WO2016119814A1

WO2016119814A1 - Resolver for a servo motor

Info

Publication number: WO2016119814A1
Application number: PCT/EP2015/051470
Authority: WO
Inventors: Fan Dai; Björn MATTHIAS
Original assignee: Abb Schweiz Ag
Priority date: 2015-01-26
Filing date: 2015-01-26
Publication date: 2016-08-04

Abstract

The invention is related to a resolver (10, 86, 90) for a servo motor (82), comprising a first (12) and a second (14) pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation, comprising an excitation coil (16) rotatable (18) around the axis of rotation which is foreseen to induce a respective first voltage V₁ in the first pickup coil (12) and a second induction voltage V₂ in the second pickup coil (14), comprising voltage supply means for supplying an alternating voltage V_E to the excitation coil (16) and comprising evaluation means (92). The evaluation means (92) are foreseen to determine in subsequent time-cycles i the revolution angle θ _M,i of the excitation coil (16) dependent on an analysis of the first V₁ and second V₂ induced voltage and to continuously perform a consistency check of the first V₁ and second V₂ induced voltage. The evaluation means are further foreseen to store continuously at least the afore determined revolution angle θ _M,i- ₁ and in case of a failed consistency check to determine whether one of the first V₁ and second V₂voltages is still correct and in this case to determine which of first V₁ and second V₂voltage is faulty and to determine the revolution angle θ _M,i based on the not faulty voltage V_{1, i} or V_{2, i},V_{E, i} and the afore determined revolution angle θ _M,i _–1. The invention is also related to a respective method.

Description

Resolver for a servo motor

Description

The invention is related to a resolver for a servo motor, comprising a first and a second pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation, comprising an excitation coil rotatable around the axis of rotation which is foreseen to induce a respective first voltage Vi in the first pickup coil and a second induction voltage V₂ in the second pickup coil, comprising voltage supply means for supplying an alternating excitation voltage V_E to the excitation coil and comprising evaluation means which are foreseen to determine in subsequent time- cycles / the revolution angle 9_{M i} of the excitation coil dependent on an analysis of the first Vi and second V₂ induced voltage and which are foreseen to continuously perform a consistency check of the first Vi and second V₂ induced voltage.

A servo drive amplifier receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically the command signal represents a velocity, a desired torque or position. A sensor attached to the servo motor reports the motor's actual status back to the servo drive. The servo drive then compares the actual motor status with the commanded motor status. It then alters the voltage frequency or pulse width to the motor so as to correct for any deviation from the commanded status. Servo drives are used in many components of industrial production such as CNC machining, factory automation, and robotics. Their main advantage over traditional DC or AC motors is the addition of motor feedback. This feedback can be used to detect unwanted motion, or to ensure the accuracy of the commanded motion.

Industrial robots for example are used in wide areas of production. A robot typically comprises a robot arm with for example six arm members, which are linked by six respective motor driven joints so that in total six degrees of freedom in movement are realized therewith. Of course robots might also have 4, 5 or 7 degrees of freedom in movement. Typically three arm members with three degrees of freedom in movement build the base part of a robot arm, so that it can be moved to a desired coordinate within the working space of the robot. In many cases a wrist with additional three degrees of freedom in movement is foreseen at the distal end of the base part of the robot arm, which provides not only the possibility to reach a coordinate within the working space but also to have a desired orientation.

A robot controller controls the motors of the joints in such a way that the tip of the robot arm respectively the tool center point (TCP) of the robot arm performs a desired movement, for example corresponding to the data of a robot program which is stored within the robot controller.

A robot arm might have a length of for example 0,5m, 1 m or 3m in total. The position of the tip of the robot arm relative to the robot base depends on the actual joint angles of the joints and the geometry of the robot arm members. The joint angles of the joints are typically determined by sensors such as resolvers or encoders, wherein each joint is typically provided with a dedicated sensor, respectively the motor for a joint is provided with a dedicated sensor.

