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Investigation into Temperature Dependence of Motor Current Measurements Applied to Monitoring and Adaptive Control
Investigation into Temperature Dependence of Motor Current Measurements Applied to Monitoring and Adaptive Control M. A. Mannan (2),S. Broms; KTH Stockholm/Sweden Received on January 14,1992 It is a crucial requirement, when using motor-current-based sensing systems. that the ratio between cutting forces and the corresponding motor currents is constant during the period of observation. Different induction motors employed in spindle and feed drives have been shown to have different sensitivities to heating cycles, depending upon their design and the magnetic materids used in them. Besides the changes in electro-magnetic losses due to temperature variations, changes in mechanical losses in different components incorporated in machine tool drives are the major contributory factors. We have experimentally studied the effect of the above-mentioned factors on the accuracy of motor current measurements. taking into account variations in temperature.
Keywords: Adaptive control,temperatures. tool wear
Introduction The capacity and potential of an unmanned machining center cannot be fully utilised without adaptive control and monitoring of machine tools. There is a great need for reliable and efficient sensors for monitoring machine tools, to ensure high metal removal rates and for taking corrective action in the event of accidents and cutting tool breakage. For the purpose of adaptive control and monitoring, a set o f sensors based on well established principles is being utilised. Different sensor systems for unmanned machining are described in refs 5 and 6. Among the sensors used in identifying the cutting process in terms of tool wear and tool breakage, motor current sensing constitutes a major method. In ref.4, feasibility of motor power and current sensing for adaptive control and monitoring is established. Monitoring systems based on motor current measurement are commercially available. The major advantage of using measuremenl of motor-related parameters such as motor powericurrent to detect malfunction in the cutting process is that the measuring apparatus does not disturb the machining process. Moreover, it can be applied in the manufacturing environment at almost no extra cost. It has also been established in ref.4 that the rate of increase in feed motor current is much higher than the rate of increase in spindle motor current. In some cases, feed motor current increased by 400 % due LO tool breakage, while the current to the spindle motor increased by 25%. Thus feed motor current is more sensitive to changes in the cutting process such as tool wear and breakage than spindle motor current.
machining center used in our study were not equipped with fans for forced cooling. Three-phase brushless synchronous motors with permanent magnetised armatures and permanent field DC motors are utilised in feed drives in modem CNC lathes and machining centers as they provide several advantages. Conscquendy our analysis is confined to the above-mentioned f&d motors. Cutting force assessment based on motor currents
In the case of indirect sensing of cutting forces by measuring motor currents, the whole drive from the motor to the tool post (turret) or spindle nose, has to be considered. A simplified diagram of the feed drive in a CNC lathe is given in fig. 1 to illustrate the point. The cutting force (feed force) acts on the cutting tool and the mechanical torque is generated by the electric motor to provide the feed motion and to overcome friction in the transmission. We are measuring the motor current and assuming that it is proportional to the force acting on the cutting tool. This assumption is very rough. There is a substantial increase in temperature, both in the frictional parts of the machine tool drive such as lead screw, guideways, bearings etc., and the electric motor itself. This rise in temperature due to frictional and electromagnetic losses violates the requirement of strict proportionality between the force acting on the tool and the input current to the drive motor.
Motor current sensing uses the drive motor itself as an indirect "sensor" of
The transmission path from the feed motor to the cutting tool can be divided into two subsystems.
cutting force. Therefore, when using a sensing system based on motor current, it is crucial that the relationship between input current and output torque is linear.
2.
Aim of the study
In this work we have conducted an experimental study into the thermal behaviour of the feed drive from the cutting tool to the electric motor with respect to feed motion in an NC lathe. The main aim of this study has been to answer the question: How sensitive are motor current estimates to changes in temperature in machine tool parts. In this study we chose an NC feed drive, mainly for the following reasons: 1.
2.
3.
4.
Current from a feed motor corresponding to the feed force is much more sensitive to tool wear and tool breakage compared with the current from the spindle drive. Often NC feed drives in modern CNC lathes and machining centers have rather similar designs, whereas in the case of spindle drives a larger variety of designs are available. The feed drive in our study belongs 'to a CNC lathe in which the z-axis feed motor is coupled directly to the lead screw through a toothed belt and a pulley drive. The other belongs to a CNC machining center in which the feed motors are directly coupled to the lead screw through a coupling. Another major difference between the feed drive and the spindle drive as far as our study is concerned is the type and design of the electric motors for the respective drives. This is particularly the case with modem feed motors based on permanent magnets which have well defined temperature characteristics, i.e. their behaviour (magnetic tlux) is affected by temperature variations in a well-defined manner. Most of the main drive motors are equipped with fans for forced cooling. The servomotors for feed drives in the CNC lathe and CNC
Annals of the ClRP Vol. 41/1/1992
1.
The mechanical system from the tool holder to the motor shaft. The mechanical and electro-magnetic system of the motor.
The transfer function between the input current(1) and the feed force (F), (CurrentlForce (IiF)) can then be subdivided into two different transfer functions such that (IiF) = @IT)* (TIF) where T is output torque of the feed motor
-
i
L
II
I
I
Current Fig.1 Consequently the transfer function between output toque from the motor and the corresponding feed force acting on the turret describes the characteristics of the mechanical transmission of the force from the motor to the cutting tool. This is affected by the characteristics of: 1. Bearings
2. Guideways
3. Pair of ball screw and nut.
Many factors have an impact on this transfer function. The influence of factors such as lubrication, load, sliding velocity, preloading method to the ball screw, temperature rise etc are exhibited mainly through changes in sliding
45 1
Analysis of losses in a feed drive
magnetic flux is lower at elevated temperatures than at room temperature. Consequently, the temperature coefficient o f Br determines the magnetic losses due to temperature rise. Thus the temperature coefficient of TK(Br) is expressed in terms of the percentage of flux density per degree Celsius of temperature rise.
