ROTARY SPEED CONTROL SYSTEM FOR ENGINE

Abstract: A rotary speed control system for controlling the rotary speed of an engine (1) through remote control of the turning of a governor lever (3). This rotary speed control system comprises: a stepping motor (6) for turning the governor lever (3); lever position detecting switches (11), (12) for detecting the abutting of the governor lever (3) against stoppers (4), (5); a pulse counter (14); and a controller (13). And, the controller (13) stores a count value of the pulse counter (14), when the position detecting switches (11), (12) are turned on, as an updatable reference value and performs rotary speed control and initial adjustment of the engine (1) on the basis of the reference value.

Description: This invention relates to a prime mover rotational speed control system according to the preamble of claim 1. Such a system is known from GB 2 192 736. The system is particularly suitable for use on a construction machine such as hydraulic power shovel or the like for controlling rotational speed of its prime mover. Generally, a Diesel engine is mounted on construction machines to serve as a prime mover for driving hydraulic pumps. In this regard, it has been the general practice for the conventional construction machines of this sort to provide a control lever in an operator's cabin and to link the control lever with a governor mechanism of the engine through a control cable, link rod and so forth for control the engine r.p.m or the rotational speed of the engine. However, the machanical linkage of the control lever with the governor mechanism through the control cable and link rod has a drawback that it requires large operating forces. With a view to eliminating such a drawback, there has been proposed an electric remote control system for governor mechanism, including an electric motor provided in the vicinity of an engine for governor adjustment, a rotational angle sensor adapted to detect the rotational angle of the governor mechanism indicative of the rotational speed of the engine, and a command means in the form of operating switches or the like provided in the operator's cabin in association with a controller like a microcomputer. The controller is adapted to control the electric motor through feedback control in such a manner as to zeroize the difference between a signal value specified by the command means and a signal value detected by the rotational angle sensor, thereby turning the governor lever of the governor mechanism to a position corresponding to the specified value. In this connection, Figs. 11 to 13 show by way of example a construction machine employing a prior art prime mover rotational speed control system with a governor mechanism of the sort as mentioned above. In these figures, indicated at 1 is a Diesel engine which is mounted on a construction machine as a prime mover (hereinafter referred to simply as "engine"), at 2 is a governor which is provided on the engine 1, the governor 2 including an elongated governor lever 3 and stoppers 4 and 5 which delimit the rotational range of the governor lever 3 by abutting engagement therewith. The governor 2 functions to adjust the rotational speed of the engine 1 according to the rotational angle of the governor lever 3 in an accelerating direction H or decelerating direction L, and, as shown in Fig. 12, to hold the engine at the lowest speed N L (idling speed) when the governor lever 3 is abutted against the stopper 4 where the value of lever rotation is 0%, while holding the engine at the maximum speed N H (full speed) when the governor lever 3 is abutted against the stopper 5 where the value of lever rotation is 100%. Designated at 6 is a reversible stepping motor which is mounted in the vicinity of the engine 1. A lever 6A which is mounted on the output shaft of the stepping motor 6 is connected to the governor lever 3 through a link 7. The stepping motor 6 is rotatable in a forward direction F or in a reverse direction R according to a control pulse signal from a controller 10, which will be described hereinlater, thereby to rotate the governor lever 3 in the accelerating direction H or in the decelerating direction L through the link 7. Even when the lever rotation is stopped by a stop signal received from the controller 10, the governor lever 3 is retained in the current angular position to operate the engine 1 at the current rotational speed. The reference 8 denotes a potentiometer which is provided in the vicinity of the engine 1 to serve as a rotational angle sensor. A lever 8A which is mounted on a rotational shaft of the potentiometer 8 is connected to the link 7. The potentiometer 8 is preadjusted such that its detection range (output range) is held in a predetermined relationship with the rotational range of the governor lever 3 as indicated by solid line in Fig. 12. The potentiometer 8 is adapted to detect the rotational angle of the governor lever 3 through the lever 8A and link 7 to produce an output signal indicative of the rotational speed of the engine 1 for supply to the controller 10. Designated at 9 is an up-down switch which is provided in the operator's cabin of the construction machine as a command means for specifying a target engine speed. The up-down switch 9 is constituted by push-button type up-switch and down-switch (both omitted in the drawing). The up-down switch 9 is adapted to supply the controller 10 with a command signal, namely, an acceleration command signal or a deceleration command signal corresponding to the extent of the depressive operation on the up- or down-switch. According to the received command signal, the controller 10 sets up a target value M which corresponds to the target rotational speed of the engine 1 as will be described hereinlater. Indicated at 10 is a controller including an arithmetic operation circuit like CPU and a memory circuit such as ROM and RAM (all omitted in the drawing). The controller 10 is provided with a memory area 10A in the memory circuit. For