This invention relates to a tachograph and more particularly to a tachograph in which a correction can be made for any phase difference between the movement of a chart carrier (such as a turntable) and a time indicator (such as a clock) in order to ensure that time markings on a tachograph chart, moved by the carrier, correspond with the time on the indicator.

Current tachographs record, for example, speed, distance, driver duty and other vehicle information on a paper chart, which is mounted on a turntable, the turntable being rotated once, every 24 hours, by a mechanism which also drives the pointers of a conventional analog clock. (Sometimes the turntable is rotated once every 25^ hours, e.g. with tachographs used for 7-day recording periods.) The clock is normally mounted on the front of the tachograph case, so that it iε clearly visible by the driver and the clock face is usually hinged to provide access to the turntable for inserting and removing charts. The turntable and the clock are mechanically linked so that they are driven synchronously and so that time markings on the chart (with respect to a time datum) are the same as the time indicated by the clock; any adjustment of the clock also being applied to the turntable. As the mechanisms of the clock and the tachograph are mounted in the same outer casing, sufficient room must be provided on the dashboard of the vehicle to accommodate such designs of tachograph. This imposes restrictions on the design of dashboards for manufacturers of commercial vehicles.

In view of the above, commercial vehicle manufacturers often request separation of the recording mechanisms of a tachograph from the display mechanisms so that, for example, a normal speedometer/odometer could be mounted on the dashboard, whilst the more cumbersome recording system could be mounted elsewhere, in a vehicle cab. This would greatly

simplify dashboard design and allow for a lighter weight and smaller instrument panel. Resiting of the recording system could also make it more accessible for changing charts. In such a system, if the clock formed part of the display, it should conform to EEC 3821, which states that a driver must have a clear view, from the driver seat, of the speedometer, distance recorder and clock. However, if the clock is remote from the turntable, mechanical synchronisation is a problem and this can be difficult to achieve in a cost effective manner. Whilst electronic positioning and control is possible, for example, using a shaft encoder attached to the turntable, to determine its angular position, and by using this information to control the clock, such a system would be complex and expensive. The invention seeks to solve this problem in a more simple and cost effective manner.

The invention provides a tachograph comprising a time indicator; a chart carrier movable with respect to time and drive means for driving the chart carrier synchronously with the time indicator whereby, when a chart is located on the carrier, time indicated by markings on chart correspond with the time shown on the indicator; characterised in that synchronising means are provided (a) for sensing the position of the carrier with respect to time, (b) for providing a correction signal to correct for any difference between the actual position of the carrier and its position corresponding with the time on the indicator; and (c) for causing the chart carrier to move automatically to a position corresponding with the time on the indicator.

The chart carrier is usually a turntable which is rotated, about an axis, by (e.g.) a stepper motor. However, circular motion is not essential as a chart could be mounted on a carrier for linear movement.

The difference between the chart carrier position and

real time can be described as a phase error. Phase errors can occur in various ways but within the context of one embodiment of the invention, a phase error occurs when the chart carrier is disengaged from the chart carrier drive to enable a chart to be inserted in the tachograph and made ready to record vehicle information. For example, the tachograph may be of a drawer-type of design, where the turntable can be slid, like a drawer, out of a casing, thereby disengaging the chart carrier drive, to enable a chart to be placed on the turntable. The drawer is then closed to re-engage the drive and to enable recording to take place. Usually, the (circular) chart fits over a pear-shaped boss on the turntable, so as to orientate time markings on the chart with the angular position of the turntable (i.e. with respect to a time datum) . The angular position is referenced to a time datum, usually the recording pens, so that when the chart is moved, time markings on the chart, represent the actual time, with respect to the datum. In some cases, this datum will be visible to the driver so that he can see immediately that the time markings on the chart "tell the correct time". However, this is not always possible, for example, in the case of the latter "drawer" construction, it would not then be possible to check that the turntable is correctly synchronised with time. If the angular position of the turntable is not "correct", because the time markings on the chart show a different time to that on the clock, the turntable would need to be rotated clockwise (or anti¬ clockwise) so as to bring the time markings on the chart into synchronism with the time shown on the clock, thereby reducing any phase error to zero. It would not be necessary to disengage the turntable drive to achieve this. For example, the turntable is usually driven by a stepper motor, having a shaft that rotates one step at a time (slowly) in order to rotate the turntable, through a given small arc of movement, for each corresponding interval of time. This is a comparatively slow movement. However, in an embodiment of the invention, the stepper motor is driven

(fast) continuously, so that the turntable then rotates quickly into the required position of corresponding time where it is driven synchronously. Besides rapidly synchronising the chart carrier drive, this embodiment enables any alteration in the time shown on the clock to be transferred automatically to the turntable. This is useful, for example, in moving from BST to GMT (and vice versa) , when using the tachograph in summer and winter.

