Load state decision apparatus of servomotor

Abstract: In a load state decision apparatus of a servomotor (16) where a load current value of a servomotor (16) during operation is sampled and compared with a prescribed value and then the load state is decided based on the result, a motor current value is detected (14) by adding the function to the existing apparatus and the detected motor current value (14) is sampled at prescribed time intervals and stored in a memory, and an effective current value is calculated from the current value data and the calculated effective current value is compared with the rated current value and the load state of the servomotor (16) is decided, thereby a converting or detecting apparatus need not be installed at the outside but the load state can be decided automatically.

Description: The invention relates to an apparatus for monitoring the load state of an electric motor, comprising means for detecting a load current value of said motor during operation; means for sampling the detected load current value at prescribed time intervals and for storing the sampled value as current value data in a memory; means for calculating an effective current value from the current value data and for comparing the calculated effective current value with a rated current value for deciding the load state. From GB-A-2 122 749 an apparatus for monitoring the conditions of an electrically driven motor is known, comprising means for detecting and sampling the load current of the motor, means for comparing calculated values with reference values and testing the significance of these results. This known apparatus permits the monitoring of various data of an electric motor during test phases. Moreover it is generally known to measure the load state of a servomotor during operation, i.e. for example the torque, power, delay, speed or the like by using a detecting apparatus. As a well-known means an oscilloscope is used, the wave forms are recorded and later compared with standard wave forms for obtaining a decision. The consumed power is measured by a watthourmeter or the like and divided by the measuring time to obtain the effective current to be compared with the rated current. These methods require technical skill and knowledge of the user, and personal errors are likely to occur when making decisions. It is an object of the present invention to provide an apparatus for monitoring the load state of a servomotor without the necessity to use complicated equipment, and by which the results of the load state can be automatically obtained. In order to attain the foregoing object, an apparatus for monitoring the load state of an electric motor as defined in the preamble of claim 1, is characterized in that the motor is a servomotor, the load state of which is controlled by a positioning apparatus and a servo control unit having data communication means respectively so that data can be transmitted and received mutually; and said monitoring apparatus further comprises a setting indicator including a start key and a stop key, the operation of the start key starting the writing of sampled current value data from the memory into a further memory and the operation of the stop key starting the reading of the current value data stored in the further memory, the calculation of the effective current value from the current value data and the comparison of the calculated effective current value with the rated current value for deciding the load state. Preferable embodiments are defined in the dependent claims. The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention. FIG. 1 is a blode diagram of a servomotor positioning control system as an embodiment of the invention; FIG. 2 is a timing chart illustrating a data sampling processing by an embodiment of the invention; FIG. 3 is a flow chart illustrating a processing part A by an embodiment of the invention; FIG. 4 is a flow chart illustrating a processing part B by an embodiment of the invention; FIG. 5 is a flow chart illustrating the processing as a whole by an embodiment of the invention; FIG. 6 is a chart illustrating a record organization within a memory of an embodiment of the invention; FIG. 7 is a characteristic diagram illustrating an example of torque or current value of a servomotor; and FIG. 8 is an explanation diagram illustrating the calculation of an effective value by an embodiment of the invention. A preferred embodiments of the invention will now be described in detail referring to the accompanying drawings. FIG. 1 shows a servomotor positioning control system as an embodiment of the invention. In FIG. 1, reference numeral 1 designates a positioning apparatus, numeral 2 designates a CPU of the positioning apparatus 1, numeral 3 designates a bus I/F for performing interface with a servo control unit 7, numeral 4 designates a memory for storing data read from two-port memory 10 of a servo control unit 7, numeral 5 designates an interface section (I/F section) to a setting indicator 6 having a display device and an operation key, numeral 7 designates a servo control unit, numeral 8 designates a CPU of the servo control unit 7, numeral 9 designates a bus I/F for performing interface with the positioning apparatus 1, numeral 10 designates a two-port memory capable of accessing the CPU 2 of the positioning apparatus 1 and the CPU 8 of the servo control unit 7, numeral 11 designates a current controller for controlling current supplied to a servo motor 16, numeral 12 designates a power amplifier for producing current supplied to a servo motor 16 according to a command of the current controller 11, numeral 13 designates an A/D converter where an analog value of current detected by a motor current detector 14 is converted into a digital value, numeral 14 designates a motor current detector for detecting the motor current transmitted from the power amplifier 12, numeral 15 designates a detector I/F for inputting the pulse signal outputted from a detector 17 and for calculating a position feedback value and a speed feedback value and transmitting the feedback values to the CPU 8, numeral 16 designates a servomotor, numeral 17 designates a detector for obtaining position information and speed information of the servomotor 16, and numeral 18 designates a bus cable connecting the positioning apparatus 1 and the servo control unit 7. Next, operation will be described. As a general operation, a command is outputted from the positioning apparatus 1 according to a command of the setting indicator 6, and power is supplied from the servo control unit 7 to the motor 16 according to the command of the positioning apparatus 1, and thereby the motor 16 is rotated. The detector 17 is used for feeding back the state of position or speed of the motor 16 to the servo control unit 7. Current supplied to the motor 16 is detected by