Patent Application
20050249042
The present invention relates to an hour meter for outdoor power equipment and, more particularly, a digital hour meter for outdoor power equipment that is powered by a magneto of an internal combustion engine and accumulates engine operating time only when the engine is actually operating.
Engine operating time hour meters for outdoor power equipment including riding lawn mowers, lawn and agricultural tractors, snowmobiles, snowblowers, jet skis, boats, all terrain vehicles, bulldozers, generators, etc. are well known. Such engine operating time hour meters are provided, among other things, to let the owner and/or manufacturer know how long the equipment has been operated, when the equipment is due for repair/maintenance service, whether the equipment is still under warranty, etc.
With the widespread use of digital circuitry, digital engine operating time hour meters generally have replaced the old style mechanical hour meters which utilized rotating wheels. Digital hour meters provide improved accuracy and a digital display of accumulated hours. One shortcoming of some digital and mechanical prior art hour meters is that they accumulate hours of use of the equipment as soon as the ignition key or switch is turned on. Such hour meters provide an inaccurate measure of engine use. There may be instances where the ignition switch is on and the engine is not running, for example, an operator may inadvertently leave the ignition key in the on position after use of the equipment is completed and the engine is off. If the hour meter is accumulating time when the ignition switch is on the accumulated hours on the hour meter will overstate the true engine operating hours. Since warranty and service intervals are generally based on hours of engine operation, accumulating hours on the hour meter when the ignition key is on will result in premature indication that maintenance is needed and/or premature expiration of warranty, both to the disadvantage and dissatisfaction of the equipment owner.
As the manufacturer and owner of power equipment generally want to know the hours that the most expensive component of the equipment, namely, the engine has been operated, it is desired to have an hour meter that accumulates hours only when the engine is actually on. Certain prior art hour meters have attempted to address this issue. Generally, such prior art hour meters include two terminals which are coupled to the engine battery (generally 12 volts DC) and further include a third or enable terminal. Such hour meters only accumulate hours when the third terminal is enabled, that is, the third terminal receives a signal indicating that the engine is operating. One prior art hour meter utilizes three terminals, two of which are coupled to an internal DC power source of the hour meter and a third terminal is coupled to a spark plug wire and only accumulates time if the spark plug is firing. A disadvantage of such three terminal hour meters is that they necessarily include three terminals, two for power and a third terminal which must be enabled for accumulation of time. An additional disadvantage of the three terminal hour meter with an internal power source is that the power source eventually runs down necessitating a new power source being installed.
What is needed is a digital hour meter that utilizes only two terminals and accumulates engine operating time only when the engine is operating. What is also needed is a digital hour meter that is powered by a magneto of the engine and only accumulates engine operating time when the magneto is powering the hour meter. What is also needed is a versatile digital hour meter that can be powered by an engine magneto or the engine battery and is polarity insensitive.
One exemplary embodiment of the present invention is directed to a digital engine operating time measuring apparatus for an internal combustion engine which accumulates hours only when the engine is operating. The operating time measuring apparatus includes:
Preferably, the digital integrated circuit is a microprocessor and, more specifically, a programmable integrated circuit (PIC) chip and the display is a liquid crystal display.
In one aspect of the present invention the power regulation circuitry includes a full wave rectifier for converting the time varying electrical signals generated by the magneto to a DC signal. In one preferred embodiment, the power regulation circuitry includes a temperature compensation circuit including a thermistor for changing a magnitude of the low voltage DC signal generated by the power regulation circuitry to compensate for a change in a magnitude of a threshold voltage of the display resulting from changing ambient temperature in the vicinity of the display.
Advantageously, the apparatus of the present invention is versatile and may be powered by an time varying signal such as an AC signal or by a DC signal and may be powered by a pulsed signal such as a magneto signal, whether the magneto output is a positive or a negative going signal. Although not preferred, the apparatus may be coupled to and powered by an engine battery and ignition switch. In this situation, the accumulated engine operating time will reflect a number of hours the battery is turned on.
These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings.
Turning to the drawings, a block diagram of an a digital engine operating time measuring apparatus of the present invention is shown generally at 100 in
Typically, the hour meter 100 is mounted on a dashboard of outdoor power equipment such as a tractor, snowmobile, riding lawn mower, personal water craft or boat to inform the owner of the number of hours that the engine has been operated since the equipment was manufactured. However, it should be understood that the hour meter 100 of the present invention can be utilized with any type of internal combustion engine and is not limited to any particular type of equipment or vehicle. For ease of installation and compactness, circuitry and the liquid crystal display 250 of the hour meter 100 is mounted to a printed circuit board 110. The circuit board 110, in turn, is conveniently plugged into a socket (not shown) disposed beneath the dashboard of the power equipment and coupled to the output of the magneto 20. The magneto 20 is a transformer device and typically the hour meter 100 is coupled to a primary side of the magneto 20. Advantageously, because of the power regulation circuitry 150 (discussed below), the circuit board 110 is polarity insensitive and may be plugged into the socket in either direction, that is, either terminal of the power regulation circuitry may be coupled to either terminal of the magneto output.
