Absolute position measuring transducer system for an industrial robot

Abstract: Absolute-measuring position transducer system for an in­ dustrial robot which has one unit for each robot axis with a resolver (32) mechanically coupled to the axis movement. The unit has a revolution counter (405) which, on the basis of the resolver signals, continuously indicates within which revolution the resolver is, and an R/D converter (406) for accurate determination of the resolver position within each revolution. During normal operation the resolver is supplied from an oscillator (403). In case of drop out of the supply voltage to the axis unit, the resolver is switched for pulse mode operation via a second oscillator (404) and the R/D converter is disconnected. The second oscillator (404) and the revolution counter (405) are then supplied from an accu­ mulator battery (402).

Description: The invention relates to an absolute position measuring transducer system for an industrial robot according to the precharacterising part of claim 1. Such a system is known from the EP-A-0 177 901. It is previously known to use resolvers as position transducers in industrial robots. To achieve the desired high accuracy of the position value obtained with the aid of such a resolver, the resolver is usually arranged such that the operating range of the axis covers a plurality of revolutions of the resolver. This can be achieved, for example, by mounting the resolver on the shaft of a motor which drives the robot axis via a gear. However, this solution has the drawback that the resolver signal does not give an unambiguous indication of the position of the axis. Therefore, the equipment must be supplemented with electronic members in the form of counters, memories, etc., which continuously keep track of within which resolver revolution the axis is at the moment. These members must be reset when putting a robot into service and thereafter after each voltage drop out or other switch-off of the voltage supply of the robot. These so-called synchronization operations require time and manual effort. Therefore, it is desired to provide so-called absolute position measuring transducer systems , i.e. systems which provide an unambiguous - and accurate - determination of the position of a robot axis without requiring any synchronization operations. It has been proposed to design such a system with two resolvers for each robot axis. One resolver is arranged so as to rotate a plurality of revolutions when the robot axis moves between the limits of its operating range, thus making possible an accurate determination of the position. The other resolver is arranged so as to rotate less than one revolution when the robot axis moves between the limits of its operating range, and with the aid of this resolver an unambiguous determination may be obtained as to within which revolution the first resolver is situated. Thus, by combining the output signals of the two resolvers, an unambiguous determination of the position of the robot axis may be obtained. However, this solution requires two resolvers per robot axis and is therefore complicated and expensive. In prior art position transducer equipment using resolvers, these have been connected to a control system common to all robot axes, which system then comprises supply and sensing members for the resolvers. Since each resolver has three windings, an extensive cable arrangement between the resolvers and the control system is then required. Such a cable arrangement will be expensive and bulky. This is particularly the case with absolute position measuring transducer systems having two resolvers per robot axis. From EP-A-177 901 it is previously known to provide an absolute position measuring transducer system for an industrial robot by arranging for each robot axis two transducers, i.e. a resolver and a pulse transducer with associated circuits for sensing the direction of movement and counting the aggregate number of revolutions of the axis. In normal operation, the number of whole revolutions for each axis, obtained from the sensing circuits of the pulse transducer, is combined in a calculation circuit with the angular position within each revolution, which is obtained from the resolver of the same axis. In case of drop out of the supply voltage, the pulse transducer with associated reading circuits is supplied from a battery and stores a value which correctly indicates the number of whole revolutions of the axis independently of movements of the robot during the voltage drop out period. The pulse transducer may be fed from an oscillator with a low on/off ratio in order to reduce the power consumption during the battery supply interval. Thus, in this known system, the resolver with its high current consumption is not used during the battery feed interval, but the resolver is supplement with another type of position transducer, a pulse transducer, which in itself has a low power consumption. The invention aims at providing an absolute position measuring transducer system for an industrial robot, which determines the axis positions with high accuracy, is of relatively simple and inexpensive design with respect to both the transducer units and the required cable arrangement, and requires only little energy to continue performance in case of a drop out of the normal supply voltage . To achieve this aim the invention suggests an absolute position measuring transducer system for an industrial robot according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention one and the same transducer, namely the resolver, is used both during normal operation and for keeping track of the number of full revolutions of the axes during battery operation. This results in a considerable cost saving and a reduction of the complexity of the system compared with the prior systems described above in which for each axis the resolver must be supplemented with an additional transducer - for example an additional resolver or a pulse transducer - with its sensing circuits. The consumption of a resolver, which per se is high and which is the reason for the above-mentioned known solution with an extra transducer, a pulse transducer with its low current consumption, for each axis, is reduced according to the invention - by the novel pulse mode operation of the resolver during battery-powered intervals - to such low values that the battery operation may be maintained also during long interruptions of the normal current supply. