The digital pulse output has the advantage of having the lowest latency - typically this is below 2ms - but it is only useful for measuring speed (frequency) and distance (counting pulses). The other advantage of the pulse output is that it can be used to replace other common speed sensors, for example fifth wheel, optical speed sensors and GPS-only sensors. In order to allow it to replace as many existing sensors as possible, with minimal change to the data acquisition system, the pulse output has been made extremely configurable, as described below.
There are two modes of pulse output: 1) fixed pulse width, or 2) 50% duty cycle. In both cases, the pulse frequency is proportional to speed and the pulse count to distance. The number of pulses per meter may be set using the configuration software. The two pulse modes are shown in the figures below. The fixed width and 50% duty cycle modes are shown in figures 7 and 8 respectively, at the same two faster and slower speeds. In the fixed width mode, the high time of the pulse is always constant, and may be varied from 10 to 1000μs using the configuration software. In the 50% duty cycle mode, the pulse high and low times are equal at any given speed, and the minimum allowable high or low time is 25μs.
For both modes, the number of pulses per meter output may be configured using the configuration software to between 1 and 400. A higher value of pulses per meter will give greater resolution, but will saturate at a lower speed. A lower value of pulses per meter will give lower resolution, but will allow a higher speed to be reached before saturation occurs. The configuration software shows the maximum speed that can be represented before saturation for any entered value of pulse configuration.
For both pulse modes, the low and high pulse levels are 0 and 5V respectively. 50Ω of output impedance is provided by the SPEEDBOX20 in order to eliminate “ringing”.
Figure 7: Pulse output at two speeds for the fixed width pulse
Figure 8: Pulse output at two speeds for the 50% duty cycle pulse