In all mechanical transmission of force, there is a certain loss of energy. In
Experimental study
and rolling friction and the changes in dimensions of the different parts. The transfer function between the input current and output torque of the motor defines the mechanical and electro-magnetic properties of the feed motor.
machine tool drives, there are a number of components used to transmit the energy from the motors to the spindle and cutting tool or machine tool table: These parts, such as different gearboxes, belt dnves of different types. pairs of ball screw and n u t etc., are characterised by a certam efficiency of force transmission. The mechanical efficiency of gearboxes, belt drives. ball screw and nut etc., is always a vanable number. For exampie the mechanical efficiency of one and the same ball screw in a machine tool feed drive may vary between 0.94 and 0.98, depending upon the changes in frictlon. even though all other parameters are kept constant. It is highly desirable that the mechanical efficiencies of components incorporated in a spindle or feed drive be constant during the period of machine tool operation, or at least that they vary in a predictable manner. In the foilowing discussion. we will be confining ourselves to the analysis of mechanical efficiency of a feed drive as shown in fig. 1, as i t is affected by the rise in temperature of all machine tool parts. The power generated by the feed motor is used to provide feed motion and to overcome friction i n transmission parts such that
While estimating the total error in motor current sensing, the effect of the changes in the frictional properties of the mechanical transmission as described by the ratio (TIF)and effect of the changes in electro-magnetic properties and mechanical properties of the feed motor have to be studied separately. A number of test series are conducted to evaluate the error in estimation of motor currents. In the first set of test series, the variations in (T/F),(IiT) and flI/F) are experimentally investigated. In the second and third sets of test senes, error in motor current estimates while machining for a longer period of time in a lathe and in a machining center, respectively. are studied. Belt Drive to
Armature Temperature, Ta Temperature. Ts
P(motor) = P(cutting) + P(losses) where P(losses) can be expressed as a sum of losses in different pans such that: P(losses) = Plbd+PIsn+Plbr+Plgw Plbd = power losses i n belt drive Plsn = power losses in ball screw-nut pair Pigw = power losses in guideways Plbr = power losses in bearings All the losses in the above expression are more or less temperature-dependent. However, losses due to friction in guideways and ball screw-nut pair are affected by temperature rise in a more straightforward manner, and changes in r are subsrantial. I n the CNC machine to& used in our their ~ w e losses study, lubrication oil is supplied to guideways and ball screw-nut and bearings at certain intervals, which leads to semi-fluid friction. The coefficient of friction depends upon the sliding velocity, viscosity of the lubncant and the nominal pressure. In the roughing operations, feed rates and loads are chosen in a rather narrow range, which makes the changes in viscosity the major parameter affecting the coefficient of friction and thereby the losses in the transmission.
Thermal behaviour of modern feed force motors The transfer function between torque and current is mainly influenced by the temperature rise i n the motor. Permanent magnets(PM) are used both in permanent field DC motors and in three-phase brushless synchronous niotors with permanent magnetised armatures. Therefore, the characteristics of the motors are strongly dependent upon the properties of the magnets. In the following table, the properties of magnets commonly used in feed motors are given.
NdFeB Sm2Co17 SECo5 SrFe203 Alnico8
Remanence
Maximum BH product
Br [TI 0.95-1.3 0.95-1.15 0.6-1.3 0.37 0.8
(BH)max [kT/m3] 170-335 150-240 60-210 25 32
Temperture coefficient of Br TK(Br) [%IT] -0.11--0.10 -0.030 -0.015--0.040 -0.2 -0.02
Curie temperature
Tc
roc] 310 800 720 450 845
In this table, the temperature coefficient of TK(Br) is the important parameter
(Strain Gauges) Fig.2 1.
Investigation of variations in (T/F), (IIT) and (IIF) as function of temperature rise in a NC f e d drive (test series I):
A special arrangement as shown in fig. 2 has been designed, which facilitates the measurement of output torque from a feed motor. (In the conventional design of a servo-motor used in the NC lathe, no space was available at or around the output shaft for the placement.of sensors for measuring torque.) The amount of the feed force, the output torque and input current to the motor were measured at different feed motor temperatures. The servo-motor. lead screw, guideways and bearings etc., were heated up through rapid travel of the feed drive. A feed rate of 9 mimin was chosen. The test arrangement according to fig. 2 was manufactured and attached between the feed motor for Z-axis motion and the belt drive to the lead screw. A number of tests were performed, where the turret was loaded by a hydraulic jack to simulate the feed force acting on the cutting tool. It is almost impossible to obtain identical force levels by actual turning because of different tool wear of the cutter and varying machinability of the work material. In these test series, output torque from the feed motor and input current to the same were measured for different loads on the turret.
A heat insulating material was wrapped around the motor housing for faster heating of the whole motor. The feed drive. was made to move at a rate of 9 mlmin continuously for a certain period of time until a motor temperature rise of about 5°C occurred. After that, load was applied to the turret by hydraulic equipment. A force transducer was placed between the turret and the hydraulic cylinder and the feed dnve was made to move against the hydraulic plunger at a feed rate of 2 mImin. The signals from the fofce Iransducer and the torque sensor and the corresponding motor current were registered on an instrumentation tape recorder for further analysis.
