Method and device for checking an electrical value of an electric machine

Abstract

The invention relates to a method for checking an electrical value of an electric machine, in particular an electric machine of a coordinate measuring device. The electric machine has an electric drive comprising a stator and a rotor. The method includes the steps of: detecting a value of a drive current delivered to the electric drive for driving the rotor, detecting a measured value of an electrical input variable of the electric machine, determining a calculated value of the electrical input variable on the basis of the detected value of the drive current and on the basis of a performance model of the electric machine, and determining a comparison value on the basis of the detected measured value and the calculated value of the electrical input variable, in order to check the detected value of the drive current.

Claims

1. A method for checking a drive current of an electric machine of a coordinate measurement device, wherein the electric machine includes an electric drive having a stator and a rotor and wherein the drive current is fed to the electric drive to drive the rotor, the method comprising: detecting a measurement value of an electrical input variable of the electric machine, wherein: the electrical input variable is aa total current consumed by the electric machine or (ii) an electrical variable corresponding to the total current consumed by the electric machine and the total current consumed by the electric machine includes current consumed by other electrical components in the electrical machine in addition to current consumed by the electric drive; detecting a value of the drive current; determining a calculation value of the electrical input variable based on (i) the detected value of the drive current and (ii) a performance model of the electric machine; and determining a comparison value based on a difference value between (i) the measurement value of the electrical input variable and (ii) the calculation value of the electrical input variable, in order to check the detected value of the drive current.

2. The method of claim 1, wherein the calculation value of the electrical input variable is calculated based also on a total voltage supplied to the electric machine.

3. The method of claim 1, wherein the performance model takes into account an electromagnetic force of the electric drive.

4. The method of claim 3, further comprising: detecting a movement variable of the rotor and determining the electromagnetic force based on (i) characteristic variable of the electric drive and (ii) the detected movement variable.

5. The method of claim 1, wherein: the electric machine includes a control unit and the performance model takes into account an electrical power loss of the control unit.

6. The method of claim 5, further comprising determining the electrical power loss of the control unit when the rotor is stationary.

7. The method of claim 1, wherein the performance model takes into account a mechanical drive power of the electric drive.

8. The method of claim 1, further comprising: determining a plurality of difference values that includes the difference value, wherein the comparison value is determined based on a sum of the plurality of difference values.

9. The method of claim 8, wherein a predefined value is subtracted from a value of the sum.

10. The method of claim 1, wherein the comparison value is determined based on an integral of the difference value over time.

11. The method of claim 10, wherein a predefined value is subtracted from a value of the integral.

12. The method of claim 1, further comprising interrupting the drive current fed to the electric drive in response to the comparison value exceeding a predefined threshold value.

13. A method for checking a drive current supplied to an electric machine of a coordinate measurement device, wherein the electric machine comprises an electric drive having a stator and a rotor, wherein the drive current is fed to the electric drive in order to drive the rotor, and wherein the electric machine obeys a conservation of energy principle, the method comprising: detecting a first measurement value that represents an instantaneous value of the drive current; detecting a second measurement value that represents a total current consumed by the electric machine, including by components of the electric machine other than the electric drive; determining a calculation value of the total current consumed by the electric machine based on (i) the first measurement value and (ii) a performance model of the electric machine, wherein the performance model is configured to model consumed power and output power of the electric machine and wherein the performance model takes into account the conservation of energy principle, and comparing the second measurement value and the calculation value in order to check the instantaneous value of the drive current.

14. An apparatus for checking a drive current of an electric machine of a coordinate measurement device, wherein the electric machine includes an electric drive having a stator and a rotor and wherein the drive current is fed to the electric drive in order to drive the rotor, the apparatus comprising: a first detection unit, that is associated with the electric drive and is configured to detect a value of the drive current; a second detection unit, which that is configured to detect a measurement value of an electrical input variable of the electric machine, wherein: the electrical input variable is aa total current consumed by the electric machine or (ii) an electrical variable corresponding to the total current consumed by the electric machine and the total current consumed by the electric machine includes current consumed by other electrical components in the electrical machine in addition to current consumed by the electric drive; a determination unit that is configured to determine a calculation value of the electrical input variable based on (i) the detected value of the drive current and (ii) a performance model of the electric machine; and a checking unit, that is configured to determine a comparison value based on a difference value between (i) the detected measurement value of the electrical input variable and (ii) the determined calculation value of the electrical input variable in order to check the detected value of the drive current.

