Most tachographs produced prior to 1 May 2006 were of the analogue type. Later analogue tachograph head models are of a modular design, enabling the head to fit into a standard DIN slot in the vehicle dashboard. This would enable a relatively easy upgrade to the forthcoming digital models, which were manufactured to the same physical dimensions.
The analogue tachograph head uses styli to trace lines on a wax coated paper disc that rotates throughout the day, where one rotation encompasses a 24-hour period. If the disc is left in the head over 24 hours, a second trace will be written onto the first, and so on until the disc is removed. It is an infringement of EU Regulation 561/2006 to use a disc for a period longer than it is designed for. Multiple overlapping traces may still be deciphered in the speed and distance fields, but it is far more difficult for the activity field where one trace can easily be obliterated by another. Analogue tachograph heads provide no indication to the driver of the need to change the disc.
Analogue data is retrieved visually, and can be assisted by manual analysis tools. Analogue discs can also be electronically scanned and analysed by computer, although this analogue to digital conversion process still requires human expert interpretation for best results, due to imperfections in the source disc such as dirt and scratch marks in the wax surface that can be incorrectly read as trace marks.
The analogue chart (EU)
The analogue chart must be EU type approved. The country of type approval can be found on the rear of the chart, i.e. a mark of E11 would indicate the chart to have been approved in the UK for use in the EU. The chart is manufactured out of heavyweight paper with a black printed face that is thinly coated with a white wax, upon which is printed a number of features. The surface can be scratched or rubbed to reveal the black paper underneath. This enables the traces to be made without the use of ink. The chart features a pear-shaped aperture in the centre, ensuring it is perfectly aligned upon insertion into the tachograph head. There is no facility to prevent it being inserted back to front, where the styli would be prevented from making contact with the wax surface.
The centrefield is used by the driver to store certain handwritten information. This includes the drivers name, the date(s) the disc refers to, the start and end odometer readings and the registration mark of the vehicle.
Three traces are made in the wax surface by the head. These traces are either made by three separate styli or a single multipurpose stylus.
The trace closest to the centrefield is the distance trace. The stylus moves up and down with distance travelled, producing a zig-zag pattern, often referred to as a 'V' trace. A complete deflection is created every 5 kilometres, and therefore each completed 'V' represents 10 kilometres travelled. By counting the zig-zags, the total distance travelled can be calculated and compared against the stated odometer readings in the centrefield. By comparing the end position of the trace for a particular day against the start position for the following day, it can be seen if the vehicle has moved in the intermediate period.
The trace in the central area is the mode trace. The driver's activity is displayed in this area, and is always displayed as either drive, other work, availability or rest. Earlier tachograph heads displayed the mode as a thin line in one of four concentric tracks within the activity band. These heads are known as manual heads as the activity was manually selected using the mode switch. Automatic heads succeeded manual heads, and differ from them in two main areas. Firstly, the automatic head will always display the drive mode when the vehicle is in motion, regardless of the setting of the mode switch. For this reason, the drive mode is no longer available to be selected by the mode switch. Secondly, the activity is displayed on the chart as a sequence of block traces of differing thickness. The rest mode appears as a thin line, availability as a slightly thicker line, other work as slightly thicker again and the drive trace being the thickest.
The trace closest to the outer edge is the speed trace. The disc is preprinted with a speed scale and the stylus produces a mark corresponding with the speed of the vehicle at any given time. It is important that the maximum speed (Vmax) specification of the chart matches that of the tachograph head for the speed to be correctly recorded. It can be expected that a high speed trace will correlate with a tightly spaced zig-zag pattern within the distance trace.
The disc is preprinted with a 24-hour scale that completes the outer circumference.
The rear face of the chart is printed with a grid that enables the driver to make handwritten additions or amendments to the information on the front.