the motor current detector 14 and is converted into a digital quantity by the A/D converter 13 and fed back. Next, operation of deciding the load state will be described. First, means are described for sampling the current value detected by the motor current detector 14 and for transferring the sampled value from the servo control unit 7 to the positioning apparatus 1. FIG. 2 shows the data sampling processing by an embodiment of the invention. In FIG. 2, a clock signal 20 is shown that is generated by a clock incorporated in the positioning apparatus 1, and which is outputted also to the servo control unit 7 through the bus cable 18. One period of the clock signal 20 is 3 - 5 ms, and data is transferred one time during this period. Processing part A is that at the side of the servo control unit 7, and when the CPU 8 of the servo control unit 7 detects a rise 21 of the clock signal 20, the execution begins, and the period of the clock signal 20 is set so that the execution begins when the CPU 8 of the servo control unit 7 detects the rise 21 of the clock signal, and it is always finished before a fall 22 of the clock signal 20. Part designated by numeral 31 represents that the processing part A is being executed. Content of the processing part A as shown in FIG. 3 is that a motor current value detected by the motor current detector 14 and converted into digital value by the A/D converter 13 is taken and is written in a determined area of the two-port memory 10. Thereby the current value data is sampled in the two-port memory 10 in one piece during one period of the clock signal 20. Next, processing part B is that at the side of the positioning apparatus 1, and the execution is started when the CPU 2 of the positioning apparatus 1 detects the fall 22 of the clock signal 20 and it is always finished before the rise 21 of next clock signal 20. That is, the period of the clock signal 20 is set so that any of the required time for the processing A and the required time for the processing B is within a half period of the clock signal 20. In actual fact, the processing to access the two-port memory 10 may be finished before the rise time of next clock signal 20, and the processing not to access the two-port memory 10 is possible irrespective of the clock signal 20. Content of the processing part B as shown in FIG. 4 is that the motor current value data is read from a determined area of the two-port memory 10 and is written in a determined area of the memory 4 of the positioning apparatus 1. The procedure that the positioning apparatus 1 reads the motor current value at the time intervals determined by the clock signal 20 has been described. FIG. 6 shows form of the motor current value data stored in the memory 4. FIG. 5 shows the whole processing of the load state decision according to an embodiment of the invention. The load state decision processing shown in FIG. 5 will be described. In order to decide the load state of the motor during the positioning operation, the start key of the setting indicator 6 is pushed. Then, the process becomes "YES" at step S1, and is transferred to the processing part B of step S2. At the processing part B, as shown in FIG. 4, the motor current value data is read from the two-port memory 10 and written in the memory 4. Next, the process proceeds to step S3, and the address to write the next motor current value is updated. If this updating is finished, the process returns to the beginning, and as long as the stop key is not pushed, the above-mentioned steps are repeated, and as shown in FIG. 6, the motor current value data is recorded in the memory 4. After lapse of the desired data taking time, a stop key of the setting indicator 6 is pushed. Then, the process proceeds from step S1 to step S4 and from step S4 to step S5, and the current value data stored in the memory 4 is read, and then the effective current value is calculated by hereinafter described means. Next, the process proceeds to step S6, and the calculated effective current value is compared with the rated current value and the load state is decided and indicated in the setting indicator 6. Or the decision result of the load state is utilized, for example, as the feedback data in the control of the motor. Next, a calculation method for the effective current value Ie will be described. Effective current is estimated by taking the mean square value of the current data. That is, each data is squared and the sum is estimated and divided by the number of data and its root is estimated to obtain the effective value. That is, the root of [(I₁² + I₂² + ... + I n ²)/n] may be calculated. n designates the number of data. Eventually, the effective current value Ie is expressed as follows: Ie = (I₁² + I₂² + ... + I n ²)/n The effective current value calculated in this manner is compared with the rated current and the decision is performed, however, since the rated current value is different depending on the capacity and condition of the motor, for example, if the rated current value is made 100% and the effective current value is expressed by %, it can be readily understood. Time of sampling the current value or the number of sampling can be set arbitrarily in that a harmonic generating converter or the like is interposed in the clock signal thereby the signal period is made variable, and the effective value being the calculated mean square value is compared with the rated current determined corresponding to the sampled time thereby the load state can be decided. Consequently, the instantaneous maximum current value is sampled and the effective value is calculated and compared with the rated value corresponding to this, thereby the load state can be decided. Since the torque and the calorific value of the motor is proportional to current, the decision of the load state becomes important for the thermal margin of the motor. According to the invention as above described, using the positioning apparatus and the servo control unit, the motor current value is sampled and the effective value is calculated and compared with the rated current and the load state is decided, thereby the decision of the load state can be performed automatically. Further according to the invention, a converting or detecting apparatus such as an oscilloscope need not be installed at the outside and the load state is decided by the positioning apparatus and the servo control unit, thereby the load state can be decided easily at low cost. Still further according to the invention, since a converting or detecting apparatus such as an oscilloscope need not be installed at the inside and the positioning apparatus and the servo control unit already existing are provided with the load state deciding function, an apparatus need not be newly added and the load state can be decided using a conventional apparatus structure as it is.

