- Email: [email protected]
Mechatronical equipment for measuring form defect by miniature shafts
Mechatronics, Vol. 3, No. 2, pp. 139-147, 1993
0957-4158/93 $6.00+0.00 Pergamon Press Ltd
Printed in Great Britain
MECHATRONICAL EQUIPMENT FOR MEASURING FORM DEFECT BY MINIATURE SHAFTS A. HALMAI,A. HUBA,L. MOLNAR Technical University Budapest, Faculty of Mech. Eng., Dept. of Precision Mechanics & Optics H-1521 Budapest, Egry J. u. 1. Abstract - This paper is concerned with problem s of the design and technology of a mechatronic measuring equipment.
This equipment is able to measure the distortion of diameter of a micromotor commutator. Actual diameters of ~ l to 1.3 mm are measured in a range of 400 pm with + 1 ~un sensitivity. The main information about the distortion of each workpiece (eccentricity maximum) is stored in a computer. The same computer controls the measuring process.
1. I N T R O D U C T I O N The drawing of the micromotor commutator to be measured is shown in Fig. 1. It's important for the production technology to be well informed about the eccentricity of the commutator related to the ~ 1 mm steel shaft. The commutator consists of 6 segments, and it is fitted onto the shaft with die-press technology. We had to develop a computer controlled instrument, for measuring the placing and deformity of commutator segments onto the steel shaft. Because of the miniature sizes and the sensitivity of the workpieces, only a non-contact measuring principle can fulfill the requirements. Angles with 1°, distances with 1 ~un sensitivity had to be measured in 2 s on each workpiece. This measuring problem couldn't solved with traditional equipment. But a really mechatronial measuring instrument inseparable uniting mechanical, electronical and optoelectronical subsystems and controlled by a computer could fulfill the high requirements.
2. T H E M E A S U R I N G P R I N C I P L E The measuring principle selected, for reasons of practicability, is the optoelectronic method. The essence of this is: if between the limited sizes of light source and limited sizes of detector an optical nontransparent object is moving (or a part of this, Fig. 2), the current on the surface of detector is changed as shown by the diagram of Fig. 3. The most important feature of this curve is it has an inflection. It's easily seen, when applying a parallel beam and adequate geometrical sizes the curve of Fig. 3 may be linearized to a considerable degree, so higher precision of measurement can be achieved. The outline construction of the optoelectronic measuring head developed is shown on Fig. 4. Parts of the measuring head are as follows: LD is the light source with a collimator, B1 and B2 are the rectangular windows in front of D~ and DE detectors. The part of the workpiece to be measured is placed between the source and detectors; and the measuring branch is the commutator and the reference branch are the basic steel-shafts. A laser diode served as a light source, with the maximum radiation of 785 nm. It's power was set to 3 mW. The collimator was a microscope objective with the focal length f = 8.16 mm. The distance between measuring and reference optical axis was 5.5 mm. The sizes of the masking windows in front of the PIN diodes 139
140
A. HALMAI
et al.
I I
Commutator Segments (6 pcs,)
-III lil III
I
I
I
. ']
Plastic
I
~1g6 1¢3
rz
Shaft
Fig. 1.
32
Detector Current
Detector Light Source
X IlL
Fig. 2.
Fig. 3.
×
Measuring form defect by miniature shafts
L~ 'IM~~ ]'-B1 i
D1 ~
141
A
B t
°oA
LD _
A
B2 T
21
_,,l ]
B2
D2
I. B
T
02
Fig. 4.
were 0.4 * 0.8 mm. The open space between the source and detectors was 40 mm, which allows a safe insertion or exchange of workpieces. For the stability the transmitted and receiver unit were placed inside a casing made of a single aluminium block. In the interest to extend the life of the laser diode and stabilise the measuring system, the temperature of the complete measuring head was controlled to 20 °C. The rotating of the workpiece is carried out by a stepping motor. The motor is 200 steps/revolution, which is used in half-step motion, so one step amounts to 0.9 ° . The axis of rotation, and the axis of the workpiece can coincide only with 10 grn accuracy. For compensation serves the reference beam. The simplified relations are shown in Fig. 5. Here Or is the rotation centre (the same as the axis of the stepping motor) R F is the radius of rotation, A is the centre of the workpiece shaft, B is the imaginary centerpoint of the commutator, rk is the imaginary radius of the commutator. During the measurement the detector detects the projection of this imaginary radius. In case of the reference beam: eref = RF sin (ao + 0 ) + r ,
where a0 is the starting angle of the rotation (this is constant). In case of the measuring signal: Pmeas = RF sin (fl0 + 0)+ rk, where fl0 is an angle, depending on placement in the collet of the workpiece (it is variable).