--
r-
0.6
which affects the torque. Torque in a permanent field DC motor is calculated from the following expression:
I .Motor Current
w
T=KIaO where K is a constant defined by the design parameters of a given motor. I, is the armature current and 5 is the magnetic flux. Thus the output torque from a permanent field DC motor is defined as the product of the armature current and the magnetic flux. The magnetic flux of the magnet system of the motor is a temperature-dependent characteristic of the magnetic material used. The
452
,Mercury Slip Rings
0
Fig.3 At least 10 load cycles were applied to the turret at each temperature increment. The force acting on the turret and the corresponding torque and
motor current are plotted i n fig. 3. The two ratios (]IT), (TiF) for the period of time chosen, represented by the two vertical lines shown in fig. 3 were then calculated. About I50 ratios for each cycle were calculated, which gave 1.500 values for the ratios (]IT) and (TIF) For each temperature increment. T h e mean of these 1,500 values are plotted in figs. 4a and 4b. T h z temperatures on the lead screw, bail nut, guideways. bearings, toothed belt. feed motor armature and stator and the lubrication oil were also measured. I n table 2 . t h e range of temperature variations as measured are given for differen1 test series. 1
Z-axis feed notor
I
1
2:.0 15.3
j
Test seres I1 111 20.3 ' 22.0 45.1 ' 50.1 ~
!
IV
!
32.0 55.2
simulated, where the tool life limit of an end mill has to be monitored. It is assumed that the monmred end mill is not used continuously until Its useful tool life limit (30 minutes) is reached. Instead, i t is used period-wise, i.e. an end mill will be cutting for a shon period of time (25 seconds) and will be replaced by another tool for a different operation and again called upon for the next cut, and so on. During the 10 -
' 0
3
2
1
4
Machining time lhoursl
Lead screw
1
20.3 30.3 10.3 30.7 21.0
19.8 29.8 19.9 30.5 19.9
,
34.1
-10.1
!
11.0 28.7
1 Nut Toothed belt Lubncating
oil
~
I
19.4 27.5
j
71.3 43.0 21.1 36.5 21.2 35.9 23.8 31.6
1 ~
I
21.0 49.6 21.6 38.5 21.3 37.9 23.8
Fig.5 time the tool life of the end mill is monitored, it is desired that the machining center be in continuous use for about two hours, which should lead to substantial heating of the feed motor observed. Two different workpieces
1
'
r 840
500
iI,820
34.1
400 In this case, it was aimed to test the effect of heating in a feed motor and other parts in the feed drive due to continuous operation during a half working shift, based on the estimation of motor currents. For this purpose, a roughing operation of long shafts was selected. The cuttlng was performed with 600 mm-long workpieces of alloyed steel SS2142 with the following cutting data: V = 240 mimin, S = 0.4 rnmirev a = 2.5 m m . Two different cutters of the same make were employed under these tests: one for the roughing operation and the other employed only when motor current and feed force measurements were taken. Each pass of roughing operation lasted for about 25 minutes and after each period of
Constant force = 4000
N
I
-
300
lSp
lmv]
s=O.l5mmitooth a4mm
__
200
7 . 011
20
0
40
Fig.6 are mounted on the table of the machining center. One for "heating" the motor and the other for testing of tool wear of an end mill. An 8Wmm -long workpiece is used for face milling operations with the following cutting data: v = 70mimin. sz= 0. I5mm. t = 5mm, b = 50mm
1400 1200
800
v=30rn/min s=O.15rnrn/toth
i
T=23min
600 Constant force = 2000 N
100
80
60
1000 Fie.4a
i'c 760
Ix irnV1
10
--
Fig.%
TJF -20
20
Temperature [ ' C ] 30
,
40
Fig.4b
2,4- IxIAl 22: 2,o 1 1,8 1 16: 1,4 : 127 1 ,o 600
y = 1.6667e-3x R"2 = 1.000
Fx[h.]
800
1000
I
1
1200
1400
Fig. 7b -4 20
3.
Temperature ['Cl
30
I
40
Fig.4~ 7-5 minutes' roughing, a 100 mm-long cut was taken with the second cutter, when the feed force and corresponding motor current were measured. The results of this test are presented in fig.5. Heating of feed motor in a machining center (test series 111).
In this test, a typical monitoring task in a machining center is
The feed motor currents are measured during both face milling and end milling. In the beginning, feed motor current is measured with a new end mill, and after that face milling is performed. After 3-4 passes of face milling again, end milling is camed out on the other workpiece. The workpiece for the end milling tests IS mounted on a dynamometer so that the cutting forces are monitored simultaneously with current measurement. The cutting data dunng the end milling test are kept constant and are given in the following values:
453
v = 30m/min, sz= O.lSmm, a,= IOmm, ar= 3mm This test proceeded continuously for approximately 127 minutes. The machine tool was.briefly (aprox. 1 minute) stopped to change the worn cutting tips of the face milling cutter. The results from these tests are plotted in fig. 6 and fig. 7. Results and Discussions
In this study, our main concern has been to estimate the degree of error in the measurement of feed motor current due to heating of the motor and the other parts in the feed drive. The experiments mentioned in the aforementioned test series were aimed at estimating the variations in frictional properties of the guideways, ball screw-nut pair and bearings etc., and the changes in the mechanical and elecro-magnetic propenies of the feed motors. One of the major goals in our study was to evaluate an approximate range within which the variations of motor current needed to maintain one and the Same feed force acting on the cutter occur. The experimental investigations presented here do not fully cover the many different situations encountered in modem manufacturing, but seek to provide explanations of various phenomena and the manner in which different changes occur. The results presented in figs. 4a - 4c. clearly show that while evaluating the accuracy of force estimation through motor current measurements, the effect of changes in the transfer function (T!F) and effect of changes in transfer function (I/T) have to be considered separately. In the case of CNC lathes we have tested, it is obvious from figures 4a - 4c that the amount of torque needed to maintain the Same level of force acting on the cutter decreases linearly as the temperature rises, whereas the amounx of input current needed to output the Same torque at.the motor shaft increases as the motor is heated up. The total effect of the temperature increase on the transfer function (UF). as shown in fig. 4c is rather modest, varying between -5% to +9%. In cases of machine tools, where a servo-motor is equipped with a fan for forced cooling or some temperature stabilization measures are applied to maintain the temperature of ball screw-nut pair, guideways and bearings etc. at a constant level, then the degree of error in motor current measurements during machining for a long period of time can be estimated by following the manner are separately in which the behaviour of transfer functions (T/F) and (In) treated. In our case, we have found that there is a relative gain of energy in mechanical transmission from the motor shaft 10 the turret (increase in total mechanical efficiency) and there is a relative loss of energy in the feed motor as the temperature increases. The total effect of temperature rise on the motor current, as shown in fig 4c, reveals that in a majority of test series the relative loss of power in feed motors is higher than the relative gain in power in the mechanical parts. The results, presented in fig. 5 correspond well with the findings in fig. 4c. The behaviour of the changes in motor current estimates due to temperature rise have been confirmed by the results obtained from another major investigation conducted at our department.