setting up a target value M which corresponds to the target rotational speed of the engine 1, the controller 10 is adapted to convert the command signal from the up-down switch 9 into a percentage target value M on the basis of a map as shown in Fig. 13, which is stored in the memory area 10A, and to store the target value M thus obtained. Then, the controller 10 compares the target value M with a value N B of governor lever rotation, which is detected by the potentiometer 8 and corresponds to the rotational speed of the engine 1, to produce a control pulse signal to the stepping motor 6. Accordingly, the stepping motor 6 rotates the governor lever 3 in the accelerating direction H or decelerating direction L to control the rotational speed of the engine 1 to the target value. With a prime mover rotational speed control system of the above-described prior art construction, the operator enters a desired engine speed through the up-down switch 9, whereupon the controller sets up a target value M of the engine speed according to the command signal from the up-down switch 9. Then, the controller 10 reads in the rotational angle of the governor lever 3 from the potentiometer 8 as a value corresponding to the current rotational speed of the engine 1, comparing the value with the target value M to produce a control pulse signal to be applied to the stepping motor 6 for rotation in the forward or reverse direction. As a result, the governor lever 3 is turned in the accelerating direction H or decelerating direction L to adjust the engine speed into conformity with the target value M. As soon as the rotational speed of the engine 1 substantially reaches the target value M, the controller 10 produces a stop signal as a control pulse signal for the stepping motor 6, which then maintains the governor lever 3 at the current rotational angle to let the engine 1 rotate at a speed corresponding to the target value. In this regard, the above-mentioned prior art is arranged to compare the target value M with a value N B of governor lever rotation, which is detected by the potentiometer 8 as an indicator of the rotational speed of the engine 1, and to adjust the rotation of the stepping motor 6 for control of the rotational speed of the engine 1. It follows that, in the entire rotational range between the minimum and maximum rotational speeds which are delimited by the stoppers 4 and 5, the governor lever 3 needs to be turned in a manner which corresponds to the detection range of the potentiometer 8 as indicated by solid line in Fig. 12. However, in the above-described prior art, the positions of the stoppers 4 and 5 differ from engine to engine, so that it becomes necessary to preadjust the range between these members by changing the setting of the link ratio or through fine adjustment of the potentiometer 8 individually for each engine. These preadjustments are very troublesome and time consuming. Besides, there is a problem that the governor lever 3 and link 7 are susceptible to loosening of mechanical parts as a result of repeated operations over a long period of time, or a problem that temperature variations might cause variations in output characteristics of the potentiometer 8, inviting discordance between the rotational range of the governor lever 3 and the detection range of the potentiometer 8, for example, as indicated by broken line in Fig. 12 to make correct control of the engine speed difficult. Furthermore, there are possibilities of noises creeping into the detection signal from the potentiometer to lower the accuracy and reliability of the engine speed control. GB 2 192 736 discloses a fuel control system for internal combustion engines. In this system, the number of steps required for a stepper motor to reach a target load is computed by dividing in proportion the number of steps required for the stepper motor to move a fuel metering member from an idle position to a full-load position by a ratio between a target load value of an electric command given by fuel supply command means and a maximum value of the electric command. There are further provided learning means for learning the number of steps required for energizing the stepper motor to move the fuel metering member from an idle position to a full-load position. Variations in the number of steps required to rotate the control lever between an idle position and a full-load position are corrected on the basis of the learned values. In view of the foregoing problems of the prior art, the object of the present invention is the provision of a prime mover rotational speed control system, which is arranged to facilitate the preadjustments to a marked degree and which possesses improved reliability in controlling the rotational speed of a prime mover stably and accurately at a target value over a long period of time. This object is solved in accordance with the features of the independent claim. The dependent claim is directed on a preferred embodiment of the invention. There is provided a prime mover rotational speed control system, including a prime mover, a governor having a governor lever to increase or reduce the rotational speed of the prime mover according to the rotational angle of the governor lever, a stepping motor adapted to turn the governor lever according to a control pulse signal, a command means for specifying a target rotational speed or the prime mover, and a controller adapted to produce a control pulse signal according to the specified value from the command means for application to the stepping motor, characterized in that: the rotational speed control system comprises a pulse counter means for counting control signal pulses to be applied to the stepping motor; and said controller comprises a memory means adapted to store a count value from the pulse counter means as a renewable reference value when the rotational speed of the prime mover is set at least at one of predetermined minimum and maximum speeds thereof, and an arithmetic operating means adapted to calculate the