Rangeable Speedometer/Odometer

The present invention relates to speedometers and odometers, and more particularly to selectable range electronic speedometers and odometers.

Traditional mechanical speedometers are connected to the transmission output shaft by a gear driven flexible rotating shaft enclosed in a cable. Except for a change in axle ratio, the speedometer cable rotation is then directly proportional to the road speed. The proportion is determined by the ratio between a transmission drive gear and the mating cable gear, as well as the tire diameter and rear axle differential ratio. The speedometer unit is attached to the other end of the speedometer cable and provides a visual indication of speed from the speedometer cable rotation according to a second fixed proportion. Since a manufacturer may make vehicles which have a rear axle ratios that vary over a nearly 2-to-1 range, and select tires which have a difference in wheel revolution per mile of nearly 20 percent (between the largest to the smallest tire), the manufacturer must stock a wide variety of speedometers or speedometer cable gears to provide the proper speed and odometer readings. Moreover, problems of mechanical failure and routing of the speedometer cable within the body of the vehicle require significant time and effort to resolve with each different vehicle produced.

In electric speedometers, a magnetic pick-up is placed close to the speedometer cable drive gear in the transmission housing to produce a periodic signal that is proportional to the vehicle speed. The periodic signal is received by an electric or electronic speedometer unit having a fixed relationship between the periodic signal and the indicated vehicle speed and distance travelled. In such speedometer units, the vehicle manufacturer must stock a unique speedometer unit for each tire size and/or axle ratio. Adjustments are made within the speedometer unit itself, and require precision adjustment and calibration of the ratio between the indicated velocity and the period of the received signal. Moreover, a separate adjustment of the odometer range is necessary. The adjustments require sophisticated operator training, tools, and calibration equipment not normally available outside of the factory.

In special purpose or in heavy duty commercial vehicles, the rear axle (differential) often includes a second gear ratio which is selectable by the vehicle operator. In order for the mechanical speedometer to accommodate the additional axle ratio, additional mechanical gear ratios are required which correspond to the appropriate rear axle ratio. Electric speedometers typically include a divider to operate the odometer counter. To accommodate a second rear axle ratio would require a counter having a second divider modulus, which second modulus is likely to be inconvenient for available digital counters.

The speedometer/odometer of the present invention has an adjustable range to indicate vehicle speed and elapsed distance for various combinations of tire size and axle ratio. The selectable range adjusts both the speedometer and odometer indications simultaneously. The range is selectable from among a large number of discrete values, to allow the vehicle manufacturer or service personal to stock a single speedometer/odometer unit for virtually all vehicle requirements. The selection process may be performed by service personal without special tools, training, or calibration equipment. In addition, the selected range can be automatically adjusted to accommodate a second rear axle ratio. The component elements of the speedometer/odometer according to the present invention may be easily reduced to integrated circuits, resulting in an economical speedometer/odometer which is easily installed, maintained, and serviced throughout the life and configuration changes of the vehicle.

These and other features of the present invention may be better understood from the following, solely exemplary, detailed description together with the drawing, wherein the single FIGURE is a block and schematic diagram of one embodiment of a speedometer/odometer embodying the invention.

With reference to the drawing, the speedometer/odometer 50 receives a periodic signal from a signal source 52. The signal source 52 is preferably a magnetic transducer located in close proximity to the speedometer drive gear within the transmission. The transmission transducer and gear assembly is well known in the art and is not discussed herein. The comparator 54 converts the periodic signal into a signal of uniform amplitude by comparing the signal of source 52 to ground potential. The amplitude of the signal produced by the comparator 54 is determined by a voltage divider comprising resistors 58, 60 and 62 connected between ground potential and a reference voltage V1 of the reference supply 56. The resistors 58 and 62 are generally selected so that the voltage at the junction 64 is approximately 60 percent of the voltage V1. The resistor 60 is selected to provide a fine adjustment when calibrating the entire speedometer/odometer instrument. When the signal from the source 52 causes the comparator 54 output to become active, the comparator 54 effectively shunts the resistor 62 to the ground potential, reducing the signal at the junction 64 to zero volts. The signal at junction 64 alternates between zero and approximately 6 volts according to the periodicity of the signal received from the signal source 52. A charge pump 66 is used as a high accuracy frequency to voltage converter. The alternating signal at common point 64 is coupled into a charge pump 66 by a timing capacitor 68. The capacitor 68 becomes charged for each pulse produced at the junction 64. More particularly, the duration of the resulting periodic signal at 64 is at least 5 time constants of the network formed by the capacitor 68 and the associated circuit resistances, including resistors 58, 60, 62 and the charge pump 66 input resistance. The charge pump 66 operates according to well known principles, wherein the resulting output signal is a result of the combination of the timing capacitor 68 and the output resistors comprising resistors 70 through 80. An integrating capacitor 69 is included to smooth the ripple of the output voltage, and is selected according to the desired level of ripple. Additional discussion of charge pump circuits is available in several texts, including Application Note 162, of National Semiconductor Corporation of Santa Clara, Calif. dated 1976, listing parts LM 2907 and LM 2917 and is herein incorporated by reference.

The speedometer 91 comprises a moving magnet air core meter, which has two orthogonal windings. The two windings receive a signal corresponding to the sine and cosine value of the desired speedometer needle position. When energized with the appropriate signal, each coil produces a magnetic field whose vector sum aligns a magnet (connected to the needle) in that direction. A function generator 82 provides the necessary sine and cosine signals to the speedometer 91 according to the signal received from the charge pump 66. Moreover, the charge pump output may also be conditioned and received by a conventional D'Arsonval meter.