142
A. HALMAI et al.
130 Of
Fig. 5.
We get eccentricity by making up the unequality:
P = ( r k - r ) + e sin (70 + ~), where the interpretation for angle 70 is Fig. 5. The equipment shows not only the maximum value of the eccentricity, but also the deformations of individual segments. This way we obtain important information: the displacement function, generated the brushes as elastic systems, while sliding on the surface of commutator segments. The commutator-brush connection is suitable, if during the rotation the brushes never move away from the commutator, moreover, the pressure force between commutator and brushes doesn't decreased under a certain value.
3. THE FUNCTIONAL ELEMENTS OF THE EQUIPMENT The more important functional elements are shown on Fig. 6. The optoelectronic measuring head 1. we discussed earlier. The control unit 2. is housing the head providing constant temperature i.e. a Peltier-element. In the analogue unit 3. can be found the amplifiers of the photo diodes, and the driver and light stabilizing circuits of the laser diode. The analogue signal is converted by a 8-bit A/D converter to digital signal sequence, controlled by the 5. computer. This computer controls also the 6. stepping motor, rotating the 13.
Measuring form defect by miniature shafts
143
12
13
I 1
7
I 6
H
]
4
9 I
I 5H
I
10
8 Fig. 6.
workpiece, by the 7. holding collet fastened on the rotating shaft of the stepping motor. The 8. keyboard belongs to the computer, and also the 9. display and 10. printer. The 11. power supply gives the necessary stabilized voltages for the units, and the 12. is a ventilator with a brushless motor, that provides the thermal balance of the equipment.
4. PROBLEMS O F MEASURING The very high requirements of precision necessitate the efficient analysis of the measuring arrangement's geometry. It's clear that the optoelectronic scanning device works as an analogue optogate and the workpiece produces shadow-patterns on the receptor diodes. These shadows pass the information about the form of the commutators as function of the angle. It's obvious that only is a special case the radius and the shadow shape correspond. But because the light beam behaves similarly to the brushes in the motor the shadow patterns hold more important information for the technology than a pure radius measuring. Dispensing with long demonstrations we've shown some relations for the error calculation. This examination was important for production too, because necessary tolerances could be deduced from it. There are four types of error that can influence the measured results: a Deflection between rotation axis and the axis of the workpiece. This error occurs in connection with angle deviation, but they have to be handled separately because of different measuring techniques (Fig. 5). b Position angle of the workpiece (Fig. 5). c Size deviations of the ~ 1 mm shafts (Fig. 7). d Angle deviation between rotation and workpiece axis (Fig. 8).
144
A. HALMAI et al.
Of
94
I
Fig. 7. a) Deflection between rotation axis and the axis of the workpiece. Due to the technology the two axis do not coincide exactly. The relations are shown of the Fig. 5: Or is the rotation axis and Rr the radius of rotation, A is the axis of the workpiece and r the radius of the shaft of the workpiece. Rotating the workpiece by the stepping motor, a sinus-shaped error results from the deflection of the two axis. This error may be eliminated by application of a reference channel. The important condition for elimination is the following: the steepness and linearity of both (measuring and reference) channels have to correspond with each other. b) A further measuring error is caused by the initial angle position of the workpiece. This is also shown in Fig. 5. The radius of the commutator to be measured is rk, the imaginary centre is point B. Between the centre (A) of the workpiece and B the eccentricity e may be measured, with the optimum level zero. The initial angle position of the workpiece is given by the angle Y0. It's visible, that by application of a reference channel, the workpiece can be precisely measured in any position. Of course, the requirements on linearity and steepness of both channels are still important. e) Measuring errors also originates from size deviations of O 1 mm shaft, Because of size tolerances of shaft - - called +AD - - the radius of rotation changes, as shown in Fig. 7. (The shaft is pressed in a prismatic holding collect by force F.) This error can also be eliminated by application of a reference channel. d) The angle deviation between rotational axis and the workpiece axis also causes a measuring error. This is systematic error, so it may be eliminated during measurement. To demonstrate this e r r o r - - instead of presenting it in a plane - - it is better to make a three-dimensional figure, as shown in Fig. 8. Because of the angle deviation between the axis, there is a 6 angle difference between the maxi-
Measuring form defect by miniature shafts
145
mums, detected by photo elements. This difference can't be eliminated by application of a reference channel, only by recording and storing in a computer the "calibration curve" of a perfect workpiece (or practically faultless). In this case the (controller) software is able to correct all the measurement results of this calibration curve. After correcting the computer can display and store the correct results of a workpiece, and can carry out statistical analysis for quality control.