current rotational speed of the prime mover on the basis of the reference value stored in the memory means and a count value of the pulse counter means at the current position of the governor lever. Preferably, the above-mentioned memory means is arranged to store a count value from the pulse counter means as a renewable minimum or maximum speed reference value when the rotational speed of the prime mover is set at the minimum or maximum speed, and the arithmetic operation means is arranged to calculate the current rotational speed of the prime mover on the basis of the stored reference value and the count value of the pulse counter means at the current position of the governor lever. With the above-described arrangement, when the rotational speed of the prime mover is set at least at the minimum or maximum speed by the command means, the governor lever is turned according to the specified rotational speed, while the memory means stores a count value from the pulse counter means, corresponding to the rotational angle of the governor lever, as a renewable reference value at the minimum or maximum speed, so that the arithmetic operating means can calculate the value of governor lever rotation corresponding to the current rotational speed of the prime mover on the basis of a current count value of the pulse counter means and the stored reference value. Further, in an arrangement where the count values of the pulse counter means at both of the minimum and maximum speeds of the prime mover are stored in the memory means as renewable minimum and maximum reference values, the arithmetic operating means can calculate the rotational value of the governor lever corresponding to the current rotational speed of the engine, on the basis of the reference values and a current count value from the pulse counter means. The invention will be described by way of examples and with reference to the accompanying drawings in which: Fig. 1 shows a general arrangement of a prime mover rotational speed control system of related art; Fig. 2 is a processing flow chart of a prime mover rotational speed control of related art; Fig. 3 is a diagrammatic illustration of conditions of a count value from a pulse counter; Fig. 4 shows the general arrangement of a prime mover rotational speed control system of related art; Fig. 5 is a processing flow chart of prime mover rotational speed control of related art; Fig. 6 is a diagrammatic illustration of conditions of count value from a pulse counter; Fig. 7 shows the general arrangement of a prime mover rotational speed control system according to an embodiment of the present invention; Fig. 8 is a processing flow chart of the inventive prime mover rotational speed control; Fig. 9 is a diagrammatic illustration indicative of conditions of detected value of a potentiometer; Fig. 10 is a diagrammatic illustration indicative of conditions of count value from a pulse counter; Fig. 11 shows the general arrangement of a prime mover rotational speed control system of the prior art; Fig. 12 is a characteristics diagram showing the relationship between the value detected by a potentiometer and the rotational angle of the governor lever; and Fig. 13 is a diagrammatic illustration of a map showing the relationship between target value and target rotational speed stored in a memory area of the controller. In the following description of the embodiments, the component parts common to the above-described prior art are designated by common reference numerals and their descriptions are omitted to avoid repetitions. Referring to Figs. 1 to 3, there are indicated at 11 and 12 lever position sensor switches in the form of limit switches located in the vicinity of a governor lever 3 correspondingly to stoppers 4 and 5. These lever position sensor switches 11 and 12 are connected to a controller 13 which will be described hereinlater. The lever position sensor switch 11 is actuated when the governor lever 3 is turned to a position (minimum speed position) where the lever 3 is abutted against the stopper 4, and the lever position sensor switch 12 is actuated when the governor lever 3 is turned to a position (maximum speed position) where the lever 3 is abutted against the stopper 5, thereby sending the controller 13 a signal that the governor lever 3 has reached the minimum speed position (rotational value N B = 0%) or the maximum speed position (rotational value N B = 100%). The reference numeral 13 denotes the controller which is provided in the operator's cabin (not shown) and which is constituted, similarly to the afore-mentioned prior art controller 10, by an arithmetic processing circuit like CPU and a memory circuit including ROM, RAM or the like (neither one of the just-mentioned circuits is shown). The controller 13 is provided with a memory area 13A in the memory circuit, storing therein a map as shown in Fig. 13. The memory circuit of the controller 13 also stores therein a program as shown in Fig. 2. Upon receiving a command signal from the up-down switch 9, the controller 13 converts the command signal into a percentage target value M with reference to the map in the memory area 13A to set up, on the basis of the command signal, a target value M which corresponds to the target rotational speed of the engine 1. Then, from a count value X of the pulse counter 14, which will be described hereinlater, and the minimum and maximum reference values X 1 and X 2 , the controller 13 calculates a percentage rotational value N B of the governor lever 3 corresponding to the current rotational speed, comparing the target value M with the rotational value N B and accordingly controlling the rotational speed of the engine 1 through adjustment of the stepping motor 6. Denoted at 14 is a pulse counter which serves as the pulse counter means, the pulse counter 14 being adapted to add up and store an added number of control signals upon application of a forward rotation signal to the stepping motor 6 from the controller 13, and to subtract pulses and