Abstract:
A tachometer has a display responsive to an rpm signal for producing an observable indication of an rpm value. A circuit responsive to an engine input develops a standard rpm signal representative of instantaneous engine rpm, and a differential Power Band amplifier circuit amplifies the difference between the standard rpm signal and an adjustable “offset” rpm signal by predetermined gain to produce an amplified differential rpm signal. A switching circuit selects the standard rpm signal or the amplified differential rpm Power Band signal for driving the display. A peak or valley rpm signal may also be developed and selected for display either in a standard mode or a differentially amplified Power Band mode.

Description:
FIELD OF THE INVENTION 
     This invention is directed generally to tachometers, and more particularly to a racing tachometer having two display modes in which two different ranges in two different sets of incremental values of engine speed in rpm are displayed. 
     BACKGROUND OF THE INVENTION 
     Automotive racing continues to be refined as the equipment and driving techniques advance toward optimum performance. In one class of racing, professional circle track racing, often, the automobile&#39;s engine performance is measured in the laboratory on a dynamo meter which measures the engines torque, speed and power characteristics. Particular engine characteristics, together with transmission design, rear-end ratios, tire parameters, suspension set-up, and other vehicle characteristics, together result in actual “on track” vehicle racing performance. While the optimum rpm for a given vehicle may be established in the laboratory, this optimum rpm value will obviously vary from one vehicle to the next. 
     In professional circle track racing, the peak vehicle performance for a given set of track conditions tends to fall within a narrow band of engine rpms, given the other vehicle components such as transmission, tires, suspension, etc. as noted above. This peak performance or optimum range of engine rpm can be established in the laboratory as also noted above. For example, a variation in engine rpm of only as much as 50 rpm while entering or exiting a corner could require a small gear change in the differential. Accordingly, it is important that a racing tachometer give an accurate reading of engine rpm in a form in which the driver can best ascertain and utilize the information. 
     During a race, the engine rpm is measured by vehicle instruments and displayed to the driver by the tachometer. A digital tachometer can be very precise and display engine rpm down to several places. However, with rapid, slight variations in engine rpm, it is difficult for the average driver to quickly read and interpret the rapidly changing display of the digital tachometer. Accordingly, an analog display of rpm information, that is an analog tachometer display, is generally preferred in racing. 
     In most analog tachometers, a pointer rotation of approximately 250 to 270 degrees is provided. The diameter of the tachometer face may vary; however, even with a relatively large diameter face of the tachometer, with the requirement that the reading go from 0 to approximately 10,000 rpm, usually the minimum usable scale division which can be marked on the face of the dial is approximately 100 rpm. Such a scale division would result in division markings approximately 120 inches apart on a 5 inch diameter tachometer dial face. However, in practice, for purposes of maintaining peak vehicle performance as mentioned above, often as little as 20 rpm variation in engine speed can be significant. Thus, with minimum usable divisions representing 100 rpm, a variation of 20 rpm is difficult or impossible to read or identify. Thus, it is difficult with present tachometers for the racing driver to determine small yet significant variations from the optimal or peak performance rpm. 
     As a related matter, the peak or maximum rpm on a vehicle generally occurs immediately prior to gear shifting. Drivers often train heavily on effective gear shifting. Not only should the driver shift quickly and cleanly, but the driver should also shift at the appropriate engine speeds to extract the maximum power and racing speed from the vehicle. In oval track racing it is unlikely that the driver would over-rev the engine in high gear, but it is possible that he may have over-reved while accelerating through the gears. This could cause damage to the valve train which will be important information for the crew chief. While various rpm memory devices have been provided in the past for recording and recalling peak engine rpm, one aspect of the present invention provides an improvement in a peak or valley rpm memory. 
     A present invention improves on the above-noted situation regarding tachometer readings in that it allows for two different operational modes. In a first mode, the tachometer functions as a traditional instrument providing analog engine rpm information in a relatively wide range, such as from 0 to 10,000 rpm as the vehicle accelerates to competitive racing speed. In a second or “Power Band” mode, after the vehicle reaches competitive racing speed, the dial operation is in effect expanded. For example, the first or “normal” tachometer mode might have major dial divisions in 1,000 rpm increments with minor dial divisions in 100 rpm increments. However, the “Power Band” mode might have a full meter range of only 1,000 rpm, such that each major division represents 100 rpm and each minor division is equivalent to 10 rpm. Thus, the driver can more accurately determine relatively small changes in rpm of the engine with the Power Band mode in effect, and therefore more accurately maintain near peak performance. 
