PAVAS Data Logging

Operations during a rally stage are very time demanding of both the driver and navigator. In many instances, a video camera is used to the record the view from the windscreen to provide analysis data. The audio may also be recorded in the vehicle. This is very useful to review the rally stages but it is difficult to incorporate speed, distance and time readings as well. The PAVAS capability to log these values, provides a solution.

The Philosophy

The design of the data logging process is always a compromise of such requirements such as storage capacity and speed. Given that is always a finite limit to the storage media available, these two elements are key to a design. With regard to storage capacity, PAVAS uses a Secure Digital (SD) card with capacities of 4,8,16 or 64 Gigabytes. It also employs a data compression technique to reduce the storage requirements for each record. If a sample rate of 0.5 seconds is used, then 24 hours of data can be stored in approximately one megabyte of data. This allows many days of data to be stored on a card. The data is stored in a RAM based buffer, under interrupt. It is then written, periodically, to the SD card.

The main design requirement for the system is that it should be intelligent and require a minimum of interaction with the driver and navigator because, otherwise, it just adds more workload to the crew. As a consequence, data retrieval should also be intelligent to allow efficient access. Though it might be desirable, external inputs such as exact stage end are not provided.

The Parameters Required

The prime parameter to log is the Real Time Clock (RTC). This important parameter is used to identify stages and to set the sample interval. PAVAS uses an integrated software RTC since this also provides the sampling clock and, being concurrent, the Real Time value is locked to the sample time. Each data sample file stores the values for a day and the RTC value is set at the start of the rally day. PAVAS stores the last RTC value set to reduce the effort to set the RTC value, since the time of day will be possibly be the same approximate value each day. This makes setting the RTC simpler. PAVAS also reads this value as part of the start up procedure and sets the RTC. The third parameter required is the sample rate. This can be set from 0.2 seconds to 10 seconds. The recommended value is 0.5 seconds because this is commensurate with a typical driver reaction time. The last menu item is the Verbose mode. This is used for troubleshooting the data logging.The menu is called Set RTC Clock data sample/File.

Setting the RTC Parameters

The first sub menu item shows the current time which is constantly updated. It can be changed by moving the cursor and pressing the UP or DOWN keys. Once the cursor is moved, the time update stops. This allows the time to be set accurately to a reference such as a watch or GPS type device. The best method is to set the time value in advance of the current reference time and press the Select button when the two times correspond. After the Select key is pressed, the display will then move to the Sample Time Interval which can be set and then adopted by pressing the Select button. The last menu is the Day Log File number. This can be set from 0 to 199. This large range permits a whole year of rally day data to be stored. If the day Number is set to zero, the data logging is turned off. If the Data Log file does not exists, one is created. A further requirement is that the PAVAS unit remain powered up, otherwise the RTC value should be reset when power is applied again. The Real Time Clock (RTC) option provides additional hardware with a battery backed up clock function, so the time is retained when the unit is powered down. It automatically sets the Data Log Day file number at the commencement of each day.

The Stop Marker

As will be apparent in further descriptions, the stop condition is crucial to allow simple manipulation of the data. The data collection algorithm does not collect any further data when the vehicle is stopped. The first effect of this is that large amounts of irrelevant data are not collected. The second aspect is that it marks the end of a stage. Lastly, it marks the start of a stage by assuming the vehicle started the stage from a stationary position. The Stop Marker requires that the vehicle be stopped after a stage is completely. In practice, a timing check point is often located at the end of a stage and requires the vehicle to stop. In some cases,the vehicle may be waved through by the officials without requiring a stop. The vehicle should be stopped when safe and premissable to do so. In practice, the vehicle may make many stops, for example at crew rest breaks or queueing for the start of a stage. With all these stop conditions, a large number of “stages” will be apparent in the data collected. The post processing in the PAVAS Terminal (PAVAS Term) program can easily filter them out. An inadvertent stop during a stage can also be easily identified and edited out from the raw data.

Startup Display

On start up, PAVAS looks for an SD card in the card holder. If one is found, it's size is displayed on the screen. The absence of an SD card is also reported. PAVAS also creates the data logging sub folder, if it is not present. It looks for the Day Log file according to the number. If it is not present, it creates the file. If the RTC option is available the Day Log file number is automatically set.