Method and apparatus for enhanced gauge resolution
FIELD OF THE INVENTION 
     This invention relates to driving analog gauges with digital gauge drivers and particularly to a method and apparatus for attaining gauge positions of a higher resolution than the gauge driver. 
     BACKGROUND OF THE INVENTION 
     Analog instruments in motor vehicles for indicating speed or fuel level, for example, generally use a gauge comprising an electrically driven armature which moves a pointer around a dial. An electrical analog signal proportional to the parameter measured ideally drives the pointer to an angular position representing the parameter. It is becoming a common practice to sample the various input signals by a microcontroller which then controls the respective gauges. 
     The microcontroller is coupled to each gauge by a respective digital driver, typically a low resolution driver which has a limited number of position outputs, so that gauge movement is incremental or step-wise rather than a smooth continuous motion. This stepping or &#34;ticking&#34; is particularly objectionable during slow gauge movement. The limited number of driver position outputs also requires the gauge, when stationary, to indicate one of these positions even though the input signal represents a value between two adjacent positions, thus causing a gauge error except when the input signal happens to coincide with one of the fixed positions. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to obtain smooth gauge movement using a low resolution gauge driver. Another object is to increase the resolution of a digital gauge system beyond the raw resolution of the gauge driver. 
     The invention is carried out by rapidly oscillating the drive angle applied to the gauge between two neighboring points, such that the average position is between the neighboring points and thus can be much closer to the desired angle. The mechanical gauge movement does not respond instantaneously to each drive command and thus performs a filtering or averaging action. The neighboring point nearest the desired point is invoked more often than the farthest one to bias the gauge to the desired angle. 
     A continuously repeated algorithm in the microcontroller determines which drive angle of the two neighboring points shall be applied at each iteration such that, over time, the average of the points will be very near the desired position. The algorithm commands reading the input signal and the desired angle requested by the input signal is expressed as a value which is M times the driver precision. A remainder determined by the previous iteration is added to the angle, and the result is divided by the value M. The integer portion of the dividend determines which drive angle shall be applied. A new remainder is calculated and the algorithm is repeated. In this way the two neighboring drive angles are applied in proportion to the proximity of the desired angle. The result is that the gauge can move to many positions between those normally afforded by the gauge driver to more accurately reflect the input signal. In the same way, when the input is slowly changing, the gauge, instead of jumping from one position to another, can move to intermediate points as well to more smoothly traverse the gauge range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein: 
     FIG. 1 is a block diagram of a gauge and its drive system for carrying out the invention; and 
     FIG. 2 is a graph of a waveform of applied gauge angles and the average value. 
    
    
     DESCRIPTION OF THE INVENTION 
     The ensuing description is directed to a scheme for enhancing gauge smoothness and accuracy developed specifically for automotive gauges such as a fuel level gauge, a speedometer, and the like. It will be recognized, however, that the invention is not limited to such automotive uses but can be applied where improved gauge accuracy is desired in microcontroller driven gauges. 
     Referring to FIG. 1, an analog input from a speed sensor or a fuel level sensor, for example, is connected to the A/D input of a microcontroller 10 which has a serial clock output, a data output and a chip select output. The outputs are carried by separate buses to a gauge driver 12 which in turn has two pairs of outputs coupled to a gauge 14. The gauge has a pointer 16 which sweeps across a dial 18 to indicate fuel level or vehicle speed. The microcontroller includes a ROM or other memory which stores programs for operating the control including an algorithm for enhancing gauge smoothness and accuracy. 
     The microcontroller 10 causes the input signal to be read and develops a digital output which represents the magnitude of the input signal within the resolution permitted by the gauge driver. The gauge driver 12 converts the digital signal to sine and cosine waveforms to drive the gauge pointer through an angle to the desired position, subject to the delay of gauge movement. If desired, the gauge driver function could be incorporated into the microcontroller. 