Measuring Beam
I
1,
f I !
sured Shaft
4
8
!
Measuring Signal Rotating Axis !
Reference Signal
\
/ \
/ Fig. 8.
5. MEASURING POSSIBILITIES AND RESULTS The control computer is an IBM compatible one, it is compatible with an AT or others with higher capacity, such as 386 or486 and with a monitor type EGA(or VGA, SVGA). The measuring program directs the complete process including the data processing and storage of results. A part of this program directs the signal sensing and prepares the graphic output of the measured values from the analysis. The output can be given in two forms: in polar co-ordinates and in rectangular co-ordinates. A further part of the software serves the serial control of sizes, for this it stores only one piece of information about a workpiece as well as the knocking of commutator head. It means the difference between maximal and minimal sizes. Figs 9 and 10 show finally the polar and rectangular diagrams of a commutator with fairly low degree of knocking. Figs 11 and 12 show a workpiece with bad results. The mechatronical measuring instrument described in this paper makes the control of a production process of micromotor commutators possible. By the help of this analysis indispensable changes may be affected for the quality of products.
146
A. HALMAI
et al.
Rectangular Diagram r
750
[p~]
-
700 -
"
-
V
"~
',/
",/
~00
i500
dO
120
180
360
240
Fig. 9.
Polar Diagram o*
<-
....6o.
/",~:........ ,
I' ,
,/ )
"
/ I
'
:
"
"--
.....,
)~.--"
'L.
--" ~,
.::c::,
",~
I
?\-:q "-. i ..i" :;,/. .L" ""
~40 °
'\
" ~
"~'4----~
"~
"'--..~"-~. i j'f~:-"/ "--~ -~"~ _......
:j.BOo Fig. 10.
/ ~
"""
......1.20o
~
360
[o1
147
Measuring form defect by miniature shafts
RectangularDiagram [P M
21
~'50
I
00-
650-
600
-
5500
~b
I~o
i~o
2~o
'3 0 0
3 6"0"
Fig. 11.
Polar Diagram o*
, ,,'60
""-../ /~
,,'
t
~, .......
7¢
.----'-Y'-i -..
:
....."i,~ ......
°
"%.
...-'".i I
,
i'x,,.,\: ~!~ ..... '~:i...... '.-,: '',.. )" ........ "',,.,
120 o
:180 o
Fig. 12.
~'J
0957-4158/93 $6.00+0.00 Pergamon Press Ltd
Printed in Great Britain
MECHATRONICAL EQUIPMENT FOR MEASURING FORM DEFECT BY MINIATURE SHAFTS A. HALMAI,A. HUBA,L. MOLNAR Technical University Budapest, Faculty of Mech. Eng., Dept. of Precision Mechanics & Optics H-1521 Budapest, Egry J. u. 1. Abstract - This paper is concerned with problem s of the design and technology of a mechatronic measuring equipment.
This equipment is able to measure the distortion of diameter of a micromotor commutator. Actual diameters of ~ l to 1.3 mm are measured in a range of 400 pm with + 1 ~un sensitivity. The main information about the distortion of each workpiece (eccentricity maximum) is stored in a computer. The same computer controls the measuring process.
1. I N T R O D U C T I O N The drawing of the micromotor commutator to be measured is shown in Fig. 1. It's important for the production technology to be well informed about the eccentricity of the commutator related to the ~ 1 mm steel shaft. The commutator consists of 6 segments, and it is fitted onto the shaft with die-press technology. We had to develop a computer controlled instrument, for measuring the placing and deformity of commutator segments onto the steel shaft. Because of the miniature sizes and the sensitivity of the workpieces, only a non-contact measuring principle can fulfill the requirements. Angles with 1°, distances with 1 ~un sensitivity had to be measured in 2 s on each workpiece. This measuring problem couldn't solved with traditional equipment. But a really mechatronial measuring instrument inseparable uniting mechanical, electronical and optoelectronical subsystems and controlled by a computer could fulfill the high requirements.