Patent document DE 10201 1078583 A1 is disclosing such a resolver which comprises a first and a second pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation, an excitation coil rotatable around the axis of rotation which is foreseen to induce a respective first voltage Vi in the first pickup coil and a second induction voltage V₂ in the second pickup coil and voltage supply means for supplying an alternating voltage V_E to the excitation coil. Evaluation means are foreseen to determine the revolution angle 9_{M i} of the excitation coil dependent on an analysis of the first Vi and second V₂ induced voltage. To achieve high reliability of fault detection, physically redundant channels for the diagnostics are used. That is, a failure in either the first or the second pickup coil can be detected reliably.

Disadvantageously within the state of the art is that in case of an invalidity of either the first or the second voltages no further revolution angle is determined by the re- solver, so that, in case of a failure of a resolver, no information is available for the control system of the robot to stop its movement in a controlled manner.

The objective of the invention is to provide a resolver for a servo motor which is able to determine the revolution angle even in that case, that one of the first or second voltages of the pickup coils is invalid. The objective of the invention is also to provide a method to operate such a resolver accordingly.

The problem is solved by a resolver for a servo motor of the aforementioned kind. This is characterized in that the evaluation means are further foreseen

• to store continuously at least the afore determined revolution angle #_M i-i and

• in case of a failed consistency check to determine whether one of the first V-i and second V₂ voltages is still correct,

• and in this case

• to determine which of first Vi and second V₂ voltage has failed and

• to determine the revolution angle 9_{M i} based on the remaining operational voltage Vi, i or V2, i ,VE, i and the afore determined revolution angle

@M.i-l -

Basic idea of the invention is to use the physically redundancy of the channels for the diagnostics, which are normally used only for a consistency check, as base for the determination of the revolution angle 9_{M i} in case of the breakdown of one of the channels.

Thus it is possible in case of the breakdown of either the first voltage \l^ or the second voltage V₂ to determine the revolution angle 9_{M i} based on the respective other voltage, the excitation voltage V_E and the afore determined revolution angle #_M i-i- The revolution angle determined afore is required in order to consider a possible change of the rotation direction compared to the revolution angle 9_{M i} determined actually.

The resolver comprises an evaluation unit, for example a computing unit with a respective software product running thereon, which is foreseen to execute the respective steps for performing the consistency check respectively for determining the revolution angle. The consistency check respectively the determination of the revolution angle is performed repeatedly every time interval At so that the actual time t can be calculated as t = i ^* At, wherein /^' is the current number of time-intervals.

The first and second pickup coils are oriented perpendicular each to each other. Thus the envelopes of the induced voltages Vi and V₂ are shifted by 90° each to each other so that trigonometric calculation functions such as sin, cos, arcsin, arcos and arctan can easily be applied.

Basic assumption for the course of the voltages V _; V₂ and V_E are the following:

V_E t) = V_E ^{0) s o)_Et,

Vi(t) = rV_E(t) cos ω_Μί = rV^ sin a^t cos ω_Μί = [sin(a)_£— ω_Μ)ί + sin(a)_£ + ω_Μ)ί ] ,

V₂ {t) = rV_E{t) sin o)_M£ = rV^ sin a^t sin a)_Mt = r - - [COS(6L)_£ — ω_Μ ^~)ί— cos(a>_£ + ω_Μ ^~)ί ] , wherein

• V_E(t) = excitation signal

• V_E °^ = amplitude of excitation signal

• r = maximum transformation ratio

• Vi(t) = signal on first— coil

• 1^( = signal on second— coil

• ω_Ε = angular frequency of excitation

• ω_Μ = angular frequency of motor rotation.

Accordingly the determination of the motor shaft angle respectively revolution angle can be done by the following equation

Θ_Μ = ω_Μΐ = arctan— in case that the cross-check between first and second voltage is valid

Depending on the methods of signal sampling or filtering, the SIN term can be eliminated, so that the square sum becomes a constant value, e.g. if we average the first and second voltages over a time interval much longer than the period of the excitation signal, then we obtain the average of the sin² factor, which is 0.5.

The consistency check performed by the evaluation unit comprises two main aspects:

• checking of relative phase of excitation frequency on excitation voltage V_E and on pickup voltage Vi and V₂,

• checking the amplitude of the pickup voltages Vi and V₂ against the excitation voltage to verify the expected transformation ratio.