store a subtracted number of control pulses upon application of a reverse rotation signal. The prime mover rotational speed control system with the above-described construction has no difference in particular from the prior art counterpart in basic operation. A reference is now made to Fig. 2 to explain a rotational speed control process which is performed by the controller 13 for the engine 1. Firstly, upon starting the processing operation, a previously stored backup value X B is set for the maximum speed reference value X 2 in the memory area of the controller 13 in Step 1, followed by flag initialization in Step 2, resetting a flag F 1 which stands as F 1 =1 when a count value X from the pulse counter 14 is set in the minimum speed reference value X 1 and a flag F 2 which stands as F 2 =1 when a count value X is set in the maximum speed reference value X 2 . In next Step 3, detection signals S 1 and S 2 are read in from the respective lever position sensor switches 11 and 12, followed by Step 4 reading in a preset target value M corresponding to a command signal from the up-down switch 9 and Step 5 of reading in a count value X (a count value at the end of a previous engine operation when freshly starting the processing operation) from the pulse counter 14 at time t (which is t 0 when starting the processing operation) as shown in Fig. 3. Step 6 makes a checkup to see if the flag F 1 is set to stand as F 1 = 1, namely, to see if the minimum speed reference value X 1 is set. In this instance, since the flag F 1 has been reset in Step 2, the result of judgement in Step 6 is "NO" and the processing goes to Step 7 to see if the position sensor 11 is actuated (on). If the result of judgement in Step 7 is "NO", which means that the governor lever 3 has not reached the minimum speed position, a reverse rotation signal is produced for the stepping motor 6 in Step 8, thereafter returning to Step 3 and turning the governor lever 3 in the decelerating direction L until the lever position sensor switch 11 is actuated. If the result of judgement in Step 7 is "YES", which means that the governor lever 3 is at the minimum speed position (rotation value N B = 0%) abutting against the stopper 4, a stop signal is produced for the stepping motor 6 in Step 9 to stop the lever rotation, thereby preventing damages to the governor lever 3 and retaining same at that rotational angle. In Step 10, a count value X of the pulse counter 14 at that time point t 1 (Fig. 3) is stored as the minimum speed reference value X 1 , setting the flag F 1 to stand as F 1 = 1 in Step 11 and continuing the operations of Step 3 and afterwards. In case the flag F 1 is set, that is, if the result of judgement in Step 6 is "YES", the processing proceeds to Step 12 to see if the flag F 2 is set to stand as F 2 = 1. In this instance, since the flag F 2 was reset in Step 2, the result of judgement in Step 12 is "NO" and the processing goes to Step 13 to see if the target value M has reached the maximum rotational speed N H (M = 100%) as shown in Fig. 13. If the result of judgement in Step 13 is "NO", the current backup value X B is left to stand as the maximum speed reference value X 2 and the processing goes to Step 19 for control of the stepping motor 6 as will be described hereinlater. If the result of judgement in Step 13 is "YES", which means that the target value M has reached the value of M = 100%, the processing proceeds to Step 14 to see if the position sensor switch 12 is on. When the result of judgement in Step 14 is "NO", which means that the governor lever 3 has not reached the maximum speed position, the processing goes to Step 15 to produce a forward rotation signal for the stepping motor 6 and returns to Step 3 to rotate the governor lever 3 in the accelerating direction H until the lever position sensor switch 12 is actuated. If the result of judgement in Step 14 is "YES", which means that the governor lever 3 has reached the maximum speed position abutting against the stopper 5, a stop signal is produced in Step 16 to stop the lever rotation, thereby preventing damages to the governor lever 13 and maintaining same at that rotational angle. The processing then goes to Step 17 to store a count value X of the pulse counter 14 at that time point t 2 as a new maximum speed reference value X 2 , shown in Fig. 3, setting the flag F 2 to stand as F 2 = 1 in Step 18 before returning to Step 3. On the other hand, if the flag F 2 is found to be set by an affirmative result "YES" in Step 12 or if the target value M is found to have not yet reached the maximum rotational speed N H (M = 100%) by a negative result "NO" in Step 13, the processing goes to Step 19 to calculate the rotational value N B of the governor lever 3 corresponding to the current rotational speed of the engine 1, on the basis of the minimum and maximum speed reference values X 1 and X 2 and a current count value X of the pulse counter 14, as follows. N B = X-X 1 X 2 - X 2 X 100 The processing then goes to Step 20 to determine the deviation of the governor lever rotational value N B from the target value M. If the current rotational value N B is found to be smaller than the target value M in Step 20, the processing goes to Step 21 to produce a forward rotation signal for the stepping motor 6 to turn the governor lever 3 in the accelerating direction H, and returns to Step 3. If the rotational value N B is found to be greater than the target value M in Step 20, the operation goes to Step 22 to produce a reverse rotation signal for the stepping motor 6, turning the governor lever 3 in the decelerating direction L, and returns to Step 3. In case the rotational value N B is found to be substantially equal to the target value M in Step 20, the operation proceeds to Step 23 to produce a stop signal for the stepping motor 6, thereby maintaining the governor lever 3 at the current rotational value N B to operate the engine 1 constantly at that speed. After the minimum and maximum speed values X 1 and X 2 are set in the