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 an industrial robot which is known per se, Figure 2 a position transducer system, known per se, with a resolver, Figure 3 how, according to the invention, a position transducer unit is connected to the robot axis and to a common control system, Figure 4 an example of the design of the position transducer unit according to the invention for one of the robot axes, Figure 5 an embodiment of the revolution counter, included in the position transducer unit according to the invention, Figure 6a, 6b and 6c some of the signals occurring in the equipment according to Figure 4 and 5, Figure 7 an embodiment of the connection of the position transducer units for the various robot axes to the common control system in a position transducer system according to the invention. Figure 1 schematically shows an industrial robot, known per se. It has a base plate 1 mounted on a foundation, a lower part 2 which is rotatable around an axis 10 perpendicular to the base plate, a lower arm 3, which is rotatable around an axis 11 perpendicular to the plane of the paper, an upper arm 4 which is rotatable in relation to the lower arm around an axis 12 perpendicular to the plane of the paper, and a wrist 5 which is rotatable in relation to the upper arm around an axis 13 perpendicular to the plane of the paper. The robot wrist 5 supports a tool, for example a gripping device with jaws 6a and 6b, for gripping an object 7. The tool is rotatable in relation to the robot wrist 5 around an axis 14. Thus, the robot has five axes of movements, each one with a drive motor and with position transducers for sensing the actual rotational angle in the corresponding axis. The robot has a control system 20 which is connected to the robot with the aid of a cable 21. The control system generates, in a known manner, desired position values for the various axes based on a program stored into the control system, compares these desired values with the actual position values for the various axes and generates, in dependence thereon, control signals for the drive motors of the axes. Figure 2 shows how, in a manner known per se, the drive and position sensing system may be designed for one of the axes of movement of the robot, for example, the axis 11. The drive motor 31 of the axis is connected to the robot axis via a gear 30, for example a reduction gear set with the gear wheels 30a and 30b. The drive motor is supplied from the control system 20 of the robot. A resolver 32 is mechanically connected to the motor shaft for sensing the shaft rotational angle position. The resolver is fed from the control system 20, and its two sensing windings are connected to the control system, which in a known manner, in dependence on the voltages induced in the two sensing windings of the resolver by the magnetic field from the excitation winding of the resolver, generates a signal corresponding to the rotational angle of the resolver. In a system of this known type, the cable arrangement is extensive, since each one of the three windings of each resolver must be connected to the control system 20. Further, the system shown in Figure 2 is not absolute-measuring and therefore, as mentioned in the introduction, a manual synchronization operation is required when putting the robot into operation or when resuming operation after a power failure. In a position transducer system of absolute-measuring type, known per se, with two resolvers for each robot axis, the cable arrangement is still more complicated. Figure 3 shows that part of a position transducer system according to the invention which belongs to a robot axis, for example, the axis 11. Each axis has an axis unit 400 comprising a resolver 32, connected to the shaft of the drive motor, and an electronic unit 40 for supplying the resolver and for processing of the output signals of the resolver. The latter are converted in the electronic unit into a digital signal which is supplied to the control system 20 via a data line 42. The control system delivers to each one of the axis unit a supply voltage via line 41. Figure 4 shows an example of the design of the axis unit 400 in Figure 3. The unit comprises an oscillator 403 for continuous supply of the resolver, an oscillator 404 for pulsed supply of the resolver, a revolution counter 405, an R/D converter 406, and a voltage-sensing relay 407. The supply input 410 of the oscillator 403 and the supply input 413 of the R/D converter 406 are directly connected to the supply line 41 from the control system 20. The supply inputs 411 and 421 of the oscillator 404 and the revolution counter 405 are connected to an accumulator battery 402, which via a diode 401 is connected to the supply line 41. During normal operation, all four units 403-406 receive supply voltage from the line 41 and are in operation. The necessary charge or compensating charge of the battery 402 may then suitably be made with the aid of a battery charger, not shown in Figure 4. When the supply voltage is present, the relay 407 is activated and connects the output 415 of the oscillator 403 via the contact 408 and the line 423 to the excitation winding 32a of the resolver. At the same time, a signal corresponding to a logical "one" is delivered via the relay contact 409 to the synchronization input 425 of the revolution counter 405. In case of drop out of the supply voltage from the control system, the relay 407 falls back into its home position, with its contacts 408 and 409 assuming the positions shown in Figure 4. The oscillator 403 and the R/D converter 406 do not receive supply voltage and stop working. The oscillator 404 and the revolution counter 405, on the other hand, do receive supply voltage from