrangement. At time t.sub.a the finger 
comes in contact with the sensor, this bringing about as in the case of 
date correction, change to state 1 also of line 109b (A.noteq.0) and of 
line 109c (A.noteq.B). As before, short pulses issued from the timing 
circuits 120 and 122 appear on lines 120a and 122a. It was seen above that 
the short pulse coming from timing circuit 121 appears on line 121a at the 
end of its delay .tau..sub.3 if line 109c is at the state 0. In the case 
with which we are now concerned, the speed of movement of the finger over 
the sensor is so high that when the end of delay .tau..sub.3 arrives line 
109c is already at the state 1. No pulse appears thus on line 121a. When 
the finger leaves the sensor at the moment t.sub.d the condition A.noteq. 
0 is no longer satisfied and line 109b passes to state 0. The short pulse 
given by the timing circuit 120 appears normally at the end of its delay 
.tau..sub.1. Line 123a is at state 0 since no pulse has been applied 
thereto by 121 via the D flip-flop 123. Thus all the conditions are 
present to enable gate 124 which emits via line 124a a pulse to the month 
counter 115a. This arrangement is shown on FIG. 5 at time t.sub.y. 
It will be understood that counter 115b to which the date display 
corresponds does not receive a clock pulse in the case under 
consideration. Effectively in order that this should be the case it would 
be necessary that at a certain moment lines 120a, 123a and 109b be at the 
state 1 which is not the case as is illustrated by the timing diagram of 
FIG. 5. 
It may also be seen through examination of FIG. 5 that at high speed there 
can be only one clock signal 124a for one passage of the finger over the 
sensor since the signal appears only when the finger leaves the sensor and 
provided that it has maintained its high speed. This is a requirement 
imposed by practice for if, as in the correction mode at slow speed, one 
could at high speed and with a single movement introduce several 
corrections the arrangement would be totally lacking in precision and 
lengthy operations would be necessary in order to stop at the desired 
value. If during its movement the speed of the finger should descend below 
the threshold speed v.sub.2, the situation again will be as explained with 
reference to FIG. 4. It will be also noted that in order to enter the 
correction mode for counter 115a (month) it is necessary to contact the 
sensor at high speed. If such were not the case (low speed or 0) the 
timing circuit 121 will emit a pulse at the end of its delay .tau..sub.3, 
this having as consequence the enabling of counter 115b (date) as has been 
explained with reference to FIG. 4. Next, if the speed of the finger were 
to increase above the threshold speed v.sub.2 during the same movement the 
situation of the timing diagram of FIG. 5 will never arrive since line 
123a will be at the state 1, this preventing the enabling of gate 124. 
This constitutes an advantage since it prevents differences in speed 
during a single traverse sometimes to act on one counter and sometimes on 
the other. 
3. Considerations common to both corrections 
The choice of corrections between one group of digits and another depends, 
as one has seen, on the speed with which the finger is moved over the 
sensor. If it is moved slowly one group of digits will be corrected and 
with as many as six corrections for a single traverse. If it is moved 
rapidly another group of digits will be corrected and with a single 
correction for each traverse. 
The threshold speed v.sub.2 which is between the slow speed v.sub.1 and the 
high speed v.sub.3 is determined by the longitudinal dimension of the 
sensor. It has been seen that the presence or absence of the short pulse 
from the timing and delay circuit 121 at the end of its delay .tau..sub.3 
conditions the correction of one group to the exclusion of the other or 
vice versa. The delay .tau..sub.3, starting at the end of delay 
.tau..sub.2, the time to be taken into consideration for the threshold 
speed is thus .tau..sub.2 +.tau..sub.3. In a practical example of the 
invention the distance separating two coded information items on the 
sensor is 2.25 mm. In the same manner it has already been mentioned that 
the delays .tau..sub.2 and .tau..sub.3 are chosen respectively to be 30 
and 25 ms, this giving for .tau..sub.2 +.tau..sub.3 a period of 55 ms. 
Under these conditions the threshold speed v.sub.2 is on the order of 4 
cm/sec. 
It must also be recalled that in both cases counters 115a and 115b may be 
augmented or diminished by line 109a (A&gt;B or A&lt;B) in accordance with the 
sense of movement of the finger from left to right or from right to left. 
In conclusion it may be mentioned that the reversible counters 115a and 
115b may be obtained through the use of standard circuits known from the 
state of the art such as the type 4029. More generally these could also be 
in a micro-processor for which the soft-ware is arranged to provide an 
equivalent to what is normally expected from a standard counter.