above-described manner, a cycle of Step 3 → Step 4 → Step 5 → Step 6 → Step 12 → Step 19 → Step 20 → Step 21, Step 22, Step 23 is repeated to effect an ordinary servo control. Thus, the lower limit value (the minimum speed position) and the upper limit value (the maximum speed position) of the rotational range of the governor lever 3 are detected by the position sensors 11 and 12, respectively, storing the count value of the pulse counter 14 as minimum and maximum speed reference values X 1 and X 2 when the governor lever 3 is at the minimum speed position (where the rotational value N B =0%) and the maximum speed position (where the rotational speed N B =100%), calculating the rotational value N B of the governor lever 3 as a percentage value corresponding to the rotational speed of the engine 1, from the respective reference values X 1 and X 2 and the current count value X of the pulse counter 14, and adjusting the stepping motor on the basis of the deviation of the rotational value N B from the target value M for the control of the rotational speed of the engine 1. Therefore, the rotational range of the governor lever 3 is automatically adjusted to coincide with the counting range of the pulse counter 14, obviating the use of the potentiometer 8 as described hereinbefore in connection with the prior art. This contributes to simplify the jobs of initial adjustments to a marked degree, while eliminating the adverse effects of the variations in output characteristics as well as the influences of noises to which the potentiometer 8 is very likely to be subjected. Since the automatic adjustment is effected on every start of the engine 1, it becomes possible to prevent development of a discrepancy between the rotational range of the governor lever 3 and the counting range of the pulse counter 14 in a reliable manner even in case the governor lever 3, link 7 or other parts have gone through mechanical wear as a result of repeated use over an extended period of time, thereby permitting to control the rotational speed of the engine 1 stably and accurately over a long time period with markedly improved reliability. Referring to Figs. 4 to 6, there is shown a tension spring to pull the governor lever toward the minimum speed position when the engine is at rest, omitting the lever position sensor switch which is provided on the side of the minimum speed position in the above-described first embodiment. More specifically, the reference 21 denotes a tension spring in the form of a coil spring which is provided in the vicinity of the engine 1. The coil spring 21 is supported at its base end by a support member, not shown, and has its fore end connected to the governor lever 3. The coil spring 21 constantly urges the governor lever 3 toward the minimum speed position, so that the governor lever 3 is abutted against the stopper 4 when the engine 1 is turned off and the stepping motor 6 is de-energized, namely, when there is no holding torque any more. Indicated at 22 is a controller which is arranged substantially in the same manner as the controller 13. The controller 22 is provided with a memory area 22A in the memory circuit to store the map of Fig. 13. Besides, a program as shown in Fig. 5, for example, is stored in the memory circuit thereby to control the rotational speed of the engine 1. Now, the rotational speed control is explained with reference to Fig. 5. Firstly, upon starting the processing operation, a previously stored backup value X B is set as the maximum reference value X 2 in the memory area 22A of the controller 22 in Step 31, followed by flag initialization in Step 32 resetting flags F 1 and F 2 . Nextly, the processing goes to Step 33 to read in a detection signal S 2 from the lever position sensor switch 12, and to Step 34 to read in a target value M which has been determined on the basis of a command signal from the up-down switch 9, reading in a count value X from the pulse counter 14 at time t as shown in Fig. 6. The processing then goes to Step 36 to ascertain if the flag F 1 is set to stand as F 1 = 1, which means that the minimum reference value X 1 is set. Since the flag F 1 was reset in Step 32, the result of judgement in Step 36 is "NO", and the processing proceeds to Step 37 to produce a stop signal for the stepping motor 6 to stop its rotation because the governor lever 3 is held in the minimum speed position under the influence of the biasing force of the spring 21, thus preventing damages to the governor lever 3 and maintaining its current rotational angle. A count value X at that time point is stored as the minimum speed reference value X 1 in Step 38, setting the flag F 1 to stand as F 1 = 1 in Step 39 and continuing the processing of Step 33 and afterwards. When the flag F 1 is set and the result of judgement in Step 36 is "YES", the processing goes to Step 40 to see if the flag F 2 is set. In this instance, since the flag F 2 was reset in Step 32, the result of judgement in Step 40 is "NO" and the processing proceeds to Step 41 to see whether or not the target value M has reached the maximum rotational speed N H (M = 100%). If the result of judgement in Step 41 is "NO", the processing then goes to Step 47 for the control of the stepping motor 6, leaving the previously set backup value X B to stand as the maximum speed reference value X 2 . When the results of judgement in Step 41 is "YES", that is to say, when the target value M is found to have reached the maximum rotational speed N H , the processing proceeds to Step 42 to see if the position sensor 12 is on. In case the result of judgement in Step 42 is "NO", which means that the governor lever 3 has not reached the maximum speed position, the processing goes to step 43 to produce a forward rotation signal for the stepping motor 6 and then returns to Step 33 to rotate the governor lever 3 in the accelerating direction H until the lever position sensor switch 12 is actuated. If the result of judgement in Step 42 is "YES", which means that the governor lever 