15. The apparatus of claim 14, wherein: the checking unit includes at least two separate checking units, each of the second detection unit and the determination unit is connected to the at least two separate checking units, and each of the at least two separate checking units is configured to independently determine the comparison value in order to independently check the detected value of the drive current.

16. The apparatus of claim 15, further comprising: a switching unit configured to interrupt an electrical power supply path to the electric drive, wherein each of the at least two separate checking units is connected to the switching unit in order to interrupt the drive current to the electric drive.

17. The apparatus of claim 14, further comprising a switching unit configured to interrupt an electrical power supply path to the electric drive based on the comparison value.

18. The apparatus of claim 17, wherein the switching unit is configured to interrupt the electrical power supply path to the electric drive in response to the comparison value exceeding a predefined threshold value.

19. The apparatus of claim 18, further comprising: at least one of (i) one or more integrated circuits and/or (ii) one or more microcontrollers, wherein the one or more integrated circuits and/or the one or more microcontrollers implement the first detection unit, the second detection unit, the determination unit, and the checking unit.

20. The apparatus of claim 14, further comprising: at least one of (i) one or more integrated circuits and/or (ii) one or more microcontrollers, wherein the one or more integrated circuits and/or the one or more microcontrollers implement the first detection unit, the second detection unit, the determination unit, and the checking unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are illustrated in the drawing and will be explained in greater detail in the following description. In the drawing:

(2) FIG. 1 shows a schematic illustration of an electric machine having a control unit for driving an electric drive and having an apparatus for checking an electrical variable of the electric machine,

(3) FIG. 2 shows a schematic detail drawing of an embodiment of the electric machine from FIG. 1,

(4) FIG. 3 shows a schematic flow chart of a method for checking an electrical variable of the electric machine, and

(5) FIG. 4 shows a schematic illustration of an embodiment of a coordinate measuring machine as an exemplary application for the electric machine from FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 1 schematically illustrates an electric machine, which is generally denoted by 10. The electric machine 10 has an electric drive 12 and a control unit 14 for actuating or for energizing the electric drive 12. The electric drive 12 can generally be configured as a rotary motor or as a linear motor and generally has a fixed stator and a moving rotor, wherein, depending on the embodiment, the rotor provides a torque or a linear motor force. The control unit 14 is electrically connected to the stator of the electric drive 12 and actuates the electric drive 12 or energizes the electric drive 12 by means of a motor current I.sub.A. The torque provided by the rotor or the motor force provided by the rotor is proportional to the motor current I.sub.A, with the result that the torque or the motor force can be controlled or limited accordingly by way of the motor current I.sub.A.

(7) The control unit 14 is connected to electrical input connections 16, 18 of the electric machine 10 in order to electrically connect the control unit 14 to an external voltage source 20, which supplies an input voltage or a supply voltage U.sub.V to the input connections 16, 18. Depending on the drive power of the electric drive 12, the control unit 14 consumes an input current I.sub.V, which forms a total consumed current I.sub.V of the electric machine 10. A measurement resistor 22 having an electrical resistance R.sub.S is arranged in an input line, which connects one of the electrical connections 16, 18 to the control unit 14, wherein a measurement voltage U.sub.S is dropped across the measurement resistor 22, said measurement voltage being detected by means of a voltmeter 24 in order to determine the total consumed current I.sub.V.

(8) An advancing force F of the rotor of the electric drive 12 or a torque M of the rotor is directly proportional to the motor current I.sub.A by way of a motor force constant or a torque constant. The motor current I.sub.A can be measured or detected by the control unit 14 or a separate ammeter and is available as a non-confirmed measurement value. If the advancing force F or the torque M are intended to be reliably determined or reliably limited with the aid of the motor current I.sub.A, the measurement of the motor current I.sub.A has to be correspondingly confirmed by a further measurement, as is explained in more detail below.

