Technical Article: Lap Timer
Live lap times and section times are essential for a driver when monitoring their performance during competitive racing. MUR has used a GPS system the past few years to calculate lap times, with varying degrees of accuracy. The update rate of this commercial system is around 10Hz, which means that the system error results in inaccuracies of up to 0.1 seconds of driving time. The 2015 MUR electrical subteam has designed a more accurate system using pairs of LED emitters and infrared receivers.
The lap timer makes use of several beacons placed in strategic locations around the track. Each beacon is encoded with a special message signal. By implementing an FSK (Frequency Shift Keying) theory we are able to generate the same pause shape but a different frequency. The infrared receiver sits on top of the car and is connected directly to the data logging system and the GPS system. The data process unit inside the receiver recognises the different messages sent by these beacons.
There are several system performance and power consumption requirements for the lap timer:
- Directional infrared LED transmitters with a narrow beam angle, a high light intensity and low power consumption;
- An infrared receiver with a long-distance detection region and high sensitivity, as well as a built-in band pass filter to accept only certain frequencies and improve the robustness of the system;
- A high speed microprocessor with a built in frequency counter;
- A switch buffer to isolate the processor output and the data logging system.
After considering both the cost and performance for the requirements above, the following components were selected:
- Osram Opto SFH 4545 (RS665-5518) with beam angle smaller than for the infrared LED, with a full load power consumption of 250mW and wavelength 950nm.
- Vishay, 38 kHz IR Receiver (RS700-5308) with peak sensitivity at 950nm, 3dB wavelength bandwidth 180nm which matches the IR transmitter. Peak sensitivity at 38 kHz, which perfectly avoids pink noise. 3dB cut off bandwidth ~0.2f with a centre frequency of 38KHz. Detection region up to 45m.
- NE555D, Precision Timer (RS785-818). By using a stable mode, adjustable digital pause shape up to MHz can be generated with tunable duty cycle, low power consumption (~100mW) and fast rise/fall delay.
Figure 1: Modulated Message Signal at 38KHz
- NXP 74HC08D, 652, AND Logic Gate (RS113-0124): Acts as mixer that modulates the message signal and the carrier signal.
- SN74HC244DW, 8-Bit Buffer, (RS663-2026) for isolating the receiver output and the data logging system
- Arduino Micro (RS779-8864) as the processing unit. A proxy board was made and tested. Total power consumption is below 100mW at 5V input which is close to the design consideration. We can last up to 4 hours by using a rechargeable 1500mAh AA battery provided by RS. The following is some data of the lap timer collected at one of our track days.
Figure 2: Lap timer data
The red dotted line indicates the lap beacon which sits beside the track. This can also be seen on the GPS map on the left of the figure. The distance between the track and the beacon is approximately 5m.
After a series of successful proxy board tests, PCBs for both Tx and Rx were designed and tested using the components we got from RS. The 3D view of the PCB board can be seen below.
Progress is ongoing, and the number and variety of components that RS have on their website has really helped us with our design and manufacture. Thank you RS for your continued support!