2. T H E M E A S U R I N G P R I N C I P L E The measuring principle selected, for reasons of practicability, is the optoelectronic method. The essence of this is: if between the limited sizes of light source and limited sizes of detector an optical nontransparent object is moving (or a part of this, Fig. 2), the current on the surface of detector is changed as shown by the diagram of Fig. 3. The most important feature of this curve is it has an inflection. It's easily seen, when applying a parallel beam and adequate geometrical sizes the curve of Fig. 3 may be linearized to a considerable degree, so higher precision of measurement can be achieved. The outline construction of the optoelectronic measuring head developed is shown on Fig. 4. Parts of the measuring head are as follows: LD is the light source with a collimator, B1 and B2 are the rectangular windows in front of D~ and DE detectors. The part of the workpiece to be measured is placed between the source and detectors; and the measuring branch is the commutator and the reference branch are the basic steel-shafts. A laser diode served as a light source, with the maximum radiation of 785 nm. It's power was set to 3 mW. The collimator was a microscope objective with the focal length f = 8.16 mm. The distance between measuring and reference optical axis was 5.5 mm. The sizes of the masking windows in front of the PIN diodes 139
140
A. HALMAI
et al.
I I
Commutator Segments (6 pcs,)
-III lil III
I
I
I
. ']
Plastic
I
~1g6 1¢3
rz
Shaft
Fig. 1.
32
Detector Current
Detector Light Source
X IlL
Fig. 2.
Fig. 3.
×
Measuring form defect by miniature shafts
L~ 'IM~~ ]'-B1 i
D1 ~
141
A
B t
°oA
LD _
A
B2 T
21
_,,l ]
B2
D2
I. B
T
02
Fig. 4.
were 0.4 * 0.8 mm. The open space between the source and detectors was 40 mm, which allows a safe insertion or exchange of workpieces. For the stability the transmitted and receiver unit were placed inside a casing made of a single aluminium block. In the interest to extend the life of the laser diode and stabilise the measuring system, the temperature of the complete measuring head was controlled to 20 °C. The rotating of the workpiece is carried out by a stepping motor. The motor is 200 steps/revolution, which is used in half-step motion, so one step amounts to 0.9 ° . The axis of rotation, and the axis of the workpiece can coincide only with 10 grn accuracy. For compensation serves the reference beam. The simplified relations are shown in Fig. 5. Here Or is the rotation centre (the same as the axis of the stepping motor) R F is the radius of rotation, A is the centre of the workpiece shaft, B is the imaginary centerpoint of the commutator, rk is the imaginary radius of the commutator. During the measurement the detector detects the projection of this imaginary radius. In case of the reference beam: eref = RF sin (ao + 0 ) + r ,
where a0 is the starting angle of the rotation (this is constant). In case of the measuring signal: Pmeas = RF sin (fl0 + 0)+ rk, where fl0 is an angle, depending on placement in the collet of the workpiece (it is variable).
142
A. HALMAI et al.
130 Of
Fig. 5.
We get eccentricity by making up the unequality:
P = ( r k - r ) + e sin (70 + ~), where the interpretation for angle 70 is Fig. 5. The equipment shows not only the maximum value of the eccentricity, but also the deformations of individual segments. This way we obtain important information: the displacement function, generated the brushes as elastic systems, while sliding on the surface of commutator segments. The commutator-brush connection is suitable, if during the rotation the brushes never move away from the commutator, moreover, the pressure force between commutator and brushes doesn't decreased under a certain value.
3. THE FUNCTIONAL ELEMENTS OF THE EQUIPMENT The more important functional elements are shown on Fig. 6. The optoelectronic measuring head 1. we discussed earlier. The control unit 2. is housing the head providing constant temperature i.e. a Peltier-element. In the analogue unit 3. can be found the amplifiers of the photo diodes, and the driver and light stabilizing circuits of the laser diode. The analogue signal is converted by a 8-bit A/D converter to digital signal sequence, controlled by the 5. computer. This computer controls also the 6. stepping motor, rotating the 13.
Measuring form defect by miniature shafts
143
12
13
I 1
7
I 6
H
]
4
9 I
I 5H
I
10
8 Fig. 6.
workpiece, by the 7. holding collet fastened on the rotating shaft of the stepping motor. The 8. keyboard belongs to the computer, and also the 9. display and 10. printer. The 11. power supply gives the necessary stabilized voltages for the units, and the 12. is a ventilator with a brushless motor, that provides the thermal balance of the equipment.
4. PROBLEMS O F MEASURING The very high requirements of precision necessitate the efficient analysis of the measuring arrangement's geometry. It's clear that the optoelectronic scanning device works as an analogue optogate and the workpiece produces shadow-patterns on the receptor diodes. These shadows pass the information about the form of the commutators as function of the angle. It's obvious that only is a special case the radius and the shadow shape correspond. But because the light beam behaves similarly to the brushes in the motor the shadow patterns hold more important information for the technology than a pure radius measuring. Dispensing with long demonstrations we've shown some relations for the error calculation. This examination was important for production too, because necessary tolerances could be deduced from it. There are four types of error that can influence the measured results: a Deflection between rotation axis and the axis of the workpiece. This error occurs in connection with angle deviation, but they have to be handled separately because of different measuring techniques (Fig. 5). b Position angle of the workpiece (Fig. 5). c Size deviations of the ~ 1 mm shafts (Fig. 7). d Angle deviation between rotation and workpiece axis (Fig. 8).