Claims: Apparatus for monitoring the load state of an electric motor (16), comprising means (14) for detecting a load current value of said motor (16) during operation; means (8, 10) for sampling the detected load current value at prescribed time intervals and for storing the sampled value as current value data in a memory (10); means (2) for calculating an effective current value from the current value data and for comparing the calculated effective current value with a rated current value for deciding the load state; characterized in that the motor is a servomotor (16), the load state of which is controlled by a positioning apparatus (1) and a servo control unit (7) having data communication means (18) respectively so that data can be transmitted and received mutually; and said monitoring apparatus further comprises a setting indicator (6) including a start key and a stop key, the operation of the start key starting the writing of sampled current value data from the memory (10) into a further memory (4) and the operation of the stop key starting the reading of the current value data stored in the further memory (4), the calculation of the effective current value from the current value data and the comparison of the calculated effective current value with the rated current value for deciding the load state. Apparatus as set forth in claim 1, wherein said load current value detecting means is installed in said servo control unit (7) and is constituted by a motor current detector (14) for detecting the load current of said servomotor (16) as analog quantity and an A/D converter (13) for converting the detected analog quantity into digital quantity, so that the load current of said servomotor (16) is detected and is sampled during a prescribed period. Apparatus as set forth in claim 1, wherein said sampling and storing means comprises a clock which is installed in said positioning apparatus (1), and a clock signal (20) of prescribed period is generated and outputted also to said servo control unit (7) through said data communication means (18), and a load current value being sychronized with the clock signal (20) and converted into a digital signal is sampled and stored. Apparatus as set forth in claim 3, wherein said sampling and storing means comprises a CPU (8) and the memory (10) installed in said servo control unit (7), and rise (21) of the clock signal (20) is detected by the CPU (8), and the load current value converted into a digital signal is sampled and written into the memory. Apparatus as set forth in claim 4, wherein said memory (10) is a two-port memory. Apparatus as set forth in claim 4 or claim 5, wherein said sampling and storing means comprises a further CPU (2) and the further memory (4) installed in said positioning apparatus (1), and fall (22) of the clock signal (20) is detected by the further CPU (2), and the load current value data is read from the memory (10) in said servo control unit (7) and also is written into the further memory (4) in said positioning apparatus (1). Apparatus as set forth in claim 1, wherein when each sample of current value data is made I₁, I₂, ..., I n , said calculating means calculates the effective current value Ie by following formula Ie = (I₁² + I₂² + ... + I n ²)/n wherein n: the number of samples Apparatus as set forth in claim 1, wherein said decision means indicates the effective current value in % of the rated current value corresponding to 100 %. Apparatus as set forth in claim 3, wherein the period of the clock signal (20) is set between 3 ms and 5 ms. Apparatus as set forth in claim 3, wherein the clock signal (20) is outputted through a harmonic generating converter, and the signal period can be varied arbitrarily.

Inventors: NISHIMURA MAKOTO C O MITSUBISH, TAKASE SHIGEAKI C O MITSUBISHI, NISHIMURA, MAKOTO C/O MITSUBISHI DENKI K.K., TAKASE, SHIGEAKI C/O MITSUBISHI ELEC. ENGIN.CO.LTD
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: G01R31
IPC codes: G05B19, H02P29, G01R21, G01R31