the battery 402. The output 416 of the oscillator 404 is connected via the contact 408 to the excitation 32a winding of the resolver. From a synchronization output 417 of the oscillator 404 a synchronization signal is delivered via the contact 409 to the synchronization input 425 of the revolution counter 405. The two sensing windings 32b and 32c of the resolver 32 are connected, via lines 420, to the signal input 421 of the revolution counter and the signal input 422 of the R/D converter. From the revolution counter 405 a digital signal n is obtained on the output 418 which is supplied via the data line 42a to a communication unit 430 and which indicates within which revolution or within which quadrant the resolver is. From the output 419 of the R/D converter 406 there is obtained during normal operation a digital signal α which indicates the rotational angle of the resolver within the revolution or within the quadrant in question and which, via the data line 42b, is supplied to the communication unit 430. This unit handles the digital communication with the control system 20 via the bus 42 and forwards measured values n and α to the control system. The oscillator 403 delivers as long as supply voltage is present on the line 41 a alternating voltage of sinusoidal or square shape with a frequency which may be 2-20 KHz. The oscillator 404 delivers on its output 416 an alternating voltage of the same type and the same frequency as the output signal from the oscillator 403. However, the oscillator 404 operates in a pulsed mode, i.e. it delivers short output signals, for example of a duration of one or a few periods of the alternating voltage, separated by lengthy pauses, for example of 2-3 ms. From the output 417 of the oscillator 404 there is obtained a synchronization signal which is a logical "one" during the brief intervals when the oscillator delivers an output signal and which is a logical "zero" during the pauses between these intervals. During normal operation, i.e. in the presence of supply voltage on the line 41, the relay 407 is activated, the resolver 32 is supplied continuously from the oscillator 403, the resolver counter 405 is continuously activated by supplying a logical "one" to its synchronization input 425 and the R/D converter 406 is in operation. By the combination of the output signals of the revolution counter and the R/D converter, a quantity can be formed in a simple manner, which within the entire operating range of the robot axis accurately indicates the actual axis position. Provided that the output signal n of the revolution counter indicates the number of quadrants (one-fourth of a revolution) that the resolver has rotated from a reference position, the actual position value of the axis in obtained:  = n· π/2 + α radians. In the event of a drop out of the supply voltage on the line 41, only the oscillator 404 and the revolution counter 405 are working. Since both this oscillator and the revolution counter are working in pulsed mode, they may be designed for very low power consumption, and their operation is therefore ensured with the aid of the accumulator battery 402 for very long periods of drop out of the supply voltage. Figure 5 shows the design of the revolution counter 405. The resolver signals from the windings 32b and 32c arrive at the revolution counter on the lines 420b and 420c. Each input signal is supplied to an amplitude detectors 51 and 52, respectively. The amplitude detectors are phase-sensitive with the oscillator signal as reference value and their output signal may thus vary between a positive and a negative maximum value. From each one of the amplitude detectors, an output signal is obtained which constitutes a measure of the amplitude of the respective resolver signal, which consists of an alternating voltage with the same frequency as the supply voltage of the resolver. Each amplitude signal is supplied to a level detectors 53 and 54, respectively, each of which delivers a logical "one" if the input signal is positive and a logical "zero" if the input signal is negative. The output signals of the level detectors are supplied to inputs of a reversible counter 55 known per se. Upon rotation of the resolver, the output signals of the two level detectors will be 90° offset in phase relative to each other, and the counter determines in a known manner the direction of rotation and hence the direction of counting on the basis of the sign of the phase difference between the two level detector signals. The combination of input signals to the counter changes upon every ninetieth degree of the rotation of the resolver, the counter counting up or down one unit, depending on the direction of rotation of the resolver. The contents of the counter are thus proportional to the number of quadrants (or revolutions) that the resolver has rotated relative to a reference position. The contents of the counter are obtained on the output 56 of the counter as a digital number n. The input 412 of the revolution counter is supplied with supply voltage from the accumulator battery 402, which supply voltage is supplied to the different units of the revolution counter. To ensure a correct function, the counter 55 is activated only during the working interval of the oscillator 404. This is achieved by supplying, during these intervals, the synchronization signal from the oscillator 404 to the so-called count-enable input of the counter. During an interruption of the operation of the robot, for example because of a power failure, the contents of the counter 55 will always provide a correct indication of the position of the robot axis, i.e. an indication within which of the quadrants of the resolver the position is at the moment independently of any changes of the axis position during the interruption of the operation. As soon as normal operation is resumed and the axis unit receives supply voltage via the line 41, the axis unit will resume its normal operation without the need of any synchronization operations. In Figure 6a the curve A shows the supply voltage from the oscillator 403 to the resolver