In this investigation, carbide tools of different makes were tested while cutting in alloyed steel SS2142. The tool wear and the corresponding motor currents were accurately measured in all tests. A couple of interesting curves from these test series are shown in fig. 8. The marked range in both curves shows a strange drop in motor current. This drop in motor currenu can be explained by the fact that the tests were terminated at the point in the afternoon when the machine tool was warm, and when they were resumed the next morning. the machine tool was cold. Similar drops in motor current, between the point of termination and restart of the tests, have been observed in several cases. In this investigation, the motor current estimates increased by 5-10 % between the cold machine (morning) and the warm machine (in the afternoon). 4.2
sets of cutting edges are shown.
In this figure, both the feed motor current and spindle motor current are plotted as a function of cutting time. This plot clearly demonstrates that the tool life of cutting tips of a face milling cutter can be successfully monitored by sensing the feed motor currents. In fig. 7a, the feed motor current and feed force measured by a Kistler piezo-electric dynamometer are shown as a function of tool flank wear. In fig. 7b, it can be seen that there is a direct proportionality between cutting force and the motor current. The error caused by the motor heating - resulting from continuous cutting for more than two hours - is found to be less than 2.001..
Temperatures of different parts incorporated in the feed drive were measured (table 2). Furthermore, the value of the maximum temperature of the magnets in a permanent field DC motor have been calculated using a finite element heat-transfer package. These calculations have shown that the magnets may reach a temperaxure of about 60°C - 75°C under &most unfavorable conditions. If we assume a maximum temperature of the magnets m'und X "C , then the temperature rise is equal to 55'C (75°C-70"C). The total decrease of magnetic tlux can now be calculated as Tk(Br) * 55'C. The value of TK(Br) varies between -O.o?%/"C and -0.2%1°C (table. 1). Thus the error attributable to magnetic losses amounu to 1.1-1 I % . In our case, feed motors in the CNC machining center have Samarium cobalt permanent magnets which give a maximum error of 1.65% in current measurements. The feed motor in the CNC lathe investigated has ferritic magnets with the TKIB,) = -O.2%'oi0Cc, which leads to a degree of error around 10 - 15%. The loss of energy within the PM motor due to temperature rise is attributable to factors other than the decrease of magnetic flux: pulsation losses on the magnet surface arising from the windings are an important contributory factor. Conclusions
I . The transfer function between the cutting force and the corresponding motor current is affected by non-linearities in the electro-magnetic system of the motor and the tribolcgical system of the feed transmission. The results obtained show that less torque is needed to perform the feed motion at higher temperatures of feed screw, guideways etc. At the Same time, it has been observed that the amount of input current to the feed motor needed to maintain a certain level of torque, increases due to temperature rise, fig. 4a and fig. 4b 2. In the case of the CNC lathe investigated, the total error in the estimation of motor current, attributable to temperature rise, has been found to vary from -4% to +9%. fig.4c. 3.The temperature-dependent characteristics of PM ked motors are governed by the propenies of the magnetic materials employed. An error of I % - 15% in current values can be expected during tool life monitoring. The results in fig. 4a and 4b, which show that the amount of increase in motor current values needed to maintain one and the same torque is in the range of 15-20%, confirm that a major part of the increase in current is attributable to magnetic losses'
(I)
(2) (3) (4)
*__-___-_
[Motor current [A] !
(5)
/
488.
(6)
Fig.8 l%e long-term cutting tests presented in fig. 6 and fig. 7 show that tool life limit can be successfully monitored even though there may be a substantial temperature rise in the motor due to continuous operation of the machine tool (under normal cutting conditions) over a longer period of time, which is actually the situation encountered in the industry. In this case,error caused by the non-linear behaviour of the motor and the transmission is estimated to be below 2%. In fig. 6, xhree different curves corresponding to three different
454
Bollinger, J.G.,Stute, G.,Van Brussel, H., 1980, Digital Control and Feed Drives State-of-the-Art and New Development, Annals of the CIRP, 2912, 497-506. Bryan, J., 1990, International Status of Thermal Error Research. Annals of the CIRP, 3912. 635-656. Koren. Y., 1984, Torque and Speed Control of DC servo-motors for robots, Annals of the CIRP, 33/1, 239-242. Mannan, M.A., Broms, S., 1989, Monitoring and Adaptive Control of Cutting Process by Means of Motor Power and Current Measurements, Annals of the CIRP, 3811. 347-350. Micheletti, G.F., Koenig, W., Victor, H.R.. 1976. In-Process tool wear Sensors for Cutting Operations, Annals of the CIRP, 2512, 483Tlusty, J., Andrews, G.C., 1983, A Critical Review of Sensors for Unmanned machining, Annals of the CIRP, 3212, 563-572.