(9) The electric drive 12 can generally be configured as an electric linear motor, as a mechanically commutated DC motor or as a brushless electrically commutated DC motor (BLDC).

(10) To confirm the detected motor current I.sub.A, the electric machine 10 has an apparatus 30 for checking the motor current I.sub.A as the electrical variable of the electric machine 10. The apparatus 30 for checking the motor current I.sub.A has a determination unit 32, which determines a calculation value U.sub.C for the measurement voltage U.sub.S on the basis of the detected motor current I.sub.A, of the input voltage U.sub.V and of a movement variable v, n of the rotor and on the basis of a performance model of the electric machine 10. The apparatus 30 for checking the motor current I.sub.A further has a checking unit 34, which compares the calculation value U.sub.C for the measurement voltage U.sub.S and the measurement value of the measurement voltage U.sub.S in order to check the detected value of the drive current I.sub.A. The checking unit 34 outputs a comparison value between the calculation value U.sub.C and the measurement voltage U.sub.S and can, on the basis of the comparison value, switch off the electric machine 10 in general or switch off the control unit 14, as is illustrated in FIG. 1.

(11) The apparatus 30 for checking the motor current I.sub.A can be configured as an integrated circuit in a chip in the form of a microcontroller.

(12) Since the determination unit 32 determines the calculation value U.sub.C from the motor current I.sub.A and from the input voltage U.sub.V and the movement variable v, n of the rotor, that is to say on the basis of measurement variables that are independent of the measured measurement voltage U.sub.S, the calculation value U.sub.C is independent of the measurement voltage U.sub.S, with the result that two redundant measurement variables can be compared and the measurement of the motor current I.sub.A can therefore be confirmed.

(13) The performance model of the electric machine 10 is based on the principle of the conservation of energy, in that an electrical input power P.sub.V of the electric machine 10 is identical to a sum of an output mechanical drive power P.sub.mech, a power loss P.sub.M of the motor and a power loss P.sub.E of the electronic components of the electric machine 10, as is shown in formula 1:
P.sub.V=P.sub.mech+P.sub.M+P.sub.E  (formula 1)

(14) The electrical input power P.sub.V can be calculated with the aid of the measurement resistor 22 and the input voltage U.sub.V according to formula 2:
P.sub.V=U.sub.V*I.sub.V  (formula 2)

(15) Alternatively, the input voltage U.sub.V and the total current I.sub.V can be detected by way of a central control unit in the region of the input connections 16, 18. Furthermore, the total current I.sub.V or the measurement voltage U.sub.S can be determined by a separate measurement unit at any position of the electrical connection line of the control unit 14, said separate measurement unit supplying the corresponding measurement results to the central control unit by means of a digital bus line.

(16) The electrical power loss P.sub.E or the self-consumption power P.sub.E of the electronic components of the electric machine 10, such as of the control unit 14, for example, can be measured in advance when the rotor is stationary, wherein the total consumed current I.sub.V is then identical to the electric current I.sub.E consumed by the electronic components. The electrical power loss P.sub.E of the electronic components results from formula 3:
P.sub.E=U.sub.V*I.sub.E  (formula 3)

(17) The power loss of the motor P.sub.M results from the motor current I.sub.A that flows through the rotor or an armature of the rotor, wherein the armature of the rotor has an armature resistance R.sub.A. The power loss of the motor P.sub.M results from formula 4:
P.sub.M=R.sub.A.Math.I.sub.A.sup.2  (formula 4)
wherein the armature resistance R.sub.A is a known motor constant and the motor current I.sub.A can be measured by the control unit 14 or an associated ammeter, as explained above.