excitation winding 32a as a function of time. The position of the resolver is assumed to correspond to the rotational angle ₁ in Figure 6b. The output signal B from the amplitude detector 51 then has the value a and the output signal C from the detector 52 the value -b. The output signal D of the level detector 53 is a logical "one", and the output signal E of the detector 54 is a logical "zero". Figure 6a shows the signal during normal operation, i.e. when the supply voltage is present on the line 41. Figure 6b shows in the top diagram the output signals F and G of the amplitude detectors 51 and 52 as a function of the rotational angle  of the resolver. Below this are shown the output signals H from the level detector 53 and K from the level detector 54. At the resolver angle ₁ the signals from the level detectors have the values 1 and 0, respectively. If the resolver is rotated so that the resolver angle comes within the next quadrant, e.g. at  = ₂, the output signal of the detector 54 is changed to a logical "one", whereupon the counter 55 counts up one unit. Figure 6c shows at in the top diagram the output voltage L of the oscillator 404 in case of a drop out of the supply voltage to the axis unit. The output signal consists of short intervals, for example a few tens or a few hundreds of µs, during which the oscillator delivers its alternating voltage output signal to the excitation winding 32a of the resolver. During the intermediate periods, which may have a duration of, for example, 2-3 ms, the oscillator is inactive. The curve M shows the synchronization signal delivered from the output 417 of the oscillator 404 to the revolution counter. The curve N shows the amplitude of the output signal from one of the resolver windings. Figure 7 shows how the axis units are connected to the common central control system 20. This comprises a rectifier 22 with terminals 25 for connection to an alternating voltage network, and delivers the supply direct voltage on the line 41 via a smoothing filter 23. The control system 20 further comprises a computer system 24, which via a digital series bus 42 communicates with and receives actual position values from the various axis units 400a, 400b ... 400i. The system has one axis unit for each robot axis. Each axis unit comprises the units shown in Figure 4, i.e. a resolver with associated supply and sensing members and an accumulator battery. Each axis unit is mounted adjacent to an associated robot axis, with the electronic units and the accumulator battery built together with the resolver into one single transducer unit, or with these units arranged in immediate proximity to the resolver. Of the various axis units, for sake of clarity, only the revolution counters 405a ... 405i, the R/D converters 406a ... 406i, and the communication units 430a, 430b ... 430i are shown. The revolution counter deliver, via the communication units and the series bus 42, the signals na, nb ... ni to the computer system 24. The R/D converters deliver, via the communication units and the series bus, the signals αa, αb ... αi to the computer system. On the basis of these signals, the computer calculates, in the manner stated above, the actual position values for each robot axis, compares these values with programmed or manually entered desired position values and, in dependence on these comparisons, controls the drive motors of the axes. The position transducer system described above only constitutes an example of a position transducer system according to the invention, and such a system may be designed in a great many alternative ways within the scope of the invention. Thus, for example, instead of an accumulator battery in each axis unit, one single accumulator battery may be arranged centrally in the control system 20 or in the robot 1. Further, instead of the two oscillators 403 and 404 in Figure 4, one single oscillator may be used, which operates continuously during normal operation and is switched to pulse mode operation in case of a drop out of the supply voltage. Also, the mechanical coupling of the resolver to the motor shaft may be carried out in other ways than that described above. The described sensing of the supply voltage drop out with the aid of voltage sensing members arranged in each axis unit may be replaced by a central sensing operation, carried out in the cabinet of the control system 20, signals for switching of the axis units to the pulse mode operation then being transmitted to these from the control cabinet.

Claims: Absolute position transducer system for determining the rotational movement of an axis of an industrial robot, comprising a resolver (32), a revolution counter (405) for counting the number of complete revolutions of the resolver axis, a main power supply (41, 403) for energizing the tranducer system during normal operation, a back-up power supply (402, 404) associated with the revolution counter for providing a power supply to the counter in case of a failure of the main power supply, characterized in that the revolution counter (405) is an electronic revolution counter having its inputs (421) connected to the output (32b, 32c, 420) of the resolver; the bac-up power supply (402, 404) includes means (404) to intermittently energize the resolver and, in synchronization therewith, to intermittently activate the revolution counter upon failure of the main power supply. Tranducer system according to claim 1, comprising signal conversion means (406), separate from said revolution counter (405) and connected to the output of the resolver (32) for conversion of the resolver output signals to a digital angle quantity (α) and means (41, 401) for continuously activating said signal conversion means (406) during normal operation of the transducer, characterized in that the tranducer system further comprises means (41, 401) for activating said signal conversion means (406) during normal operation only.

Inventors: HOLMAR ANDERS, MYKLEBUST EINAR, HOLMAR, ANDERS, MYKLEBUST, EINAR
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1990
Main classification: G01B7
IPC codes: G01B7, B25J13