Keywords: Adaptive control,temperatures. tool wear
Introduction The capacity and potential of an unmanned machining center cannot be fully utilised without adaptive control and monitoring of machine tools. There is a great need for reliable and efficient sensors for monitoring machine tools, to ensure high metal removal rates and for taking corrective action in the event of accidents and cutting tool breakage. For the purpose of adaptive control and monitoring, a set o f sensors based on well established principles is being utilised. Different sensor systems for unmanned machining are described in refs 5 and 6. Among the sensors used in identifying the cutting process in terms of tool wear and tool breakage, motor current sensing constitutes a major method. In ref.4, feasibility of motor power and current sensing for adaptive control and monitoring is established. Monitoring systems based on motor current measurement are commercially available. The major advantage of using measuremenl of motor-related parameters such as motor powericurrent to detect malfunction in the cutting process is that the measuring apparatus does not disturb the machining process. Moreover, it can be applied in the manufacturing environment at almost no extra cost. It has also been established in ref.4 that the rate of increase in feed motor current is much higher than the rate of increase in spindle motor current. In some cases, feed motor current increased by 400 % due LO tool breakage, while the current to the spindle motor increased by 25%. Thus feed motor current is more sensitive to changes in the cutting process such as tool wear and breakage than spindle motor current.
machining center used in our study were not equipped with fans for forced cooling. Three-phase brushless synchronous motors with permanent magnetised armatures and permanent field DC motors are utilised in feed drives in modem CNC lathes and machining centers as they provide several advantages. Conscquendy our analysis is confined to the above-mentioned f&d motors. Cutting force assessment based on motor currents
In the case of indirect sensing of cutting forces by measuring motor currents, the whole drive from the motor to the tool post (turret) or spindle nose, has to be considered. A simplified diagram of the feed drive in a CNC lathe is given in fig. 1 to illustrate the point. The cutting force (feed force) acts on the cutting tool and the mechanical torque is generated by the electric motor to provide the feed motion and to overcome friction in the transmission. We are measuring the motor current and assuming that it is proportional to the force acting on the cutting tool. This assumption is very rough. There is a substantial increase in temperature, both in the frictional parts of the machine tool drive such as lead screw, guideways, bearings etc., and the electric motor itself. This rise in temperature due to frictional and electromagnetic losses violates the requirement of strict proportionality between the force acting on the tool and the input current to the drive motor.
Motor current sensing uses the drive motor itself as an indirect "sensor" of
The transmission path from the feed motor to the cutting tool can be divided into two subsystems.
cutting force. Therefore, when using a sensing system based on motor current, it is crucial that the relationship between input current and output torque is linear.
2.
Aim of the study
In this work we have conducted an experimental study into the thermal behaviour of the feed drive from the cutting tool to the electric motor with respect to feed motion in an NC lathe. The main aim of this study has been to answer the question: How sensitive are motor current estimates to changes in temperature in machine tool parts. In this study we chose an NC feed drive, mainly for the following reasons: 1.
2.
3.
4.
Current from a feed motor corresponding to the feed force is much more sensitive to tool wear and tool breakage compared with the current from the spindle drive. Often NC feed drives in modern CNC lathes and machining centers have rather similar designs, whereas in the case of spindle drives a larger variety of designs are available. The feed drive in our study belongs 'to a CNC lathe in which the z-axis feed motor is coupled directly to the lead screw through a toothed belt and a pulley drive. The other belongs to a CNC machining center in which the feed motors are directly coupled to the lead screw through a coupling. Another major difference between the feed drive and the spindle drive as far as our study is concerned is the type and design of the electric motors for the respective drives. This is particularly the case with modem feed motors based on permanent magnets which have well defined temperature characteristics, i.e. their behaviour (magnetic tlux) is affected by temperature variations in a well-defined manner. Most of the main drive motors are equipped with fans for forced cooling. The servomotors for feed drives in the CNC lathe and CNC
Annals of the ClRP Vol. 41/1/1992
1.
The mechanical system from the tool holder to the motor shaft. The mechanical and electro-magnetic system of the motor.
The transfer function between the input current(1) and the feed force (F), (CurrentlForce (IiF)) can then be subdivided into two different transfer functions such that (IiF) = @IT)* (TIF) where T is output torque of the feed motor
-
i
L
II
I
I
Current Fig.1 Consequently the transfer function between output toque from the motor and the corresponding feed force acting on the turret describes the characteristics of the mechanical transmission of the force from the motor to the cutting tool. This is affected by the characteristics of: 1. Bearings
2. Guideways
3. Pair of ball screw and nut.
Many factors have an impact on this transfer function. The influence of factors such as lubrication, load, sliding velocity, preloading method to the ball screw, temperature rise etc are exhibited mainly through changes in sliding
45 1
Analysis of losses in a feed drive
magnetic flux is lower at elevated temperatures than at room temperature. Consequently, the temperature coefficient o f Br determines the magnetic losses due to temperature rise. Thus the temperature coefficient of TK(Br) is expressed in terms of the percentage of flux density per degree Celsius of temperature rise.