3 is in the maximum rotational speed position in abutting engagement with the stopper 5, the processing goes to Step 44 to produce a stop signal for the stepping motor 6 to stop the governor lever rotation, thereby preventing damages to the governor lever 3 and maintaining same at that rotational angle. In Step 45, the maximum speed reference value X 2 is renewed with a count value X of the pulse counter 14 at time t 2 as shown in Fig. 6. The flag F 2 is set to stand as F 2 = 1 in Step 46 before returning to Step 33. On the other hand, in case the result of judgement in Step 40 is "YES", which means that the flag F 2 is set, or in case the result of judgement in Step 41 is "NO, which means that the target value M has not yet reached the maximum rotational speed N H , the processing goes to Step 47 to determine the rotational value N B of the governor lever 3, corresponding to the current rotational speed of the engine 1, from the minimum speed reference value X 1 , maximum speed reference value X 2 and a current count value X of the pulse counter according to Equation (1) given hereinbefore. Nextly, the processing goes to Step 48 to see if there is a deviation between the target value M and the rotational value N B of the governor lever 3, which are both expressed in percentage. If the current rotational value N B of the governor lever 3 is smaller than the target value M, the processing goes to Step 49 to produce a forward rotation signal for the stepping motor 6 to turn the governor lever 3 in the accelerating direction H, and then returns to Step 33. When the rotational value N B is found to be greater than the target value M in Step 48, the processing goes to Step 50 to produce a reverse rotation signal for the stepping motor 6 to turn the governor lever 3 in the decelerating direction L, and then returns to Step 33. In case the rotational value N B is found to be substantially equal with the target value M in Step 48, the processing proceeds to Step 51 to produce a stop signal for the stepping motor 6, maintaining the governor lever 3 at the current rotational angle to operate the engine 1 constantly at that speed. After the minimum and maximum speed reference values X 1 and X 2 are set in the above-described manner, the processing repeats the cycle of Step 33 → Step 34 → Step 35 → Step 36 → Step 40 → Step 47 → Step 48 → Step 49, Step 50, Step 51 for an ordinary servo control. Thus, in addition to the operational effects substantially similar to those mentioned before, this arrangement, including the spring 21 which urges the governor lever 3 constantly toward the minimum speed position, permits to omit the lever position sensor switch 11 provided as mentioned before to detect the location of the governor lever 3 in the minimum speed position, and therefore contributes to reduce the production cost of the prime mover speed control system all the more. Referring now to Figs. 7 to 10, there is shown an embodiment of the invention, a feature of which resides in that a torque limiter is provided within the length of the link in place of the lever position sensor switches as mentioned before. In these figures, the reference 31 denotes a stopper which is similar in construction to the stopper 4 of the prior art mentioned hereinbefore, and arranged to abut against the governor lever 3 to delimit the rotational range thereof. In this instance, however, it is located in such a position that the rotation of the engine 1 is stopped as soon as the governor lever 3 comes into abutting engagement with the stopper 31. Namely, as shown in Fig. 7, the stopper 31 limits the rotation of the governor lever 3 in the accelerating and decelerating directions H and L to a rotational range  in cooperation with the stopper 5. When the governor lever 3 is abutted against the stopper 31, the rotational speed of the engine 1 is dropped substantially to zero to stop its rotation. When abutted against the stopper 5, the rotational speed of the engine 1 is increased to the maximum speed N H . From the minimum speed position for the minimum rotational speed N L , which is indicated by solid line in Fig. 7, the governor lever 3 is rotatable until it is abutted against the stopper 5, adjusting the rotational speed of the engine 1 within a control range  C . Indicated at 32 is a torque limiter which is inserted in the link 7 at a position between the lever 6A of the stepping motor 6 and a lever 34A of a potentiometer 34 which will be described hereinlater. For example, the torque limiter 32 is constituted by a coil spring or the like. The torque limiter 32 acts as a rigid body when the stepping motor 6 is turned in the forward direction F or reverse direction R, for transmitting the rotation of the stepping motor 6 to the governor lever 3 through the link 7, and acts as a buffer when the governor lever 3 is abutted against the stopper 31 or 5, preventing damages to the governor lever 3 which might be caused by overmuch rotation of the stepping motor 6. Denoted at 33 is a controller which is substantially same in construction as the controllers 13 and 22 of the foregoing first and second one's, including a memory area 32A in its memory circuit to store the map of Fig. 13 along with a predetermined value V 1 which will be described hereinlater. In this embodiment, a program as shown in Fig. 8 is stored in the memory circuit of the controller 32 to control the rotational speed of the engine 1. Upon stopping the engine 1, the controller 22 rotates the stepping motor 6 in the reverse direction R thereby to abut the governor lever 3 against the stopper 31. Further, indicated at 34 is a potentiometer which serves as a rotational angle sensor means for detecting the rotational angle of the governor lever 3 through the link 7. The potentiometer 34 is arranged substantially in the same manner as the potentiometer 8 of the prior art in general construction, including a lever 34A. In this instance, however, the potentiometer 34 is preadjusted such that, when the governor lever 3 