     Moreover, the present invention makes possible the selection of a nominal peak performance rpm as a center value of the Power Band range. Thus, any peak performance rpm which has been established in a laboratory or elsewhere can be selected as the “Power Band” center value on the meter in the present invention. The Power Band range will then read positive or negative increments to either side of this peak performance center value. 
     OBJECTS OF THE INVENTION 
     It is a general object of this invention to provide a novel and improved racing tachometer which overcomes the above-noted problems of the prior art. 
     A related object is to provide a racing tachometer which has two display modes in which the marked divisions on the meter or dial face represent two different ranges, of engine speed, in different increments. 
     A related object is to provide a racing tachometer wherein a nominal peak performance rpm may be set in as a center value of a range of meter or dial face readings. 
     A related object is to provide a racing tachometer which includes an improved peak and/or “valley” rpm memory. 
     SUMMARY OF THE INVENTION 
     Briefly, and in accordance with the foregoing objects, a tachometer in accordance with the invention comprises a display responsive to an rpm signal for producing an observable indication of an rpm value, a circuit responsive to an engine input for developing an rpm signal representative of instantaneous engine rpm, and a differential amplifier circuit for amplifying the difference between said rpm signal and an adjustable or selectable “offset” rpm signal by a predetermined gain to produce an amplified differential rpm or “Power Band” signal, and a switching circuit for either engine rpm or said amplified differential rpm or “Power Band” signal for driving said display. A peak or valley rpm signal may also be developed and selected for display either in a standard mode or a differentially amplified Power Band mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a racing tachometer in accordance with the invention; 
     FIG. 2 is a functional block diagram of the circuitry for producing the tachometer display in accordance with the invention; 
     FIGS. 3 a - 3   e  form a schematic circuit diagram showing further details of a portion of the circuitry of FIG. 2; 
     FIGS. 4 a - 4   e  form a schematic circuit diagram showing another portion of the circuitry of FIG. 2; and 
     FIG. 5 is a wiring diagram showing an additional portion of the circuit of the invention and connections between the respective circuit portions. 
    
    
     DETAILED DESCRIPTION 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular details disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     Referring now to the drawings, and initially to FIG. 1, there is illustrated the face or dial of a meter  10  in accordance with the invention. The meter face includes a plurality of major scale divisions  12  which are numbered 0-10 in the illustrated embodiment. Each of the major scale divisions is also divided into a number of minor scale divisions  14 , there being 9 such divisions  14  within each major division  12 . These divisions  12  and  14  are located about an outer circumference of the meter face. A second or “Power Band” set of meter divisions  16  is located on a second inner circumference of the meter face. The second group  16  of divisions defines a center or zero scale marking or point  18 , a group of major divisions  20  labeled as +100, +200, etc. extending in a clockwise direction from the center or zero point  18  and a second group of major divisions  22  designated as −100, −200, etc. extending in a counterclockwise direction from the zero or center scale marking  18 . 
     These “Power Band” divisions  16 ,  20  and  22  merely serve as a reference guide while the increment rpm readings in the “Power Band” mode are read on the outer scale divisions  12  and  14 , in terms of rpm above or below the center point  18 . 
     In normal operation, the outer major divisions  12  each represent an increment of 1,000 rpm, with the minor divisions  14  each representing an increment of 100 rpm. The pointer  24  in the illustration of FIG. 1 is pointing at the 6500 rpm mark of the dial. Thus, the outer markings  12  and  14  indicate engine speed and rpm during one mode of operation. 