The hour meter 10 of the present invention 100 advantageously is powered by and senses an output signal of a magneto 20 of the engine 10 and accumulates engine operating time only when the output signal of the magneto is sensed. The magneto 20 essentially is an AC generator with one or more permanent magnets that produce a pulsed, time varying AC output signal. The magneto output signal is typically on the order of 150-400 volts peak and of a duration of up to 2 milliseconds. A secondary side of the magneto 20 is coupled to a spark plug or plugs 30 of the engine 10 and utilized to fire the spark plug(s) 30. By grounding the magneto 20, the engine 10 is turned off because the spark plugs 30 will not fire. Thus, operation of the magneto 20 is necessarily concurrent with operation of the engine 10.
As noted above, the hour meter 10 only accumulates engine time when operation of the magneto 20 is sensed, thus, the hour meter 100 necessarily accumulates time only when the engine is operating. Further, since the hour meter 100 is also powered by the magneto output signal, it is advantageously a two terminal device, eliminating the need for a third terminal which is enabled by a logic or other signal to accumulate engine operating time.
Additionally, as will be explained below, the hour meter 100 includes robust power regulation circuitry 150 that allows the hour meter 100 to operate on an AC or DC input and with a wide range of input signal magnitudes, from 35 V AC to a pulsed 600 V peak and from 7-50 V DC. The power regulation circuitry 150 of the hour meter can operate regardless of polarity, that is, it can operate on AC signals having either positive or negative going waveforms and on DC signals which are positive or negative with respect to ground. Since the power regulation circuitry 150 of the hour meter 100 will operate on a DC signal, if a manufacturer of a piece of equipment desired, the hour meter 100 could be coupled to a DC battery 40 and ignition switch of the equipment and, in such a configuration, would accumulate time that the battery is turned on., i.e., the ignition key or switch is turned to the on position. The hour meter 100 is advantageously backwards compatible with existing prior art hour meters. Alternately, the hour meter 100 could be coupled to an alternator 50 of the equipment and would accumulate time when engine 10 and, therefore, the alternator are on.
In one preferred embodiment, the hour meter 10 is a high impedance, low operating current digital device that includes the power regulation circuitry 150. An output of the power regulation circuitry is coupled to and provides power to a digital integrated circuit 200 and to a display 250, preferably a liquid crystal display. The display 250 will display an elapsed time that the engine 10 has been operated. Although the display 250 is preferably a liquid crystal display, one of ordinary skill in the art will recognize that any display suitable for operation by digital output from the digital integrated circuit 200 could be utilized, for example, an LED display or plasma display.
Additionally, as will be discussed below, the digital integrated circuit 200 is preferably a microprocessor and, more particularly, a PIC chip. However, it should be recognized that other types of digital circuits and digital integrated circuits, know to those of skill in the art, could equally well be used such as, for example, without limitation, microcontrollers, programmable controllers, application specific integrated circuits (ASIC) and field programmable gate arrays (FPGA). Although the term “microprocessor” will be used herein in connection with reference number 200, it should be understood that the term microprocessor should be broadly interpreted to encompass any type of digital circuitry or digital integrated circuit.
In addition to the display of accumulated engine time, advantageously, the display 200 may include one or more visible service indicator displays. In the illustrated embodiment, two service indicator displays 300, 302 (
Power Regulation Circuitry 150
The hour meter 100 is a two terminal device, as such, the power regulation circuitry 150 includes a pair of terminals 152, 154 that are coupled to positive and negative primary side outputs of the magneto 20. Depending on the electrical characteristics of the magneto 20, the pulsed output voltage, VM, of the magneto 20 may be comprised of positive-going voltage pulses, negative-going voltage pulse, or AC-type pulses, that is, pulses that include both positive and negative-going voltage components. The pulsed output voltage, VM, is present across the terminals 152, 154. The terminals 152, 154 are coupled to a full wave bridge rectifier 160 of the power regulation circuitry 150. A suitable bridge rectifier is sold by Fairchild Semiconductor Corporation as part no. MB8S (Fairchild Semiconductor Corporation, South Portland, Me., web site—www.fairchildsemi.com). The bridge rectifier 160 rectifies magneto output voltage, VM, and outputs a positive DC voltage signal, VR, at node 162. The rectifier 160 is coupled to ground G at node 164.