is turned to the minimum speed position of Fig. 7 at time t 1 as shown in Fig. 9, for example, its detection value V takes a value corresponding to the predetermined value V 1 stored in the memory area 33A of the controller 33. A description on the rotational speed control by this embodiment is given below with reference to Fig. 8. Firstly, upon starting the processing operation, a previously stored backup value X B in the memory area 33A of the controller 33 is set as the maximum speed reference value X 2 in Step 61, which is followed by Step 62 of resetting flags F 1 and F 2 , Step 63 of reading in a target value M, Step 64 of reading in a count value from the pulse counter 14, and Step 65 of reading in a detection value V from the potentiometer 34. The processing then goes to Step 66 to ascertain whether or not the flag F 1 is set to stand as F 1 = 1. In this instance, the flag F 1 was reset in Step 62 so that the result of judgement in Step 66 is "NO" and the processing proceeds to Step 67. In Step 67, since the governor lever 6 is abutted against the stopper 31 at time t 0 as shown in Fig. 9, a forward rotation signal is produced for the stepping motor 6, turning the governor lever 3 in the accelerating direction H until the result of judgement in Step 69 becomes affirmative. Nextly, the predetermined value V 1 , which was stored in the memory area 33A in the stage of preadjustment of the rotational speed control system, is read out in Step 68, followed by Step 69 checking up whether or not the detection value V from the potentiometer has reached a value substantially equal to the predetermined value V 1 . If the result of judgement in Step 69 is "YES", which means that the governor lever 3 is in the minimum speed position indicated by solid line in Fig. 7, a stop signal is produced for the stepping motor 6 in Step 70 to stop rotation of the lever, thereby retaining the governor lever 3 at the current rotational angle. The processing then goes to Step 71 to store a count value X of the pulse counter 14 at time t 1 , as shown in Fig. 10, as the minimum speed reference value X 1 , and to Step 72 to set the flag F 1 to stand as F 1 = 1, before returning to Step 63. On the other hand, when the flag F 1 has been set and the result of judgement of Step 66 is "YES", the processing proceeds to Step 73 to see whether or not the flag F 2 is set. In this instance, the flag F 2 was reset in Step 62, so that the result of judgement in Step 73 is "NO" and the processing goes to Step 74 to ascertain whether or not the target value M has reached the level of M = 100% (see Fig. 13) which corresponds to the maximum rotational speed N H . If the result of judgement in Step 74 is "NO", the processing goes to Step 80 for controlling the stepping motor 6 as will be described hereinlater, leaving the current backup value X B to stand as the maximum speed reference value X 2 . In case the result of judgement in Step 74 is "YES", which means that the target value M has reached the level of M = 100%, the processing goes to Step 75 to produce a forward rotation signal to turn the governor lever 3 in the accelerating direction H until the result of next Step 76 becomes affirmative. Step 76 checks up whether or not the detection value V from the potentiometer 34 has become constant. If the result of judgement in Step 76 is "YES", which means that the governor lever 3 is in the maximum speed position in abutting engagement with the stopper 5 and the detection value V from the potentiometer 34 has become constant by actuation of the torque limiter 32, the processing goes to Step 77 to produce a stop signal for the stepping motor 6 thereby stopping the lever rotation and retaining the governor lever 3 at that rotational angle, and then to Step 78 to renew the maximum speed reference value X 2 with a count value X read in from the pulse counter 14 at that time point t 2 (Fig. 10), setting the flag F 2 to stand as F 2 = 1 in Step 79 before returning to Step 63. On the other hand, in case the result of judgement in Step 73 is "YES", which means that the flag F 2 is set, or in case the result of judgement in Step 74 is "NO", which means that the target value M has not reached the level of M = 100%, the processing goes to Step 80 to determine the rotational value N B of the governor lever corresponding to the current rotational speed of the engine 1 according to Equation (1) on the basis of the minimum speed reference value X 1 , maximum speed reference value X 2 and current count value X of the pulse counter 14. Nextly, the processing goes to Step 81 to see if there is a deviation between the rotational value N B of the governor lever 3 and the target value M, and, if the rotational value N B is found to be smaller than the target value M, goes to Step 82 to produce a forward rotation signal for the stepping motor 6. In case the current rotational value N B is found to be greater than the target value M, the processing goes to Step 83 to produce a reverse rotation signal for the stepping motor 6. When the rotational value N B is found to be substantially equal with the target value M, the processing proceeds to Step 84 to produce a stop signal for the stepping motor 6, retaining the governor lever 3 at the current rotational angle to operate the engine 1 at that speed. After the minimum speed reference value X 1 and maximum speed reference value X 2 are set, a cycle of Step 63 → Step 64 → Step 65 → Step 66 → Step 73 → Step 80 → Step 81 → Step 82, Step 83, Step 84 is repeated to effect an ordinary servo control. In addition to the operational effects similar to those in the foregoing first and second embodiments, the third embodiment with the above-described arrangement, including the torque limiter 32 inserted within the length of the link 7, can prevent damages to the governor lever 3 or other components even when the governor lever 3 is pressed against the stopper 5 by forward rotation of the stepping motor 6 in the direction of arrow F, without the provision of the