Depending on the respective quadrant QN of the revolution angle, the carrier signal of the induced voltage, which has the same frequency than the excitation voltage V_E, is either in phase or in 180° opposition to the excitation voltage V_E. Thus the analysis of the phasing of the carrier signal of the induced voltages Vi respectively V₂ compared with the excitation voltage V_E provides additional information about the quadrant of the determined value of the revolution angle, so that a possible loss of information, which is caused by using arcos, arcos or arctan functions, can be compensated. There is a defined relationship between the phases of the excitation voltage V_E and the induced voltages Vi and V₂:

The expected relationship between V_E and Vi is, for revolution angle Θ_Μ \

• For Θ_Μ6 [o,^] (Qi) we have V^ and V_E in phase

• For Θ_Μ6 [j, π (Q₂) we have Vi and V_E 180° out of phase

• For Θ_Μ6 π, (Cfe) we have Vi and V_E 180° out of phase

• For Θ_Μ6 \—, 2π] (Q₄) we have V^ and V_E in phase The expected relationship between V_E and V₂ is, for revolution angle Θ_Μ:

• For Θ_Μ6 [o,^] (Qi) we have V₂ and V_E in phase

• For Θ_Μ6 , π] (Q₂) we have V₂ and V_E in phase

• For Θ_Μ6 (Cfe) we have V₂ and V_E 180° out of phase

• For Θ_Μ6 [ , 2π] (Q₄) we have V₂ and V_E 180° out of phase

In other words, the V₂ signal is in-phase with the excitation signal for motor angles between 0° and 180°, and in anti-phase between 180° and 360°. The Vi signal is phase-shifted by 90° with respect to the V₂ signal.

The evaluation unit can check the phases of each signal channel according to the above relationships either with analog electronics, or by computation using digitized signal values. With stored last valid value of revolution position and speed, it is possible to determine, in which value range the actual revolution position is, and then check, if the phase is correct. Note: In case of very high revolution speed, i.e. if the signal sampling rate is lower than half the rotation of a motor for example, the determination of quadrant from earlier measurement can no longer be used, because the causality of the phase relationship is lost. In this case, alternative methods can be applied to determine the current quadrant, i.e. by counting the passes of maximum amplitude of Vi and V₂ signals.

The phase check can become difficult at very low speed for the motor positions at the switch points between in- and anti-phase, since the amplitude of the signal will be very low, and there can also be change of direction of motor rotation. This can be overcome by checking the other signal which is actually 90° shifted and has the highest amplitude, if this signal is available. If it is not, issuing an uncontrolled stop would be a simple way to maintain safety. Note: as consequence, this method is not reliable for the case, if both channels fail and position control at zero speed should be achieved. However, this is not required by the safety standards.

In case that the channels are not consistent it is checked whether only one of the channels for Vi or V₂ has failed and then the revolution angle 9_{M i} is calculated based on the not defect voltage Vi, i or V2, i VE, i and the afore determined revolution angle θ_{Μ ί}_ .

Thus the resolver is providing a correct revolution angle even in case of the breakdown of one channel of first respectively second voltage. In this case there is no redundancy anymore. In order to keep safety on the highest level the resolver should be repaired after the breakdown of one of the information channels anyhow, but for a limited time immediately after the breakdown the safety is increased in an advantageous way therewith. So it is possible for example to stop a robot in a controlled manner along its planned movement path instead of initializing an emergency break with a partly uncontrolled movement of the robot during decelerating.

According to another embodiment of the invention the evaluation means are foreseen to determine the revolution angle Θ_Μ in the fault free case according to the equation Θ_Μ = ω_Μΐ = arctan ^, respectively Θ_Μ = arccos ^- and Θ_Μ = arcsin^-, which can derived from the trigonomical relationships.

When the resolver is functioning properly, the magnetic field produced by the excitation coil is picked up by the first and second coils. This can be considered as a transformer arrangement. Thus, there is an expected ratio of amplitudes among V _; V₂ and V_E. Note that this also gives the possibility of an alternative determination of the revolution angle from these data. It is important to note that the correct branch of the arccos and arcsin functions must be chosen, i.e. there must be independent knowledge of the quadrant that the revolution angle is expected to be in. This expectation can be based on information on the quadrant from earlier measurement.