The odometer 92 comprises a numerical counting meter which increments a numeric indicator according to the number of pulses received. The odometer 92 is driven by a pulse train generated by a voltage to frequency converter 84, which produces a periodic pulse signal proportional to the analog charge pump 66 output signal. The frequency output from the converter 84 is received by a counter 86 which divides the signal by 3600. The output signal then corresponds to one pulse per tenth of mile. The signal pulse width is then normalized by a one-shot monostable multivibrator 88 which provides a uniform pulse width over the entire frequency (and velocity) of the received signal range. The resulting pulse is of sufficient duration and amplitude to advance the odometer 92.

Some, if not all elements of the present invention may be reduced to an integrated circuit. For instance, the particular embodiment shown here incorporates elements 54, 56, 66 and 82 in a single integrated circuit 104.

In operation, the rangeable speedometer/odometer of the present invention works as follows. With a 16 tooth gear connected to the output shaft of the vehicle transmission, 16 pulses are produced per revolution of the propeller drive shaft. Therefore, the signal from the signal source 52 can be represented by the following equation:

The axle ratios vary at least from 3.7 to 6.67, and the tire revolutions per mile typically vary from 456 to 555. In the extremes, corresponding to the smallest tire size with the highest axle ratio and the largest tire size with the smallest axle ratio, the full scale frequency from the source 52 varies between 600 Hz and 1330 Hz. Defining the timing capacitor 68 as Ct and the output resistor Rs as the sum of resistors 70 through 80, the transfer function for the circuit of the present invention is generally given by:

where K is an arbitrary constant associated with the particular circuit embodiment.

To provide an instrument which is precisely adjustable in the field, it must first be calibrated in manufacture. The resistor 60 is selected to provide the reading on the meter face of 80 MPH at 1333 Hz frequency from source 52 and with Rs set to 69.8 Kohms (all switches closed). Next the V-to-F converter 84 is adjusted. The output of the V-to-F converter is adjusted for zero output for a zero frequency signal from source 52 by adjusting a second reference voltage V.sub.2. The voltage V.sub.2 is received by the V-to-F converter 84 to offset any DC bias in the output signal of the charge pump 66. With Rs set to 83K plus 69.8K ohms and the signal from source 52 at 449 Hz (for 60 MPH), the V-to-F converter 84 is set to provide an output of 600 Hz. The 600 Hz output is set by adjusting resistor 85. The counter 86 divides the output signal of the convertor 84 output by 3600. Each resulting pulse is received by the odometer and corresponds to 0.1 miles; therefore at 60 mph an output pulse is produced every 6 seconds. The factory calibration is now complete.

The ranging of the finished speedometer/odometer 50 is provided by adjusting Rs with the resistors 71 through 80 by selection of switches 81 through 90, respectively. Resistor 70 is typically 69.8K ohms. The resistors 71 through 80 are shown in a decrementing sequence, providing resistances of 40K, 20K, 10K, 8K, 4K, 2K, 1K, 500, 250 and 125 ohms respectively; other sequences of resistance values are possible.

The linear frequency-to-voltage charge pump converter 66, has a slight offset in the relationship between voltage output signal (for a given input frequency) and total resistor Rs value. A relationship shown below (3) includes the offset to accurately describe the total Rs value (in K ohms):

Moreover, other distortions or nonlinearities in the relationship between Rs and the charge pump 66 output signal may be anticipated and corrected by adjusting slightly the selected switch combination. The different (corrected) switch combinations are incorporated in the selection chart (Table 1) so that the speedometer/odometer installer will not need to calibrate the instrument after the range network is switch adjusted. Shown below in Table 1 are the switches closed to obtain the total Rs values selected according to both the number of tire revolutions per mile and the rear axle ratio:

The range of the adjustment possible by the switched network is determined by the total resistance value of the resistors 71 through 80 in relationship to the resistance of resistors 70 through 80 inclusive. Moreover, the resolution is determined by the number of switches and resolution of included resistors. In the example shown with 10 switches having resistances ranging from 40K to 125 ohms, a resolution of at least 0.15% is obtained, and is more than adequate in most instances.

An additional feature of the present invention is the accommodation of a dual ratio rear end axle. The axle switch 94 is connected to the rear axle change mechanism to become closed when the lower axle ratio is engaged. The switch is connected to a local battery voltage and resistors 98 and 100, and causes a field effect transistor 96 to become conductive. The transistor 96 in combination with resistor 102 provides an additional shunt to resistor 62 of the voltage divider, such that the voltage at circuit point 64 is reduced by a value corresponding to the ratio between the lower axle ratio and the higher axle ratio. Typically, the difference is likely to be 73.07% of the higher axle ratio; correspondingly, the activation of the field effect transistor 96 and the resistor 102 causes the voltage across the juction 64 to be reduced to 73.07% of the signal corresponding to the higher axle ratio. Typically, the transistor 96 offers small resistance when turned on, especially in comparison to resistor 102. A resistor 102 is selected after the resistor 60 has been selected.

Alternate embodiments of the present invention by one skilled in the art are within the scope of the present invention. Moreover, other signal sources, such as alternator signals may be used by the present invention, in such instances where an engine RPM is desired. Therefore, the present invention in not to be limited except by the following claims.