lever position sensor switches 11 and 12 in the first embodiment, setting both of the minimum speed reference value X 1 and the maximum speed reference value X 2 upon each start of the engine 1 to ensure accurate control of rotational speed of the engine 1. In the foregoing arrangements, the arithmetic operating means is embodied into Steps 19, 47 and 80 of the programs of Figs. 2, 5 and 8, and the memory means is embodied into Steps 10, 17, 38, 45, 71 and 78. The pulse counter 14 which serves as a pulse counting means is provided outside the controller 13, 22 or 33 in the foregoing arrangements. However, according to the present invention, the controller is not restricted to such an arrangement and may be arranged to include a pulse counter if desired. In place of the up-down switch which is employed in the foregoing arrangements, the command means may be constituted by a mode selector switch, a fuel lever or the like. On the other hand, in the foregoing arrangements, the target value M is converted into a percentage value according to the map of Fig. 13, for comparison with the rotational value N B , a percentage value which is calculated according to Equation (1) on the basis of the minimum sped reference value X 1 , maximum speed reference value X 2 and current count value X. However, for the purpose of comparison, the target value M and rotational value N B may be expressed by a numerical value of from 0 to 1 if desired. Further, in place of the limit switches which are employed as the lever position sensor switches 11 and 12 in the first above mentioned arrangement to determine both of the minimum speed reference value X 1 and the maximum speed reference value X 2 , approaching switches or other sensor switches may be used as the lever position sensor switches, or alternatively a lever sensor switch may be provided only on the side of the minimum speed position to obtain the minimum speed reference value X 1 while setting a backup value X B for the maximum speed reference value X 2 . If desired, the coil spring or tension spring 21, which is employed in the second embodiment to constantly urge the governor lever 3 toward the minimum rotational speed position, may be substituted with a compression spring which is arranged to bias the governor lever 3 constantly toward the minimum speed position. Furthermore, in the above-described third embodiment, the location of the governor lever 3 at the maximum speed position is detected by abutting the governor lever 3 against the stopper 5 while ascertaining whether or not the detection value V from the potentiometer 34 has become constant in Step 76. Alternatively, approaching switches may be provided for this purpose, for example, on the torque limiter 32 to detect the location of the governor lever 3 at the minimum and maximum speed positions, or a rotary encoder or the like may be used as a rotational angle sensor means. Moreover, although the above embodiment is arranged to detect the location of the governor lever 3 at the minimum speed position on the basis of the detection value V from the potentiometer 34, it may alternatively employ, for example, an approaching switch, a limit switch or the like for detection of the governor lever 3 arriving at the minimum speed position. On the other hand, a biasing spring which constantly urges the governor lever 3 toward the minimum speed position may be provided in the first and third embodiment, or a torque limiter may be provided in the above arrangements if desired.

Claims: Prime mover rotational speed control system including a prime mover (1), a governor (2) having a governor lever (3) to increase or reduce the rotational speed of the prime mover (1) according to the rotational angle of said governor lever (3), a stepping motor (6) adapted to turn the governor lever (3) according to a control pulse signal, a command means (9) for specifying a target rotational speed (M) of the prime mover (1), a controller (13, 22, 33) adapted to produce the control pulse signal in accordance with the target rotational speed (M) and the current rotational speed of the prime mover (1) for application to the stepping motor (6), and a pulse counter means (14) for counting control signal pulses to be applied to the stepping motor (6) by adding pulses for a forward rotation and subtracting pulses for a reverse rotation, thereby calculating a count value (X) corresponding to the current position of the stepping motor and to the current rotational speed, a rotational angle sensing means (34) for detecting the rotational angle of the lever (3), a maximum speed position stopper (5) defining the maximum rotational speed position of the governor lever, and an arithmetic operating means adapted (step 19, 47, 80) to calculate the current rotational speed on the basis of a maximum speed reference value (X 2 ), the count value (X) and a minimum speed reference value (X 1 ) for the minimum speed of the prime mover (1), characterized in that a torque limiter (32) is provided between the governor lever (3) and the stepping motor (6) for limiting the torque applied to the governor lever (3), and the controller (13, 22, 33) is adapted, after setting the target rotational speed (M) of the prime mover (1) at the maximum speed of the operating speed range, to forwardly turn the governor lever (3) until it abuts against the stopper (5), to detect whether the output of the sensing means (34) has become constant, and to store, in a memory means (13A, 22A, 33A), the count value (X) as a renewable maximum speed reference value (X 2 ) when due to the function of the torque limiter a constant output of the sensing means (34) was detected.

Inventors: EGASHIRA MASAKI, HAGA MASAKAZU, HIRATA TOUICHI, TOMIKAWA OSAMU ROOM, EGASHIRA, MASAKI, HAGA, MASAKAZU, HIRATA, TOUICHI, TOMIKAWA, OSAMU, TOMIKAWA, OSAMU, ROOM 102,
Applicants: HITACHI CONSTRUCTION MACHINERY, HITACHI CONSTRUCTION MACHINERY CO., LTD.
Application year: 1991
Main classification: F02D1
IPC codes: F02D1, F02D29, F02D31, F02D41