In the preferred embodiment of the invention, which employs a turntable as a chart carrier, the turntable is provided with means which represent different angular positions. Such means may be, for example, tabs or protuberances having different arc lengths, each of which represents a predetermined angular position. These tabs are arranged circumferentially about the axis of rotation and on the underside of the turntable. Opto-sensors produce beams which are intercepted by these tabs. When the turntable is rotated, the length of the tab is sensed so as to generate a signal which represents the respective angular position of the turntable. Alternatively, other angular or position sensing means may be employed. It would also be possible to read markings on the chart, so as to determine the position of the turntable. This enables the position of the carrier to be sensed with respect to real time, because the chart is always located on the turntable in a reference time position. For example, the chart has a pear-shaped hole that fits over a boss, having the same shape, on the turntable. Other means of referencing the chart to a reference time position could be used.

The time indicator can be a digital or an analog clock. The clock has an output, for example, pulses representing real time. However, any suitable inputs/outputs can be used which represent time. A microprocessor on-board the tachograph, receives inputs from the means for sensing chart carrier position and from the time indicator, and it then calculates any phase difference before producing an output

signal to drive the turntable to its correct angular position. In the correct position, the time markings on the chart correspond with the real time shown by the clock.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

Fig. 1 is a perspective view of a drawer-type of tachograph in which the invention can be embodied,

Fig. 2 is a plan view of a turntable used in the tachograph of Fig. 1 and showing (in broken line) tabs on the underside of the turntable,

Fig. 3 is a section, on line 3-3 of Fig. 2.

Fig. 4 is a schematic circuit diagram.

Referring to the drawings, a tachograph 1 comprises a turntable 2 which is rotatably supported on a slide or drawer 4 which slides into and out of a housing 5. The circumference of turntable 2 is provided with teeth 3 which engage worm gear 6, fast with a shaft 7. A stepper motor 9 rotates shaft 7 whereby turntable 2 moves through given angular increments. When the drawer 4 is open (Fig. 1) the teeth 3 are disengaged from worm gear 6. However, when the drawer 4 is closed, the teeth engage the worm gear 6 (Fig. 2) .

A pear-shaped boss 12 fast with turntable 2 fits a similarly shaped hole in a tachograph chart (not shown) when placed on the turntable 2. The boss 12 ensures that each tachograph chart is located on the turntable 2 so that time markings on the chart are referenced to the same datum. This datum is usually, for example, the recording pen (or pens) with respect to which the turntable 2 rotates. Time markings on the periphery of the chart, opposite the datum, usually show the same time as a digital or analog clock on

the fascia 13 of the tachograph. (A visible datum is not essential, because the turntable 2 rotates the chart (e.g. over a 12 or 24 hour cycle) with respect to any relatively fixed point. Rotation could also be over alternative periods, e.g. as in the case of using 7-day charts.)

Clock 8, which may have either an analogue or digital display, is of conventional construction and provides an indication, to the driver, of the time of day. It also provides an output representing the time, to a micro¬ processor (17) on board the tachograph. This output could be a pulse output which can be processed by microprocessor 17, but other timing signals may be used.

The underside of turntable 2 has a plurality of protuberances, tabs or flags 14, which have different arcuate lengths. The flags 14 pass between parts of an optical switch 15 (of known construction) . Switch 15 may comprise, for example, a transmitter and a receiver whereby an optical beam is interrupted when one of the flags 14 passes by the switch 15 as the turntable 2 rotates. The switch 15 effectively turns a counter 16 (Fig. 4) on and off so that, as the flag 14 passes the switch the counter counts the number of drive pulses fed, via drive pulse generator 18, to the stepper motor 9. Thus, during the interval that the flag interrupts the beam, the length of the flag is effectively "measured". As the lengths of the flags are different and as each respective length represents the respective angular position of the turntable 2, with respect to a given datum, the angular position of the turntable 2 can be sensed and appropriate signal supplied to the on¬ board microprocessor 17. The microprocessor is programmed to determine if any phase error then exists between the angular position of the turntable 2 and the real time shown by the digital clock 8. If such an error exists, the microprocessor causes drive pulses to be sent from drive 18 to the stepper motor 9 to cause the turntable to be driven rapidly to a position at which the time markings on the

tachograph chart correspond with the time shown on the digital clock 8, whereby the phase error is reduced to zero.

