Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-08-03
2001-04-17
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S147000
Reexamination Certificate
active
06219624
ABSTRACT:
TECHNICAL FIELD
This invention relates to vehicle speed measurement and, more particularly, to a method of vehicle speed measurement that provides an improved response to slow speeds and loss of speed signal.
BACKGROUND OF THE INVENTION
Instrument clusters used in automobiles commonly use microprocessors to measure the vehicle speed signal and display the speed on an air-core gauge. Two common measurement techniques are used in the prior art. One, a pulse period measurement technique uses a high-resolution timer to store the time when each speed pulse edge occurs. The pulse period is the difference in 2 consecutive values. The high-resolution timer typical in current instrument clusters are 16-bits with a 2 microsecond resolution. The maximum period that can be measured without considering overflows is:
Max Period=$10000*2 usec=131.072 milliseconds
If overflows are considered, then the period is a relationship between the time difference and the number of overflows accumulated. An overflow is a count of how many times the timer has overflowed from $FFFF to $0000. Using an overflow counter allows longer pulse periods to be measured. Pulse period measurement has the advantages of high measurement accuracy and fast response, but exhibits a “steppy” response at low speeds and a perceived pause upon signal loss.
A second measurement technique counts the number of pulses per time interval. A timer provides a time base for counting speed pulses. Each speed pulse count is placed into a finite length buffer and periodically summed yielding a frequency. This frequency is then converted to vehicle speed. While this technique produces a smooth response over the entire range, the resolution of this method requires extensive filtering to achieve acceptable values, severely affecting the response.
SUMMARY OF THE INVENTION
The present invention combines the benefits of both the aforementioned methods while eliminating the disadvantages. The present invention provides accurate speed measurement, fast response, improved low speed response, and quick response to loss of signal. According to the present invention an input capture timer interrupt responds to the rising edge of the speed signal and is used to synchronize an output compare timer interrupt that is scheduled to occur at a fixed time interval (every 2.048 milliseconds). Each time an input capture interrupt occurs a new measurement period is started. Each time the output compare interrupt occurs, a time of 2.048 milliseconds is added to a current speed period value. On the next speed signal interrupt, the difference between the captured time and the time that the last output compare interrupt occurred is calculated. This difference is then added to the current speed period value to obtain the period measurement. Each period measurement is input to a multistage speed buffer. If the period measurement reaches a threshold then a maximum period value is input to the buffer. The speed buffer is updated using the 2.048 millisecond interrupt and prevents the long delays associated with using timer overflows. The filtering used to calculate the speed is therefore not executing on ‘static’ data. This prevents the pause or hitch seen in the prior art pulse period measurement techniques when the signal is removed.
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Assouad Patrick
Mollon Mark L.
Visteon Global Technologies Inc.
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