According to a preferred embodiment of the invention the evaluation means respectively its consistency check unit are foreseen to determine the revolution angle 9_{M i} according to the following steps, wherein Q_N determines the quadrant: in case that Vi and V₂ are correct:

• determine θ_{Μ ί} = a)_Mt_t = arctan—

vi,i

> in case that Vi went defect: • determine a = arcsin with r _ ^yli-i^+yli-i)

i-iV_Eii i-l

• in case that 9_{M i} is in Q_i then 9_{M i} ^■■= a

• in case that 9_{M i} is in Q₂ or Q₃ then 9_{M i} ^■■= π - a

• in case that 9_{M i} is in Q₄ then 0_{M i} ^■■= a + 2π

• determine 9_{M i}

• determine based on 9_Mii--y and 9_{M i} if rotation direction same or changed

• depending on rotation direction and on angles 9_{M i}__x and 9_{M i} determine new quadrant Q_n and recalculate 9_{M i}

> in case that V₂ went defect:

• determine β = arccos—— with r?_ = ^u~) _Ί1' and β ε [Ο, π] ri-l^VE,i \ ^VE ]

• in case that 9_{M i} is in Q₁ or Q₂ then 9_{M i} ^■■= β

• in case that 9_{M i} is in Q₃ or Q₄ then 9_{M i} ^■■= 2π - β

• determine 9_{M i}

• determine based on 0_Mii--1 and 9_{M i} if rotation direction same or changed

The consistency check unit is foreseen not to update r in case that the consistency check failed. In this case the previously calculated value for ris used.

The problem of the invention is also solved by a method for determining the revolution angle of a resolver for a servo motor wherein the resolver comprises

• a first and a second pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation,

• an excitation coil rotatable around the axis of rotation which is fore-seen to induce a respective first voltage Vi in the first pickup coil and a second induction voltage V₂ in the second pickup coil,

• voltage supply means for supplying an alternating voltage V_E to the excitation coil,

characterized by the following steps: • continuously determining in subsequent time-cycles /the revolution angle θ_{Μ ί} of the excitation coil dependent on an analysis of the first \l^ and second V₂ induced voltage and

• continuously performing a consistency check of the first V-i and second V₂ induced voltage,

• storing continuously at least the afore determined revolution angle #_M i-i and

• in case of a failed consistency check to determine whether one of the first Vi and second V₂ voltage is still correct,

• and in this case determining which of first Vi and second V₂ voltage is faulty and determining the revolution angle 9_{M i} based on the not faulty voltage Vi, i or V2, i ,VE, i and the afore determined revolution angle #_M i-i-

The advantages of this method correspond to the advantages of the resolver according to the invention and have already been explained before.

The problem of the invention is also solved by a method for operating a robot with at least one resolver according to one of the claims 1 to 3, wherein the at least one re- solver is operated according to the method of claim 4, comprising the following steps:

• operating the robot in a regular mode using the revolution angle of the at least one resolver until a defect of either the first Vi or second V₂ voltage of the at least one resolver is detected,

• in this case bringing the robot to a controlled stop while furthermore using the revolution angle from the at least one resolver.

So it is possible to stop a robot in a controlled manner along its planned movement path instead of initializing an emergency break with a partly uncontrolled movement of the robot during decelerating.

Further advantageous embodiments of the invention are mentioned in the dependent claims.

The invention will now be further explained by means of an exemplary embodiment and with reference to the accompanying drawings, in which: Figure 1 shows an exemplary resolver for a servo motor,

Figure 2 shows exemplary correlated voltage curves,

Figure 3 shows further exemplary correlated voltage curves,

Figure 4 shows an overview on mathematical background of determination of 9_{M i},

Figure 5 shows steps of method for determination of 9_{M i} in case that V-i is invalid,

Figure 6 steps of method for determination of 9_{M i} in case that V₂ is invalid,

Figure 7 shows exemplary components of a system with resolver,

Figure 8 shows an exemplary robot with resolver and evaluation unit and

Figure 9 shows details of consistency check

Figure 1 shows an exemplary resolver 10 for a servo motor in a principal arrangement. A first pickup coil 1 2 and a second pickup coil 14 are arranged perpendicular each to each other around a center axis. An excitation coil 1 6 is arranged rotatable around the same center axis, wherein a direction of rotation is indicated with the arrow 1 8. The revolution angle of the excitation coil 1 6 is the value of interest which has to be determined.