     In accordance with the invention, a second or “Power Band” mode of meter operation is defined wherein the inner scale markings  16  are also utilized in connection with the pointer  24 . In the Power Band mode, a selected optimum or “peak performance” rpm value is selected and is designated as the center or zero value, represented by the zero marking  18  of the scale  16 . In the Power Band mode, the outer scale markings  12  and  14  are also used to determine increments of rpm, however, in this mode the major scale markings  12  each represent a 100 rpm increment and the minor scale markings  14  each represent a 10 rpm increment. Moreover, the inner scale markings  16  indicate in 100 rpm increments whether the engine rpm is above or below the designated optimum or peak performance rpm at the center point  18 . These inner scale markings can be used as a reference together with the outer scale markings  14  to determine in 10 rpm increments the amount by which the current engine rpm exceeds or fall short of this optimum or peak performance value. 
     Accordingly, in the Power Band mode, the position of the pointer  24  relative to the markings  12 ,  14  and  16  will indicate in 100 rpm increments (with respect to markings  12  and  16 ) and 10 rpm increments (with respect to the markings  14 ) the amount by which the current engine rpm exceeds or falls short of the selected peak performance rpm value. A lamp or LED  25  may be illuminated to indicate when the meter  10  is operating in the “Power Band” mode. FIG. 1 also shows a recall switch  26  and an erase switch  28  for use in connection with a “peak” and/or “valley” rpm memory to be described below. 
     Summarizing, the tachometer of the invention allows for two operational modes. In the first mode, the tachometer functions to provide engine rpm information from 0-10,000 rpm (in the illustrated embodiment) as the vehicle accelerates to racing speed. However, in the Power Band mode, the meter of the invention operates in a second or expanded dial mode, herein referred to as the “Power Band” mode. This mode expands dial operation, such that the major dial divisions in this example are equal to 100 rpm and the minor dial divisions represent 10 rpm increments as explained above. Other applications are envisioned wherein a different differential amplifier gain and different scaled rpm divisions for the “Power Band” mode would be used. A Power Band on/off or “mode” switch  30  is provided for selecting the first or normal mode when the switch is in its “off” position and the second or Power Band mode when the switch is moved to its “on” position. In addition to manual switching by use of the switch  30 , the invention contemplates that other methods of switching might be utilized without departing from the invention, such as an activation switch on a high gear linkage of the vehicle transmission, or an rpm threshold sensing circuit, or the like. 
     In accordance with another aspect of the invention, the value of the center point of the Power Band represented by the zero marking  18  of the scale  16  may be selected as desired to coincide with any desired rpm value, for example an optimum peak performance value as determined (such as by dynamo meter testing) for a particular vehicle. Thus, for example, the center point  18  of the dial in the Power Band mode may be selected to coincide with 7,200 rpm, such that in the Power Band mode, the meter dial will show readings of up to plus or minus 500 rpm from the center reading (i.e., from 6,700 to 7,700) rpm in 10 rpm increments of the minor dial markings  14 . Thus, in this example, in the power band mode, the pointer  24  as shown in FIG. 1 now indicates 7200 rpm+150 rpm=7350 rpm. 
     Referring now to FIG. 2, a functional block diagram of components for operating the meter  10  in the fashion described above, is illustrated. Initially, an engine speed input  40  receives an input or “frequency signal” from the engine  50  representative of engine rpm. This input is usually taken from the ignition system of the vehicle, such as from the distributor side of the coil. This “frequency signal” is converted by a signal conditioning circuit  42  into a pulse train which feeds into a low pass filter  44 . The low pass filter  44  converts the signal to a voltage which corresponds to the actual or instantaneous vehicle rpm, and which is fed to one input of a differential amplifier circuit  46 . A second input to the differential amplifier circuit comes from an adjustable “offset” rpm voltage signal from a Power Band adjust circuit  48 . 
     The Power Band adjust circuit  48  outputs a voltage signal which is proportional to the selected “center” rpm (meter dial marking  18 ) in the Power Band mode, that is, the selected optimum or the peak performance rpm of the vehicle. The difference between the selected peak performance rpm and the actual rpm of the vehicle is amplified (by a factor of 10 in the illustrative embodiment) by the differential amplifier  46  and fed out to switching logic  50 . The “actual” vehicle rpm voltage from the low pass filter is also fed to a second input  52  of the switching logic  50 . The mode switch  30  determines which of these two signals is fed by the switching logic into meter drive circuitry  60 . This meter drive circuitry  60  includes a low pass filter circuit  62 , a level shifting circuit  64  and a meter driver component  66 , which drives the meter  10 . The meter  10  and circuitry  60  together comprise a “display portion”  65 . 