     Since the gauge driver is digital it inherently has a limited number of fixed gauge positions which it can control to. Thus a 10 bit driver would have 1024 fixed gauge positions, and if these were distributed over 360 degrees the positions would be spaced about 0.35 degrees apart. The effective resolution can be made much finer by controlling the gauge to points between the fixed gauge positions. This is accomplished by reading the desired gauge position from the input signal and expressing the position with a resolution greater than that of the gauge driver, and energizing the driver circuit to alternately supply signals for the two discrete gauge positions nearest the desired position. When this algorithm is repeated rapidly the signals are averaged by the gauge to achieve the desired gauge position. A repetition every 7.8 msec has been found to be suitable. 
     In greater detail the algorithm is as follows: 
     1. START 
     2. READ THE INPUT SIGNAL 
     3. DETERMINE THE DESIRED ANGLE (WITH M TIMES THE PRECISION OF THE GAUGE DRIVER) 
     4. SET DESIRED ANGLE=DESIRED ANGLE+REMAINDER 
     5. CALCULATE REMAINDER=DESIRED ANGLE MOD M 
     6. GAUGE POSITION =DESIRED ANGLE DIV M 
     7. DRIVE GAUGE TO GAUGE POSITION 
     8. REPEAT STEPS 4-7 AT A FIRST RATE AND STEPS 2-3 AT A 
     SECOND RATE 
     where DIV is an integer division operator and any remainder is truncated, and MOD is the mathematical operator modulo which returns the remainder after a division operation. The first and second rates may be equal, however satisfactory results are obtained at a second rate at, say, one fifth the first rate to greatly reduce microcontroller burden. 
     As an example, assume a 10 bit gauge driver, M is equal to 32, and the previous remainder is 25. A full scale reading would then be 32768. If in step 3 the desired angle is determined (by digitizing the input signal) to be 3000, step 4 will yield a desired angle of 3025. The calculated remainder in step 5 will be 3025 MOD 32=17. In step 6 the gauge position is the integer value of 3025 DIV 32=94 and the gauge will be driven to that value (the 94th increment from zero). In the next iteration, the desired angle (step 4) will be 3017, the new remainder will be 9, and the gauge position will still be 94. After many iterations for the same input signal the algorithm will yield three times as many 94 values as 93 values which the gauge averages to 93.75, which is between the fixed gauge positions 93 and 94. 
     A simplified form of the gauge operation is shown in FIG. 2 which covers a period of several algorithm iterations where adjacent fixed gauge positions are x and x+1, and M equals 3. For an input signal requesting a gauge position of x+1/3, the drive angle output by the gauge driver is x two-thirds of the time and x+1 one-third of the time so that the gauge assumes the average position of x+1/3. 
     The above operations are best implemented in a microcontroller by setting the value M to 2 N , carrying out the DIV operation by shifting the binary value of the desired angle N positions to the right, and executing the MOD operator by a bitwise AND operation on the binary value of the desired angle and (2 N  -1). Then the algorithm becomes: 
     1. START 
     2. READ THE INPUT 
     3. DETERMINE THE DESIRED ANGLE (WITH 2 N  TIMES PRECISION OF THE GAUGE DRIVER) 
     4. SET DESIRED ANGLE=DESIRED ANGLE+REMAINDER 
     5. CALCULATE REMAINDER=DESIRED ANGLE AND (2 N  -1) 
     6. GAUGE POSITION=DESIRED ANGLE SHIFTED RIGHT N TIMES 
     7. DRIVE GAUGE TO THE GAUGE POSITION 
     8. REPEAT STEPS 4-7 AT A FIRST RATE AND STEPS 2-3 AT A SECOND RATE 
     It will thus be seen that the algorithm of either form disclosed here is simple in execution and yet greatly enhances gauge operation by allowing physical gauge position between the fixed angles of the driver output. This results in more accurate gauge indications when the input is a steady state signal and smooth movements when the input is slowly changing. The incorporation of the relatively simple algorithm thus permits the operation of an expensive gauge system to be mimicked by less expensive hardware.