In all mechanical transmission of force, there is a certain loss of energy. In
Experimental study
and rolling friction and the changes in dimensions of the different parts. The transfer function between the input current and output torque of the motor defines the mechanical and electro-magnetic properties of the feed motor.
machine tool drives, there are a number of components used to transmit the energy from the motors to the spindle and cutting tool or machine tool table: These parts, such as different gearboxes, belt dnves of different types. pairs of ball screw and n u t etc., are characterised by a certam efficiency of force transmission. The mechanical efficiency of gearboxes, belt drives. ball screw and nut etc., is always a vanable number. For exampie the mechanical efficiency of one and the same ball screw in a machine tool feed drive may vary between 0.94 and 0.98, depending upon the changes in frictlon. even though all other parameters are kept constant. It is highly desirable that the mechanical efficiencies of components incorporated in a spindle or feed drive be constant during the period of machine tool operation, or at least that they vary in a predictable manner. In the foilowing discussion. we will be confining ourselves to the analysis of mechanical efficiency of a feed drive as shown in fig. 1, as i t is affected by the rise in temperature of all machine tool parts. The power generated by the feed motor is used to provide feed motion and to overcome friction i n transmission parts such that
While estimating the total error in motor current sensing, the effect of the changes in the frictional properties of the mechanical transmission as described by the ratio (TIF)and effect of the changes in electro-magnetic properties and mechanical properties of the feed motor have to be studied separately. A number of test series are conducted to evaluate the error in estimation of motor currents. In the first set of test series, the variations in (T/F),(IiT) and flI/F) are experimentally investigated. In the second and third sets of test senes, error in motor current estimates while machining for a longer period of time in a lathe and in a machining center, respectively. are studied. Belt Drive to
Armature Temperature, Ta Temperature. Ts
P(motor) = P(cutting) + P(losses) where P(losses) can be expressed as a sum of losses in different pans such that: P(losses) = Plbd+PIsn+Plbr+Plgw Plbd = power losses i n belt drive Plsn = power losses in ball screw-nut pair Pigw = power losses in guideways Plbr = power losses in bearings All the losses in the above expression are more or less temperature-dependent. However, losses due to friction in guideways and ball screw-nut pair are affected by temperature rise in a more straightforward manner, and changes in r are subsrantial. I n the CNC machine to& used in our their ~ w e losses study, lubrication oil is supplied to guideways and ball screw-nut and bearings at certain intervals, which leads to semi-fluid friction. The coefficient of friction depends upon the sliding velocity, viscosity of the lubncant and the nominal pressure. In the roughing operations, feed rates and loads are chosen in a rather narrow range, which makes the changes in viscosity the major parameter affecting the coefficient of friction and thereby the losses in the transmission.
Thermal behaviour of modern feed force motors The transfer function between torque and current is mainly influenced by the temperature rise i n the motor. Permanent magnets(PM) are used both in permanent field DC motors and in three-phase brushless synchronous niotors with permanent magnetised armatures. Therefore, the characteristics of the motors are strongly dependent upon the properties of the magnets. In the following table, the properties of magnets commonly used in feed motors are given.
NdFeB Sm2Co17 SECo5 SrFe203 Alnico8
Remanence
Maximum BH product
Br [TI 0.95-1.3 0.95-1.15 0.6-1.3 0.37 0.8
(BH)max [kT/m3] 170-335 150-240 60-210 25 32
Temperture coefficient of Br TK(Br) [%IT] -0.11--0.10 -0.030 -0.015--0.040 -0.2 -0.02
Curie temperature
Tc
roc] 310 800 720 450 845
In this table, the temperature coefficient of TK(Br) is the important parameter
(Strain Gauges) Fig.2 1.
Investigation of variations in (T/F), (IIT) and (IIF) as function of temperature rise in a NC f e d drive (test series I):
A special arrangement as shown in fig. 2 has been designed, which facilitates the measurement of output torque from a feed motor. (In the conventional design of a servo-motor used in the NC lathe, no space was available at or around the output shaft for the placement.of sensors for measuring torque.) The amount of the feed force, the output torque and input current to the motor were measured at different feed motor temperatures. The servo-motor. lead screw, guideways and bearings etc., were heated up through rapid travel of the feed drive. A feed rate of 9 mimin was chosen. The test arrangement according to fig. 2 was manufactured and attached between the feed motor for Z-axis motion and the belt drive to the lead screw. A number of tests were performed, where the turret was loaded by a hydraulic jack to simulate the feed force acting on the cutting tool. It is almost impossible to obtain identical force levels by actual turning because of different tool wear of the cutter and varying machinability of the work material. In these test series, output torque from the feed motor and input current to the same were measured for different loads on the turret.
A heat insulating material was wrapped around the motor housing for faster heating of the whole motor. The feed drive. was made to move at a rate of 9 mlmin continuously for a certain period of time until a motor temperature rise of about 5°C occurred. After that, load was applied to the turret by hydraulic equipment. A force transducer was placed between the turret and the hydraulic cylinder and the feed dnve was made to move against the hydraulic plunger at a feed rate of 2 mImin. The signals from the fofce Iransducer and the torque sensor and the corresponding motor current were registered on an instrumentation tape recorder for further analysis.