     In accordance with one embodiment of the invention, the meter  10  may comprise an air core meter, such that the meter driver comprises an air core driver. The meter  10  may be one of the type made by Nu-Tech Engineering, Inc., while the meter driver  66  may be an air core driver of the type made by Cherry Semiconductor Corporation, 2000 South County Trail, East Greenwich, R.I. 02818-1530. Moreover, in the illustrated embodiment, as will be seen with reference to FIG. 4, the meter  10  comprises a differential air core meter, for example Nu-Tech MAC gauge No. 97020, also available as Auto Meter Part No. 3858-24-31, from Auto Meter Products, Inc., 413 W. Elm St., Sycamore, Ill. 60178, USA, while the driver  66  comprises a differential air core driver, for example Cherry Semiconductor Part No. CS8190. Other types of meters and meter drivers may be utilized without departing from the invention. 
     In accordance with a further, optional feature, a peak (and/or valley) rpm memory  70  may also be provided. This peak or valley rpm memory  70  receives, via the swithching logic  52 , the voltage from the low pass filter  44  representative of engine rpm. The erase switch  28  may be used to erase the contents of the peak (and/or valley) rpm memory  70 . The recall switch  26  is coupled with the switching logic  50 , as is an output of the peak (and/or valley) rpm memory  70 . Operation of the recall switch  26  will cause the switching logic  50  to display the contents of the peak (and/or valley) rpm memory on the meter  10  by way of the meter drive circuitry  60 . Also, mode switch  30  permits selecting the peak (and/or valley) rpm memory rpm to be displayed in either the standard rpm mode or the “Power Band” mode. A second differential amplifier  47  is shown in FIG. 2 for this latter purpose. Detailed circuits for accomplishing this are not shown, but modifications or additions to the circuits illustrated to accomplish this will be readily apparent. For example, additional switching circuitry could be used to substitute the peak (and/or valley) signal for the engine rpm signal at the input to the differential amp  46  (rather than adding a second differential amp  47 ) so as to display the peak (and/or valley) signal in a “Power Band” mode, i.e. using the expanded meter scale. 
     In order to select the center rpm value (meter marking  18 ) in the Power Band mode, for example, such as an optimum or peak performance rpm for the particular vehicle, the input  40  may be uncoupled from the engine  50  and connected to a test signal generator  75 . The test signal generator outputs a frequency signal which generally mimics the engine signal, and which may be set to a value representing the desired or peak performance or optimum rpm value. When the test signal generator has been set to this peak performance value, the Power Band adjust circuit  48  is adjusted (with the mode switch  30  in the Power Band “on” position) to a voltage value such that the meter pointer  24  points to the center marking  18  on the scale, which is also the same as the “5” marking of the outer circumference major scale divisions  12  in the meter face shown in FIG.  1 . It is noted, however, that in the Power Band mode this value does not necessarily represent 5,000 rpm, but rather, may be any selected rpm value in accordance with the invention. 
     Referring now to FIGS. 3 a - 3   e  and  4   a - 4   d , schematic circuit diagrams of one form of the circuits shown in FIG. 2 are illustrated. The portions of the circuits of FIGS. 3 a - 3   e  and  4   a - 4   d  which correspond to the various blocks in FIG. 2 have been indicated in FIGS. 3 a - 3   e  and  4   a - 4   d  by like reference numerals. 
     Briefly, in FIG. 3 a , the signal conditioning circuit  42  is built from discrete components, however, equivalent integrated circuit components could be used if desired. In FIG. 3 b , the low pass filter  44  is a two pole active filter. In FIG. 3 c , an additional buffer  100  provides a further “tach output” signal from the filter circuit  44 , which may be used in connection with other equipment which is not a part of the present invention. The differential amplifier circuit  46  (FIG. 3 c ) and the analog switching circuit  50  (FIG. 3 d ) are assembled from integrated circuit components and discrete circuit components. The Power Band adjust circuit  48  comprises a potentiometer  110  which is shown in FIG.  5  and which is coupled at like-designated terminals to the circuit of FIGS. 3 a - 3   e . The Power Band on/off switch  30  is also shown in schematic form in FIG.  5 . FIG. 3 e  shows a DC power supply. 