Not shown voltage supply means are foreseen for supplying an alternating excitation voltage V_E to the excitation coil 1 6. Under normal operation conditions the frequency of the excitation voltage is significant - for example by factor 3 to 10 - higher than the typical rotation frequency of the excitation coil 1 6. Excitation coil 1 6 and pickup coils 12, 14 can be seen as a kind of variable transformer arrangement, so a respective first voltage Vi is induced in the first pickup coil 12 and a respective second voltage V₂ is induced in the second pickup coil 14. The amplitude of the induced voltages depends on the current revolution angle of the excitation coil 1 6, so that - a rotation of the excitation coil assumed - a sinusoidal envelope of the induced voltages Vi and V₂ is generated. The respective mathematical relations and definitions are also shown in this fig.

Fig. 2 shows the course of exemplary correlated voltage curves in a sketch 20, namely the excitation voltage V_E, 22 the first voltage Vi 24 and the second voltage V₂ 26. All voltages have the same frequency of the carrier signal, wherein Vi and V₂ have a respective sinusoidal envelope which are shifted by 90° each to each other. Fig. 3 shows further exemplary correlated voltage curves in a sketch 30. Dependent on the course of the sinusoidal envelope of first voltage (marked with SIN) and second voltage (marked with COS), the carrier signal is either in phase or anti-phase with the excitation voltage.

Fig. 4 shows an overview on mathematical background of determination of 9_{M i} and 9_{M i} in a sketch 40 for the case of a valid consistency check and in the case that either the first or the second voltage is invalid.

Fig. 5 shows the steps of a method for determination of 9_{M i} and 9_{M i} in case that V-i is invalid in a sketch 50 and Fig. 6 shows the steps of a method for determination of 9_{M i} and 9_{M i} in case that V₂ is invalid in a sketch 60.

Fig. 7 shows exemplary components of a system with resolver in a sketch 70. A motor 2) that is controlled by a drive 1 ) is connected over its shaft with a resolver 3). The Drive 1 ) provides an excitation voltage to the resolver 3), which itself provides a first and a second output voltage generated by internal pickup coils.

A signal processing unit 4), a consistency check 5) unit and a data storage 6) are part of an evaluation unit, which analyzes the provided measurement values and calculates the revolution angle of the resolver 2) therefrom. In case of a failed consistency check the revolution angle is still calculated without redundancy and a controlled stop of the motor 2) is initiated.

Fig. 8 shows an exemplary robot 82 with resolvers 86, 90 and evaluation unit 92 in a sketch 80. The robot 82 comprises two exemplary joints 84, 88 which both are equipped with the dedicated respective resolver 86, respectively 90. The evaluation unit is a computing unit with signal processing unit and data storage.

Fig. 9 shows details of consistency check in an overview 1 00. At the lower right side of the overview a movement path of the tip of a robot arm is shown, wherein the points along the movement path represent a respective point in time, where a new calculation cycle for the consistency check is initiated, The current index of the time interval is represented with the letter i, wherein according to the invention also values of the past (i-1 , i-2, ..) are required for the calculation of the current revolution angle.

List of reference signs

10 exemplary resolver for a servo motor

12 first pickup coil

14 second pickup coil

1 6 excitation coil

18 exemplary direction of rotation

20 exemplary correlated voltage curves

22 voltage curve of excitation voltage V_E

24 voltage curve of first voltage Vi

26 voltage curve of second voltage V₂

30 exemplary correlated voltage curves

40 overview on mathematical background of determination of 9_{M i}

50 steps of method for determination of 9_{M i} and 9_{M i} in case that Vi is invalid

60 steps of method for determination of 9_{M i} and 9_{M i} in case that V₂ is invalid

70 exemplary components of a system with resolver

80 exemplary robot with resolver and evaluation unit

82 robot

84 first joint of robot

86 resolver for first joint

88 second joint of robot

90 resolver for second joint

92 evaluation means

100 details of consistency check

Claims

1 . Resolver (10, 86, 90) for a servo motor (82), comprising

• a first (12) and a second (14) pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation,