(18) In addition to the power loss of the motor P.sub.M and the power loss P.sub.E of the electronic components, the actual motor power or the mechanical advancing power P.sub.Mech also has to be taken into account, which consists of a component proportional to the speed of rotation or speed and of a component proportional to the torque or to the force. The component U.sub.EMK proportional to the speed of rotation or speed corresponds to the electromotive force (back EMF) and can be calculated as a movement variable of the electric drive 12 using a motor constant K.sub.EMK, which is known or can be measured for each electric drive, and the speed v of the rotor or a speed of rotation n of the rotor. The maximum speed of an electric drive 12 results from the fact that the electromagnetic counter-voltage U.sub.EMK reaches a value of the maximum available voltage, that is to say the link circuit voltage.
U.sub.EMK=K.sub.EMK*v; U.sub.EMK=K.sub.EMK*n  (formula 5)

(19) The component proportional to the torque or to the force corresponds to the motor current I.sub.A. The actual motor power or the mechanical advancing power P.sub.Mech therefore results from formula 6:
P.sub.mech=U.sub.EMK*I.sub.A  (formula 6)

(20) The following thus results for the performance model from formula 1:
U.sub.V*I.sub.V=U.sub.V*I.sub.E+R.sub.A*I.sup.2.sub.AK.sub.EMK*v*I.sub.A  (formula 7)

(21) For the static case, where the rotor is stationary and therefore no mechanical power is output, the following results for the motor current:
I.sub.A√{square root over (U.sub.V.Math.(I.sub.V−I.sub.E)/R.sub.A)}  (formula 8)
and for the dynamic case, taking into account the mechanically output power P.sub.Mech, the following results for the total current I.sub.V:
I.sub.V=(U.sub.V.Math.I.sub.E+R.sub.A.Math.I.sup.2.sub.A+K.sub.EMK.Math.v.Math.I.sub.A)/U.sub.V  (formula 9)
wherein the calculation value U.sub.C can be calculated by way of the resistance R.sub.S of the measurement resistor 22:
U.sub.C=R.sub.S*I.sub.V  (formula 10)

(22) The measured motor current I.sub.A can be independently or redundantly checked by way of the comparison value between the calculation value U.sub.C and the measurement voltage U.sub.S, since the calculation value U.sub.C is based on the motor current I.sub.A and independent variables and the measurement voltage U.sub.S can be detected metrologically at the measurement resistor 22. The simple measurement of the motor current I.sub.A can therefore be confirmed by way of the performance model and can be used for safety-relevant applications and variables, such as limiting the advancing force, for example.

(23) If the difference value between the calculation value U.sub.C and the measurement voltage U.sub.S exceeds a predefined value, the measurement of the motor current I.sub.A is erroneous, with the result that the measured motor current I.sub.A can no longer be used to confirm the advancing force or to confirm the drive torque. In this case, the electric machine 10 can generally be switched off by way of the control unit 14 or a separate switch or contactor.

(24) Since usually smaller differences often briefly arise between the calculation value U.sub.C and the measurement voltage U.sub.S, the difference value can be averaged and the difference value can be summed or integrated for the purpose of identifying an error. Furthermore, a predefined value can be subtracted regularly or continuously from the calculated sum value or calculated integral value, with the result that an error in the motor current detection is identified only after reaching a limit value.

(25) Alternatively thereto, it is also possible to form a moving average value with a tolerance range or the like.

(26) It goes without saying that the total current I.sub.V can also be measured by way of the measurement resistor 22, provided a plurality of control units 14 are connected via the measurement resistor 22 to the input connection 16.

(27) FIG. 2 schematically illustrates an embodiment of the electric machine 10. Identical elements are denoted by identical reference numerals, with only the special features being explained here.

(28) In this embodiment, the electric drive 12 is configured as an electrically commutated DC machine that has three phases U, V, W. The control unit 14 is configured as a three-phase inverter, which has three half-bridges each having two controllable switches, in order to convert the input voltage U.sub.V to a three-phase AC voltage. The control unit 14 is connected to the three phases of the electric drive 12 by way of three connection lines and accordingly supplies three phase currents I.sub.1, I.sub.2, I.sub.3, in order to drive the rotor of the electric drive 12.

(29) The electric machine 10 furthermore has a first checking unit 36 and a second checking unit 38, which are configured separately and are each preferably configured as an integrated circuit in the form of a microcontroller. The first checking unit 36 and the second checking unit 38 together form the apparatus 30.