The magnitude of the DC voltage, VC, applied to a collector 170 of an NPN bipolar junction switching transistor 172 is clamped or limited to a maximum of 24 V DC by a 24 volt zener diode 170 coupled between the collector 170 and ground G. When there is no voltage output by the engine magneto 20 (engine 10 is off), there is no voltage, VB, at the base of the switching transistor 172 and the switching transistor is off. The output voltage, VOUT, present at output node 199 of the power regulation circuitry accordingly is zero. When the engine 10 is on, there is a 150-400 V pulsed voltage output by the magneto 20, this results in a sufficient voltage, VB, at the transistor base 174 to turn the transistor 172 on and establish an output voltage, VOUT, at node 199. At room temperature (25° C.) the output voltage, VOUT, is 3.0 V DC.
When the magneto 20 is on, a shunt regulator 180 establishes a reference voltage, VREF, of 1.24 V DC at node 188. One acceptable shunt regulator is National Semiconductor Corporation part no. LMV431AI (National Semiconductor Corporation, Santa Clara, Calif., web site—www.national.com). The reference voltage, VREF, of 1.24 V DC at node 188 causes current to flow upwardly through a pair of series coupled resistors 182, 184 (200 KΩ and 309 KΩ, respectively) and through a parallel combination of a 100KΩ thermistor 192 and a 309 KΩ resistor 194. The current flow through the resistors caused by the shunt regulator 180 establishes a voltage value of 3.6 V DC at the node 196 and, therefore, establishes the same voltage value (VB=3.6 V) at the base 174 of the transistor 172.
The signal present at an emitter 176 of the transistor 172 defines the output voltage, VOUT, of the power regulation circuitry 150. As can be seen, the output node 199 of the power regulation circuitry 150 is coupled to the transistor emitter 176. A 47 microfarad charging filter capacitor 188 is coupled between the output node 199 of the power regulation circuitry and ground G.
One advantageous feature of the power regulation circuitry 150 is temperature compensation. It has been found that the threshold voltage of the liquid crystal display 250 changes with temperature, namely, as the ambient temperature in the vicinity of the liquid crystal display 250 increases, the threshold voltage of the display decreases and as the ambient temperature decreases, the threshold voltage of the display increases. Since the expected operating temperature range of the hour meter is −30° C. to +80° C. the temperature sensitivity of the liquid crystal display 250 can be a problem.
If the output voltage of the power regulation circuitry 150 does not compensate for the changing threshold voltage of the liquid crystal display as the engine of the power equipment on which the hour meter 10 is installed heats up during use, the threshold voltage may drop low enough that all the segments of the liquid crystal display 250 will be energized by nominal background voltage thereby resulting in a nonsensical meter reading.
Therefore, a temperature compensation circuit 190 is provided as part of the power regulation circuitry 150. The temperature compensation circuit 190 includes the 309 KΩ resistor 184 coupled in series with the thermistor 192 and the 309 KΩ resistor 194, which are coupled in parallel. The thermistor 192 is a device with a high negative temperature coefficient of resistance meaning that as its temperature increases, its resistance decreases. One suitable thermistor is Murata Electronics part no. NCP18WF104J03R (Murata Electronics North America, Inc., Smyrna, Ga., web site—www.murata-northamerica.com).
At 25° C. (room temperature), the voltage, VB, present at the base 174 of the transistor 170 is 3.6 V DC. The output voltage, VOUT, present at output node 199 is the base voltage, VB, less the base-emitter junction voltage drop of approximately 0.6 V. Therefore, at room temperature, the output voltage, VOUT, of the power regulation circuitry is approximately 3.0 V DC.
Because of the negative coefficient of resistance of the thermistor 192, as the ambient temperature in the immediate vicinity of the hour meter 100 increases (and, therefore, the temperature of the thermistor 192 necessarily increases), the base voltage, VB, will decrease from the nominal 3.6 V value, thereby decreasing the output voltage, VOUT. Thus, as the threshold voltage of the liquid crystal display 250 decreases with an increase in temperature in the vicinity of the hour meter 10, the power regulation circuit output voltage, VOUT, also decreases to avoid undesired energization of all display segments. Conversely, as the ambient temperature in the vicinity of the hour meter 100 decreases, the base voltage, VB, will increase above the nominal 3.6 V value, thereby increasing the output voltage, VOUT of the power regulation circuit 150. Thus, as the threshold voltage of the liquid crystal display 250 increases with a decrease in temperature in the vicinity of the hour meter 10, the power regulation circuit output voltage, VOUT, also increases to compensate.