--
r-
0.6
which affects the torque. Torque in a permanent field DC motor is calculated from the following expression:
I .Motor Current
w
T=KIaO where K is a constant defined by the design parameters of a given motor. I, is the armature current and 5 is the magnetic flux. Thus the output torque from a permanent field DC motor is defined as the product of the armature current and the magnetic flux. The magnetic flux of the magnet system of the motor is a temperature-dependent characteristic of the magnetic material used. The
452
,Mercury Slip Rings
0
Fig.3 At least 10 load cycles were applied to the turret at each temperature increment. The force acting on the turret and the corresponding torque and
motor current are plotted i n fig. 3. The two ratios (]IT), (TiF) for the period of time chosen, represented by the two vertical lines shown in fig. 3 were then calculated. About I50 ratios for each cycle were calculated, which gave 1.500 values for the ratios (]IT) and (TIF) For each temperature increment. T h e mean of these 1,500 values are plotted in figs. 4a and 4b. T h z temperatures on the lead screw, bail nut, guideways. bearings, toothed belt. feed motor armature and stator and the lubrication oil were also measured. I n table 2 . t h e range of temperature variations as measured are given for differen1 test series. 1
Z-axis feed notor
I
1
2:.0 15.3
j
Test seres I1 111 20.3 ' 22.0 45.1 ' 50.1 ~
!
IV
!
32.0 55.2
simulated, where the tool life limit of an end mill has to be monitored. It is assumed that the monmred end mill is not used continuously until Its useful tool life limit (30 minutes) is reached. Instead, i t is used period-wise, i.e. an end mill will be cutting for a shon period of time (25 seconds) and will be replaced by another tool for a different operation and again called upon for the next cut, and so on. During the 10 -
' 0
3
2
1
4
Machining time lhoursl
Lead screw
1
20.3 30.3 10.3 30.7 21.0
19.8 29.8 19.9 30.5 19.9
,
34.1
-10.1
!
11.0 28.7
1 Nut Toothed belt Lubncating
oil
~
I
19.4 27.5
j
71.3 43.0 21.1 36.5 21.2 35.9 23.8 31.6
1 ~
I
21.0 49.6 21.6 38.5 21.3 37.9 23.8
Fig.5 time the tool life of the end mill is monitored, it is desired that the machining center be in continuous use for about two hours, which should lead to substantial heating of the feed motor observed. Two different workpieces
1
'
r 840
500
iI,820
34.1
400 In this case, it was aimed to test the effect of heating in a feed motor and other parts in the feed drive due to continuous operation during a half working shift, based on the estimation of motor currents. For this purpose, a roughing operation of long shafts was selected. The cuttlng was performed with 600 mm-long workpieces of alloyed steel SS2142 with the following cutting data: V = 240 mimin, S = 0.4 rnmirev a = 2.5 m m . Two different cutters of the same make were employed under these tests: one for the roughing operation and the other employed only when motor current and feed force measurements were taken. Each pass of roughing operation lasted for about 25 minutes and after each period of
Constant force = 4000
N
I
-
300
lSp
lmv]
s=O.l5mmitooth a4mm
__
200
7 . 011
20
0
40
Fig.6 are mounted on the table of the machining center. One for "heating" the motor and the other for testing of tool wear of an end mill. An 8Wmm -long workpiece is used for face milling operations with the following cutting data: v = 70mimin. sz= 0. I5mm. t = 5mm, b = 50mm
1400 1200
800
v=30rn/min s=O.15rnrn/toth
i
T=23min
600 Constant force = 2000 N
100
80
60
1000 Fie.4a
i'c 760
Ix irnV1
10
--
Fig.%
TJF -20
20
Temperature [ ' C ] 30
,
40
Fig.4b
2,4- IxIAl 22: 2,o 1 1,8 1 16: 1,4 : 127 1 ,o 600
y = 1.6667e-3x R"2 = 1.000
Fx[h.]
800
1000
I
1
1200
1400
Fig. 7b -4 20
3.
Temperature ['Cl
30
I
40
Fig.4~ 7-5 minutes' roughing, a 100 mm-long cut was taken with the second cutter, when the feed force and corresponding motor current were measured. The results of this test are presented in fig.5. Heating of feed motor in a machining center (test series 111).
In this test, a typical monitoring task in a machining center is
The feed motor currents are measured during both face milling and end milling. In the beginning, feed motor current is measured with a new end mill, and after that face milling is performed. After 3-4 passes of face milling again, end milling is camed out on the other workpiece. The workpiece for the end milling tests IS mounted on a dynamometer so that the cutting forces are monitored simultaneously with current measurement. The cutting data dunng the end milling test are kept constant and are given in the following values:
453
v = 30m/min, sz= O.lSmm, a,= IOmm, ar= 3mm This test proceeded continuously for approximately 127 minutes. The machine tool was.briefly (aprox. 1 minute) stopped to change the worn cutting tips of the face milling cutter. The results from these tests are plotted in fig. 6 and fig. 7. Results and Discussions
In this study, our main concern has been to estimate the degree of error in the measurement of feed motor current due to heating of the motor and the other parts in the feed drive. The experiments mentioned in the aforementioned test series were aimed at estimating the variations in frictional properties of the guideways, ball screw-nut pair and bearings etc., and the changes in the mechanical and elecro-magnetic propenies of the feed motors. One of the major goals in our study was to evaluate an approximate range within which the variations of motor current needed to maintain one and the Same feed force acting on the cutter occur. The experimental investigations presented here do not fully cover the many different situations encountered in modem manufacturing, but seek to provide explanations of various phenomena and the manner in which different changes occur. The results presented in figs. 4a - 4c. clearly show that while evaluating the accuracy of force estimation through motor current measurements, the effect of changes in the transfer function (T!F) and effect of changes in transfer function (I/T) have to be considered separately. In the case of CNC lathes we have tested, it is obvious from figures 4a - 4c that the amount of torque needed to maintain the Same level of force acting on the cutter decreases linearly as the temperature rises, whereas the amounx of input current needed to output the Same torque at.the motor shaft increases as the motor is heated up. The total effect of the temperature increase on the transfer function (UF). as shown in fig. 4c is rather modest, varying between -5% to +9%. In cases of machine tools, where a servo-motor is equipped with a fan for forced cooling or some temperature stabilization measures are applied to maintain the temperature of ball screw-nut pair, guideways and bearings etc. at a constant level, then the degree of error in motor current measurements during machining for a long period of time can be estimated by following the manner are separately in which the behaviour of transfer functions (T/F) and (In) treated. In our case, we have found that there is a relative gain of energy in mechanical transmission from the motor shaft 10 the turret (increase in total mechanical efficiency) and there is a relative loss of energy in the feed motor as the temperature increases. The total effect of temperature rise on the motor current, as shown in fig 4c, reveals that in a majority of test series the relative loss of power in feed motors is higher than the relative gain in power in the mechanical parts. The results, presented in fig. 5 correspond well with the findings in fig. 4c. The behaviour of the changes in motor current estimates due to temperature rise have been confirmed by the results obtained from another major investigation conducted at our department.