In practice, some intermediate portion of a flag may intercept the light beam of the opto-switch 15 when the turntable drive gearing 9a is re-engaged after closing the drawer. However, the microprocessor 17 is programmed to respond to this condition. For example, drive pulses can be generated and supplied to the stepper motor 9 so as to rotate the turntable first in a clockwise direction, until one end edge of the flag no longer interrupts the beam of the opto-switch. The stepper motor is then reversed to drive the turntable anti-clockwise to a position where the other end edge of the flag no longer interrupts the beam. The total length of the flag can thus be "measured" by the microprocessor 17 in terms of stepper motor drive pulses. Microprocessor 17 can be programmed so as to identify precisely the angular position of the turntable with respect to the number of drive pulses that correspond with the arcuate length of each tab and with respect to the angular position of each tab. The microprocessor can therefore cause the stepper motor to be pulsed until the turntable reaches the precise target position of time synchronisation. This target position can be located (or verified) by counting an appropriate number of stepper motor drive pulses from a leading edge of the flag nearest the target position to the actual target position (i.e. which may be intermediate the end edges of the flag) . Thus, the micro¬ processor can first determine which is the nearest flag (approximate position) and then use the opto-switch, if required, to sense its leading edge before counting down an appropriate number of pulses from the leading edge to the target position.

The microprocessor 17 can also be programmed so that it determines which direction (clockwise or anti-clockwise) is the faster route for rotating the turntable from an "out- of-phase" to a "synchronised" target position.

Clearly, the microprocessor may be programmed to store data and to process pulses in different ways in order to cause the stepper motor to be driven continuously (faster) for rotating the turntable to a position synchronised with time. However, those skilled in the art of digital processing will understand how such processing can be used to perform respective embodiments of the invention.

A proximity switch (not shown) also responds to drawer closure so as to cause stepper motor 9 to be energised, in response to a microprocessor output, whereby the turntable 2 is rotated through a sufficient arc of rotation to enable the (nearest) flag 14 to be measured. Such rotation will also occur if a "gap" between the flags 14, is between switch 15. Reception of the light beam may be used, in this instance, to initiate a motor "on" signal to drive the turntable 2. (The length of arcuate gaps can alternatively be measured to determine turntable position.)

Slots 10a, 10b, on the front of case 5 receive respective cards lla, lib. These cards may be of different types, one preferably being a "smart card", which is capable of both recording information, as well as enabling information to be read. The cards lla, lib may contain information relating to the operation of a vehicle and, in particular, to identify and authorise of the driver. When either card lla, lib is inserted, a card reader (not shown) within the casing 5 scans the card and produces appropriate signals for use by the microprocessor. Data read from the smart card may include, for example, a time reference which "tells" the microprocessor 17 which time zone the vehicle is in. Thus, if the turntable is synchronised with GMT (which may be displayed on the clock 8) , the microprocessor 17 will cause the turntable to be rotated by an appropriate angular amount to synchronise the tachograph chart markings with the real time in a country having a time zone different by, e.g. an integral number of hours from GMT. This (initial) phase correction is an "offset" for the respective time zone (or

zones) in which the vehicle is driven and thus automatically compensates for time zone differences. The microprocessor may, in this case, run its own GMT clock for reference purposes. The offset can be taken into account with any phase error (due to disengaging the stepper motor drive) so that only one phase correction is made as described below.

In operation, the driver presses a button to cause the drawer to open and then places a tachograph chart on table 2. He then presses the button to cause the drawer to close and this movement causes worm gear 6 to engage with the teeth 3 on the periphery of turntable 2. (Alternatively, the drawer is manually closed and the button is used to open it.) The stepper motor 9 is then energised so as to cause the turntable 2 to rotate until one of the flags 14 intercepts the beam of light in the optical switch 15. At this point, a counter (not shown) is actuated in order to count the number of pulses supplied to the stepper motor as explained above. The digital clock 8 also provides an output representing the correct time and thiε information is used, by the microprocessor, together with the turntable position information, to calculate the number of steps that the stepper motor 9 has to make, so as to rotate the turntable 2 to its correct "in phase" position. The time markings on the chart correspond with the time shown on the clock 8 when the turntable is "in phase" with the clock.

The above form of tachograph imposes less restraints on the designer of a dashboard of a commercial vehicle. A further advantage is that the design greatly reduces the possible of tampering, by the driver, or some other person, since the chart is automatically rotated into the correct time position when the drawer 4 is closed.