The SPEEDBOX20 is provided with a configurable pulse output proportional to combined speed. This allows the unit to be used as a direct replacement for ‘5th wheel’ systems, which typically also output a pulse train. The frequency of the digital pulse is proportional to speed. Alternatively, the pulses can be counted if a measure of distance is required.
The pulse output has a very wide dynamic range from DC to 50kHz with exceptionally low error. Crucially the pulse output is also “glitch free”. The width of the pulses can be set between 10us and 1ms. Alternatively, a fixed 50% duty cycle may be selected to maximize compatibility with other hardware.
Tests to assess the accuracy of the pulse output are very difficult to carry out because they require a known constant speed. As this situation does not exist under normal test conditions a custom version of the SPEEDBOX20 firmware has been produced which enables the unit to fix the output at a particular known speed irrespective of the actual speed. The frequency of the pulse output is then measured using a dedicated desktop frequency counter, with an external high precision off-air standard as the frequency reference. Test results for a fixed 25us high time and 50% duty cycle pulse are shown below.
Measured vs. calculated pulse frequency for a range of speed and pulse per meter settings, for a fixed width pulse with 25us high time.
Measured vs. calculated pulse frequency for a range of speed and pulse per meter settings, for a 50% duty cycle pulse.
The frequency counter used for the tests described above measures the frequency over a period of 10 seconds for each measurement.
Noise and “glitch” testing
By using the NI6210 data-acquisition device, logged data can be viewed over time to check for any glitches or noise on the frequency output. Figure 14 shows a continuous sweep of frequency from 0Hz to 18kHz logged to check for glitches on the output.
SPEEDBOX pulse output frequency sweep
The five graphs below in Figure 15 show the measured pulse output of the SPEEDBOX20 at target fixed frequencies of 500Hz, 1kHz, 5kHz, 10kHz and 50kHz respectively.
SPEEDBOX20 pulse output frequency accuracy at five target settings
The pulse output frequency is highly accurate, with no significant error for a given steady-state speed, and there is no discernible noise present. Note that these tests measure only the stability and accuracy of the pulse output relative to the target frequency. They are not a measurement of the accuracy of the speed reading which is documented elsewhere in this report.
The pulse output has been designed to give a clean signal which is completely free from ringing with good drive capabilities. The pulse output is under-damped with a rise time of approximately 200ns and standard TTL logic levels (nominally 0V to 5V). The graphs below show the pulse rise and fall characteristics sampled at 60MHz.
SPEEDBOX pulse output rise time
SPEEDBOX pulse output decay characteristics