To whom it may Concern,
I would like to inquire on some advice and wisdom on the solution to a problem I am having with ADXL326 3AXIS ACCEL BREAKOUT BRD using one axis only to provide an analogue voltage DC input signal into a Lascar SGD43A touch screen to calculate my speed reference, especially at 0 km/h.
This speed reference signal is to be part of the internal programmed software interface to achieve a zero speed that can be used to toggle another command sequence.
I have built a electronic brake testing instrument using a Lascar SGD43A touch screen with support sensors to measure the deceleration in a motor vehicle during emergency braking.
Currently, I have installed a GPS speed sensor (SAAS) which provides a pulse range (1 > 50,000ppk) with a speed range (1km/h > 300km/h) connected to a Status SEM1600F Frequency to Analogue Converter ( -5VDC to +5PVDC Output) that connects to one of the analogue inputs on the Lascar SGD43A Touch screen instrument.
The way basically the brake test instrument works is that the GPS sensor via some devices delivers an analogue input signal into the Lascar SGD43A touch screen which translates this to an actual reference speed in Km/h. A further pressure sensor which sits on the vehicle’s brake pedal is also connected to another input in the Lascar SGD43A touch screen. At speed, say 100km/h, once the vehicle brake pedal is activated the Lascar SGD43A which then looks for a zero (0 Km/h) speed reference and using its internal software and preprogramed mathematics calculates the deceleration in m/sec².
The problem is that everything is working reasonably well, except that there is a lag (delay) in the delivery of the GPS speed sensor. This delay/lag is a variable not constant due to poor GPS reception, physical location on the day. Delay Example: Like in a typical digital speedo in a car where after stopping the vehicle the speedo takes a second or two to display the zero-reference speed (0 km/h).
To address this GPS senor lag time error, it was my intention to install an ADXL326 3AXIS ACCEL BREAKOUT BRD directly with just one of the axis analogue output into the Lascar SGD43A Touch screen and using the internal clock and software take samples and calculate the speed and in particular 0 km/h at rest. I don’t want to interface an Arduino and alleviate any computation time variables it may generate.
However, after installing the ADXL326 3AXIS ACCEL BREAKOUT BRD, I am not sure on the correct formula to use for the internal software and time clock.
SAAS GPS Specifications
Operating voltage range = 8V-36V Speed Pulse rate = 1 > 50,000ppk Speed Range = 1km/h > 300km/h Accuracy = 0.02% deviation
SEM1600F Status Frequency Convertor Specifications
Input Type: Frequency Range = 0.01 to 65000 Hz Min measuring Value = 0.01 Hz Min cut off = 0.01 Hz Min pulse width = 50 uS Sample Time = 0.1 S I Output Type: Output Types = Current /Voltage General: Update time = 100 mS Response time = 200 mS
Lascar SGD43A Touch Screen Specification
4.3” TFT with 262k colours Touchscreen Capacitive Resolution 480 x 272px Processor Freescale i.MX283 (454MHz, 32bit, ARM 9) Analogue Inputs 4 x ±40V or 4-20mA (16bit ADC with 0.05% ±1mV typical accuracy*) Digital I/O 8 x DIO, 2 x open-collector alarm outputs, 4 x 8 bit PWM outputs Serial Buses RS232, SPI**, I2C** Expansion Boards*** RS485, Ethernet Memory 1Gbit DDR2 SDRAM and 2GB SD card Operating Temperature 0 to 40°C (32 to 104°F) Supply 5 to 30V d.c. (400mA typical at 5V d.c.)
Therefore, it would be appreciated if you can advise what is the best way forward to utilise the ADXL326 3AXIS ACCEL BREAKOUT BRD and the formula requirement for the Lascar SGD43A touch screen to achieve a speed reference for this application.
Look forward to your reply.
John Oste

If the objective is to measure deceleration, an accelerometer such as the ADXL326 can do so directly.

Estimation of speed using an accelerometer is troublesome since it requires integration of acceleration data. Any offset errors in the measurement will also be integrated, leading to substantial errors in estimated speed.

Furthermore, rotation around any of the three axes will result in corruption of speed estimates derived from linear acceleration data. Some variation in pitch at least is to be expected from the environment described, if not yaw and roll also; making reasonably accurate speed estimates from inertial measurements in such a scenario would call for use of a 6-axis inertial measurement unit (IMU).

The analog outputs of the ADXL326 can interface to the analog inputs of your touch screen device directly. Because their scale is proportional to the supply voltage provided to the part however, it is also necessary to measure that voltage in order to to interpret the sensor’s outputs with maximum accuracy.