Microprocessor 200
As can best be seen in
Output voltage node 199 of the power regulation circuitry 150 is coupled to line 202 of the microprocessor. Coupled to a reset line 204 of the microprocessor is a delay circuit 206 that provides for resetting the microprocessor 200 prior to power loss from the power regulation circuitry 150 when the engine 10 is shut off. An external timer or clock circuit 220 including a crystal 222 that oscillates at 32768 Hertz is coupled to the microprocessor 200 to permit the microprocessor to accurately accumulate elapsed time that the engine 10 is on, that is, the time that the microprocessor 200 is powered up.
The microprocessor 200 is programmed with custom programming code, a flow chart of which is shown in
At step 600, only one interrupt is used. This interrupt occurs when the microprocessor internal timer T1 reaches a predetermined value that is set during initialization. The internal timer T1 uses the external crystal 222 as its frequency source. During the interrupt routine, the internal timer T1 is stopped, reset, and restarted. Also, variables that are used to keep track of accumulated engine operating time and keep track of ‘writing’ to the EEPROM are checked, incremented, or cleared based on their respective values at that time.
At step 700, the logic of the main loop is shown. The main loop controls the updating of the liquid crystal display data (30 segment engine operating time display, two single segment service indicators 300, 302, and the single segment decimal point 267) and the writing of the engine operating hours data to the EEPROM. It also controls sending data to the LCD display 250 in a multiplexing fashion. Once the above functions have been addressed, the main loop puts the microprocessor 200 to “sleep”. The microprocessor 200 will only “wake up” when the internal timer T1 interrupt occurs.
When 256 interrupts have occurred, a total time of approximately one second has passed and the variable eewr ctr (EEWRITE Counter) will be incremented. When eewr ctr has been incremented 36 times, a total of 36 seconds, or one hundredth of an hour, has passed. At this time eewr ctr will be reset to zero and the accumulated time date will be incremented and stored in EEPROM memory. The data stored includes: thousands, hundreds, tens, ones, tenths and hundredths of an hour. To keep a low power level in the module, the accumulated time data is programmed into the EEPROM memory using the variable wr intbl (EEWRITE interval) in a “time staggered” fashion. The display data used to drive the segments of the display 250 are updated when the eewr ctr variable is reset to zero 100 times. This is equivalent to one tenth of an hour. When the “tenths” data is incremented, the microprocessor 200 will check to see if either of two service indicators 300, 302 and/or a decimal point 267 should be turned on. Then the new time data will be shown on the display 250. The microprocessor 200 will enter the sleep mode until another time T1 interrupt occurs.
At step 800, the interrupt routine is shown which wakes up the microprocessor 200. Sleep mode is a function internal to the microcontroller 200. It puts the microprocessor 200 in a very low power consumption condition by disabling several of it internal functions. This low power mode is used to keep the amount of energy taken from the magneto output signal to be as low as possible. The microprocessor 200 spends approximately 75% of its time in sleep mode. When internal timer T1 reaches its maximum value, it triggers an interrupt to occur internal to the microprocessor 200. This interrupt, occurring every 3.906 milliseconds, ‘wakes up’ the microprocessor 200 and allows it to perform all its regular functions again. The timer T1 runs all the time, while the 1 megahertz internal oscillator only runs when the microprocessor 200 is not in the sleep mode. Timer T1 is the only part of the microprocessor 200 that is running during the sleep mode.
Display 250
The liquid crystal display 250 includes a display of five digits 260, 262, 264, 266, 268 (
The microprocessor 200 drives the liquid crystal display 250 via four control lines 230, 232, 234, 236 coupled to a control line bus 238 and nine data lines 240, 242, 244, 246, 248, 250, 252, 254, 256 coupled to a data line bus 258. The data lines 240, 242, 246, 248, 250, 252, 254, 256 are coupled to the display segments via a front plate of the liquid crystal display. A 4:1 multiplex driving arrangement is used by the microprocessor 200 to drive each of the 33 display segments with the nine data lines and the four control lines. The control lines 230, 234, 236, 236 are each connected to one of four common backplanes (schematically shown as 290, 292, 294, 296 in
Power is applied to the liquid crystal display 250 through a set of bias or offset resistors 270 which act as 2:1 voltage dividers. The 1.5 V output from each of the four voltage dividers 272, 274, 276, 278 is coupled to a respective one of the control lines 230, 234, 236, 238.
While the present invention has been described with a degree of particularity, it is the intent that the invention includes all modifications and alterations from the disclosed design falling with the spirit or scope of the appended claims.