     Referring briefly to FIG. 4 a , the low pass filter  62  is a two-pole active filter circuit similar to the filter circuit  44  of FIG. 3 b . The level shifting circuit  64  (FIG. 4 a ) feeds the differential air core driver, Cherry Semiconductor Part No. CS8190 or equivalent, which feeds the differential air core meter  10 , such as Nu-Tech Part No. 97020 or equivalent, as indicated above. 
     Some additional circuits in FIGS. 4 c  and  4   d  comprise a peak rpm memory circuit and associated components. A Schmitt trigger circuit  120  in FIG. 4 c  is coupled with the erase switch  28  to provide a pulse input in suitable form to the switching logic circuits  50  of FIG. 3 d . A clock oscillator circuit  122  is gated by a clock gate  124  and a second gate  126  (FIG. 4 d ) to a twelve-bit binary ripple counter  128  which forms a memory portion of the peak memory circuit  70 . A comparator  130  gates through the clock signal at gate  126  if the voltage at its positive input is higher than the voltage at its negative input. The voltage at the negative input is taken from a resistor network  140  which provides an analog voltage value corresponding to the rpm value currently at the counter  128 , that is, the present peak rpm value. The voltage at the positive input comes from the filter circuit  44 , and thus, as described above comprises a voltage which corresponds to the instantaneous engine rpm signal as processed by the circuits  42  and  44 . Thus, if this instantaneous engine rpm voltage is higher than the peak rpm voltage in memory, the clock signal is gated through to allow the counter to continue to count up. The counter  128  can be reset by the erase switch  28  by way of an inverter  132  (FIG. 4 c ). 
     The resistor network  140  has its effective resistance value selected by the output of the counter  128 . Thus, as the counter  128  counts up, the voltage at the output of the resistor network increases correspondingly. The provision of the 12-bit binary ripple counter and the resistors of the resistor network  140  provide a relatively large number of incremental voltages available to represent a correspondingly large number of incremental rpm values. Some other form of memory device, such as a latch, ROM, RAM, etc. may be utilized to determine or retain a peak or valley rpm value from the output of the counter without departing from the invention. 
     The value in the memory will be retained for so long as sufficient power is applied to the ripple counter to hold its output count. In order to prolong this period, during which the peak value is held in memory, for example during a power outage, a relatively large (e.g., one microfarad) capacitor  142  is provided in a voltage supply circuit  144  which supplies the counter  128 , as shown in FIG. 4 e . The peak rpm value from the resistor network  140  of the memory  70  is input to the switching logic  50  where it may be selected for display by operation of the memory recall switch  26  as described above. 
     While a “peak” memory is described in detail above, it will be appreciated that a “valley” memory for retaining a lowest or “valley” rpm value will be substantially similar in its construction and operation. Due to the nature of racing, the valley rpm memory feature would have a selectable “window-of-time” within which the valley rpm would be recorded. The driver could be provided with a button or switch to activate this “window.” 
     What has been illustrated and described herein is a novel and improved racing tachometer which provides a first rpm range from 0 -10,000 rpm in 1,000 rpm and 100 rpm marked increments, and a second or Power Band range in 100 rpm and 10 rpm marked increments using the same incremental markings of the meter dial. The Power Band range is presented as plus or minus deviations from a center or optimum or “peak performance” rpm value. This peak performance value may in turn be selected and set into the meter or changed, as desired. In accordance with an optional feature, the meter may also selectively display a peak or “valley” rpm value which is updated as peak rpm increases or as “valley” rpm decreases. This peak and/or “valley” rpm may be recalled and displayed in either the standard mode or the “Power Band” mode when desired and may also be erased so that new peak and “valley” rpms may be determined, updated, and recalled from time to time.