(30) The measurement voltage U.sub.S and the input voltage U.sub.V are supplied to the first checking unit 36 as measured input variables. The input voltage U.sub.V, the resistance R.sub.S of the measurement resistor 22 and the measurement voltage U.sub.S as input variables from the first checking unit are supplied as input variables to the second checking unit 38. Furthermore, the second checking unit receives measurement values of the three phase currents I.sub.1, I.sub.2, I.sub.3 and the speed of rotation n and the predetermined current consumption I.sub.E as further input variables. The second checking unit 38 calculates the total current I.sub.V as explained above with reference to formula 9 on the basis of the input variables and furthermore calculates the calculation value U.sub.C by means of the resistance value R.sub.S of the measurement resistor 22 and compares the measured measurement voltage U.sub.S with the thus determined calculation value U.sub.C. The second checking unit 38 is connected to the control unit 14 by way of a signal line 42. If the difference value between the calculation value U.sub.C and the measured measurement voltage U.sub.S exceeds a predefined threshold value, a corresponding switch-off signal is transmitted to the control unit 14 via the signal line 42 and the controllable switches of the half-bridges are opened, in order to disconnect the electric drive 12.

(31) The second checking unit 38 transmits the calculated total current I.sub.V to the first checking unit 36 and the first checking unit 36 determines the calculation value U.sub.C with reference to the resistance value R.sub.S of the measurement resistor 22 and compares the calculation value U.sub.C with the measured measurement voltage U.sub.S independently of the second checking unit 38. The first checking unit is connected to a switch 35 in the connection line by way of a signal line 40. If the difference value between the measurement voltage U.sub.S and the calculation value U.sub.C exceeds a predefined threshold value, a corresponding switch-off signal is transmitted to the controllable switch 35 and the controllable switch 35 is accordingly opened.

(32) The first checking unit 36 and the second checking unit 38 are preferably physically separated from one another and exchange the calculated and measured data by way of a bus system 44 or a security bus system 44, as illustrated in FIG. 2. Further redundancy can be established by the physical separation, with the result that, in the event of an error, there are two independent evaluations and switch-off means for the electric drive 12.

(33) FIG. 3 shows a schematic flow chart of a method for checking an electrical variable of the electric machine 10. In FIG. 3, the method is generally denoted by 50.

(34) The method 50 begins with the measurement of the motor current I.sub.A, as is shown in step 52, wherein the measurement of the motor current I.sub.A as explained above has to be confirmed for safety-relevant applications. In step 54, the measurement voltage U.sub.S across the measurement resistor 22 is measured and the total current I.sub.V is determined. In step 56, the input voltage U.sub.V is measured or, in the event of the input voltage being stable and constant, is supplied. In step 58, the movement variable of the rotor is detected, said movement variable being the rotor speed v or the speed of rotation n depending on the embodiment of the electric drive 12.

(35) In step 60, the calculation value U.sub.C is determined on the basis of the performance model of the electric machine 10. For calculating the calculation value U.sub.C, motor constants such as KEMK and R.sub.A are provided by a storage means 62 and the previously measured current consumption I.sub.E is provided, as is shown in 64. In step 66, the calculation value U.sub.C is compared with the measured measurement voltage U.sub.S. In step 68, the thus determined comparison value is compared with a threshold value and, if the comparison value is greater than the threshold value, in step 70, the electric machine 10 is isolated from the electrical power supply or the electric drive 12 is disconnected. If the comparison value is lower than the predefined threshold value, the method 50 returns to step 52, as is shown by the feedback loop 72, and detects the motor current I.sub.A.

(36) It goes without saying that steps 52 to 58 can also be carried out in parallel or in any other desired order and that the method 50 can proceed continuously. FIG. 4 illustrates a coordinate measuring machine as an exemplary application for the electric machine 10, said coordinate measuring machine generally being denoted by 80. The coordinate measuring machine 80 serves to measure a workpiece 82 and has a tactile sensor 84 for measuring the workpiece 82. The coordinate measuring machine 80 comprises a carrier structure 86, which can be moved relative to a base plate 88 in the Y-direction. The carrier structure 86 furthermore has a slide 90 and a sleeve 92, which are each mounted in a movable manner and can be moved by means of electric drives. The carrier structure 86 and the slide 90 and the sleeve 92 can be driven by means of an electric machine 10 in accordance with the present invention. The drive forces of the individual elements can be safely limited by confirming the motor current I.sub.A.