In this investigation, carbide tools of different makes were tested while cutting in alloyed steel SS2142. The tool wear and the corresponding motor currents were accurately measured in all tests. A couple of interesting curves from these test series are shown in fig. 8. The marked range in both curves shows a strange drop in motor current. This drop in motor currenu can be explained by the fact that the tests were terminated at the point in the afternoon when the machine tool was warm, and when they were resumed the next morning. the machine tool was cold. Similar drops in motor current, between the point of termination and restart of the tests, have been observed in several cases. In this investigation, the motor current estimates increased by 5-10 % between the cold machine (morning) and the warm machine (in the afternoon). 4.2
sets of cutting edges are shown.
In this figure, both the feed motor current and spindle motor current are plotted as a function of cutting time. This plot clearly demonstrates that the tool life of cutting tips of a face milling cutter can be successfully monitored by sensing the feed motor currents. In fig. 7a, the feed motor current and feed force measured by a Kistler piezo-electric dynamometer are shown as a function of tool flank wear. In fig. 7b, it can be seen that there is a direct proportionality between cutting force and the motor current. The error caused by the motor heating - resulting from continuous cutting for more than two hours - is found to be less than 2.001..
Temperatures of different parts incorporated in the feed drive were measured (table 2). Furthermore, the value of the maximum temperature of the magnets in a permanent field DC motor have been calculated using a finite element heat-transfer package. These calculations have shown that the magnets may reach a temperaxure of about 60°C - 75°C under &most unfavorable conditions. If we assume a maximum temperature of the magnets m'und X "C , then the temperature rise is equal to 55'C (75°C-70"C). The total decrease of magnetic tlux can now be calculated as Tk(Br) * 55'C. The value of TK(Br) varies between -O.o?%/"C and -0.2%1°C (table. 1). Thus the error attributable to magnetic losses amounu to 1.1-1 I % . In our case, feed motors in the CNC machining center have Samarium cobalt permanent magnets which give a maximum error of 1.65% in current measurements. The feed motor in the CNC lathe investigated has ferritic magnets with the TKIB,) = -O.2%'oi0Cc, which leads to a degree of error around 10 - 15%. The loss of energy within the PM motor due to temperature rise is attributable to factors other than the decrease of magnetic flux: pulsation losses on the magnet surface arising from the windings are an important contributory factor. Conclusions
I . The transfer function between the cutting force and the corresponding motor current is affected by non-linearities in the electro-magnetic system of the motor and the tribolcgical system of the feed transmission. The results obtained show that less torque is needed to perform the feed motion at higher temperatures of feed screw, guideways etc. At the Same time, it has been observed that the amount of input current to the feed motor needed to maintain a certain level of torque, increases due to temperature rise, fig. 4a and fig. 4b 2. In the case of the CNC lathe investigated, the total error in the estimation of motor current, attributable to temperature rise, has been found to vary from -4% to +9%. fig.4c. 3.The temperature-dependent characteristics of PM ked motors are governed by the propenies of the magnetic materials employed. An error of I % - 15% in current values can be expected during tool life monitoring. The results in fig. 4a and 4b, which show that the amount of increase in motor current values needed to maintain one and the same torque is in the range of 15-20%, confirm that a major part of the increase in current is attributable to magnetic losses'
(I)
(2) (3) (4)
*__-___-_
[Motor current [A] !
(5)
/
488.
(6)
Fig.8 l%e long-term cutting tests presented in fig. 6 and fig. 7 show that tool life limit can be successfully monitored even though there may be a substantial temperature rise in the motor due to continuous operation of the machine tool (under normal cutting conditions) over a longer period of time, which is actually the situation encountered in the industry. In this case,error caused by the non-linear behaviour of the motor and the transmission is estimated to be below 2%. In fig. 6, xhree different curves corresponding to three different
454
Bollinger, J.G.,Stute, G.,Van Brussel, H., 1980, Digital Control and Feed Drives State-of-the-Art and New Development, Annals of the CIRP, 2912, 497-506. Bryan, J., 1990, International Status of Thermal Error Research. Annals of the CIRP, 3912. 635-656. Koren. Y., 1984, Torque and Speed Control of DC servo-motors for robots, Annals of the CIRP, 33/1, 239-242. Mannan, M.A., Broms, S., 1989, Monitoring and Adaptive Control of Cutting Process by Means of Motor Power and Current Measurements, Annals of the CIRP, 3811. 347-350. Micheletti, G.F., Koenig, W., Victor, H.R.. 1976. In-Process tool wear Sensors for Cutting Operations, Annals of the CIRP, 2512, 483Tlusty, J., Andrews, G.C., 1983, A Critical Review of Sensors for Unmanned machining, Annals of the CIRP, 3212, 563-572.