• an excitation coil (1 6) rotatable (18) around the axis of rotation which is foreseen to induce a respective first voltage Vi in the first pickup coil (12) and a second induction voltage V₂ in the second pickup coil (14),

• voltage supply means for supplying an alternating voltage V_E to the excitation coil (1 6),

• evaluation means (92) which are foreseen

• to determine in subsequent time-intervals / the revolution angle 9_{M i} of the excitation coil (1 6) dependent on an analysis of the first Vi and second V₂ induced voltage and

• to continuously perform a consistency check of the first Vi and second V₂ induced voltage,

characterized in that

the evaluation means are further foreseen

• and in this case

• to determine which of first Vi and second V₂ voltage went faulty and

• to determine the revolution angle 9_{M i} based on the not faulty voltage

Vi, i or V2, i ,VE, i and the afore determined revolution angle #_M i-i-

2. Resolver according to claim 1 , characterized in that the evaluation means (92) are foreseen to perform a consistency check according to the equation,

wherein r describes the maximum transformation ratio and the amplitude of excitation signal.

3. Resolver according to claim 1 or 2, characterized in that the evaluation means (92) are foreseen to determine the revolution angle Θ_{Μ t} according to the following steps: in case that Vi and V₂ are correct:

determine θ_{Μ ί} = ω_Μ^ = arctan

1,1 in case that Vi went faulty:

• determine a = arcsin— ^^ with rf_, = ' '

rt-iV_Eii ^{1 1} o)

• in case that 9_{M i} is in ^ then θ_{Μ ί} ^■■= a

• in case that 9_{M i} is in Q₂ or Q₃ then 6_{M i} ^■■= π - a

• in case that 9_{M i} is in Q₄ then θ_{Μ ί} ^■■= a + 2π

• determine θ_{Μ ί}

• determine based on 0_M,;_i and θ_{Μ ί} if rotation direction same or changed

• depending on rotation direction and on angles Θ_Μ _ and θ_{Μ ί} determine new quadrant Q_n and recalculate θ_{Μ ί} in case that V₂ went faulty:

• determine β = arccos

• in case that 9_{M i}\s in Q₁ or Q₂ then 6_{M i}■= β

• in case that 0_M<£is in Q₃ or Q₄ then θ_{Μ ί} ^■■= 2π - β

• determine 9_{M i}

• determine based on 0_M,;_i and 9_{M i} if rotation direction same or changed

• depending on rotation direction and on angles 0_M,t-i ^{an cl Θ}Μ,Ι determine new quadrant Q_n and recalculate B_{M i}

4. Method for determining the revolution angle of a resolver (10, 86, 90) for a servo motor (82), wherein the resolver comprises

• a first (12) and a second (14) pickup coil which are arranged in perpendicular orientation each to each other around an axis of rotation, • an excitation coil (1 6) rotatable (18) around the axis of rotation which is foreseen to induce a respective first voltage V1 in the first pickup coil (12) and a second induction voltage V2 in the second pickup coil (14),

characterized by the following steps:

• continuously determining in subsequent time-cycles / the revolution gle 9_{M i} of the excitation coil (1 6) dependent on an analysis of the first Vi and second V₂ induced voltage and

• continuously performing a consistency check of the first Vi and second V₂ induced voltage,

• in case of a failed consistency check to determine whether one of the first V-i and second V₂ voltage is still correct,

• and in this case determining which of first Vi and second V₂ voltage went defect and determining the revolution angle 9_{M i} based on the not defect voltage

Vi, i or V2, i _,VE, i and the afore determined revolution angle #_M i-i-

5. Method for operating a robot (82) with at least one resolver (10, 86, 90) according to one of the claims 1 to 3, wherein the at least one resolver (10, 86, 90) is operated according to the method of claim 4, comprising the following steps:

• operating the robot (82) in a regular mode using the revolution angle of the at least one resolver (10, 86, 90) until a defect of either the first Vi or second V₂ voltage of the at least one resolver is detected,

• in this case bringing the robot (82) to a controlled stop while furthermore using the revolution angle from the at least one resolver (10, 86, 90).