144
A. HALMAI et al.
Of
94
I
Fig. 7. a) Deflection between rotation axis and the axis of the workpiece. Due to the technology the two axis do not coincide exactly. The relations are shown of the Fig. 5: Or is the rotation axis and Rr the radius of rotation, A is the axis of the workpiece and r the radius of the shaft of the workpiece. Rotating the workpiece by the stepping motor, a sinus-shaped error results from the deflection of the two axis. This error may be eliminated by application of a reference channel. The important condition for elimination is the following: the steepness and linearity of both (measuring and reference) channels have to correspond with each other. b) A further measuring error is caused by the initial angle position of the workpiece. This is also shown in Fig. 5. The radius of the commutator to be measured is rk, the imaginary centre is point B. Between the centre (A) of the workpiece and B the eccentricity e may be measured, with the optimum level zero. The initial angle position of the workpiece is given by the angle Y0. It's visible, that by application of a reference channel, the workpiece can be precisely measured in any position. Of course, the requirements on linearity and steepness of both channels are still important. e) Measuring errors also originates from size deviations of O 1 mm shaft, Because of size tolerances of shaft - - called +AD - - the radius of rotation changes, as shown in Fig. 7. (The shaft is pressed in a prismatic holding collect by force F.) This error can also be eliminated by application of a reference channel. d) The angle deviation between rotational axis and the workpiece axis also causes a measuring error. This is systematic error, so it may be eliminated during measurement. To demonstrate this e r r o r - - instead of presenting it in a plane - - it is better to make a three-dimensional figure, as shown in Fig. 8. Because of the angle deviation between the axis, there is a 6 angle difference between the maxi-
Measuring form defect by miniature shafts
145
mums, detected by photo elements. This difference can't be eliminated by application of a reference channel, only by recording and storing in a computer the "calibration curve" of a perfect workpiece (or practically faultless). In this case the (controller) software is able to correct all the measurement results of this calibration curve. After correcting the computer can display and store the correct results of a workpiece, and can carry out statistical analysis for quality control.
Measuring Beam
I
1,
f I !
sured Shaft
4
8
!
Measuring Signal Rotating Axis !
Reference Signal
\
/ \
/ Fig. 8.
5. MEASURING POSSIBILITIES AND RESULTS The control computer is an IBM compatible one, it is compatible with an AT or others with higher capacity, such as 386 or486 and with a monitor type EGA(or VGA, SVGA). The measuring program directs the complete process including the data processing and storage of results. A part of this program directs the signal sensing and prepares the graphic output of the measured values from the analysis. The output can be given in two forms: in polar co-ordinates and in rectangular co-ordinates. A further part of the software serves the serial control of sizes, for this it stores only one piece of information about a workpiece as well as the knocking of commutator head. It means the difference between maximal and minimal sizes. Figs 9 and 10 show finally the polar and rectangular diagrams of a commutator with fairly low degree of knocking. Figs 11 and 12 show a workpiece with bad results. The mechatronical measuring instrument described in this paper makes the control of a production process of micromotor commutators possible. By the help of this analysis indispensable changes may be affected for the quality of products.
146
A. HALMAI
et al.
Rectangular Diagram r
750
[p~]
-
700 -
"
-
V
"~
',/
",/
~00
i500
dO
120
180
360
240
Fig. 9.
Polar Diagram o*
<-
....6o.
/",~:........ ,
I' ,
,/ )
"
/ I
'
:
"
"--
.....,
)~.--"
'L.
--" ~,
.::c::,
",~
I
?\-:q "-. i ..i" :;,/. .L" ""
~40 °
'\
" ~
"~'4----~
"~
"'--..~"-~. i j'f~:-"/ "--~ -~"~ _......
:j.BOo Fig. 10.
/ ~
"""
......1.20o
~
360
[o1
147
Measuring form defect by miniature shafts
RectangularDiagram [P M
21
~'50
I
00-
650-
600
-
5500
~b
I~o
i~o
2~o
'3 0 0
3 6"0"
Fig. 11.
Polar Diagram o*
, ,,'60
""-../ /~
,,'
t
~, .......
7¢
.----'-Y'-i -..
:
....."i,~ ......
°
"%.
...-'".i I
,
i'x,,.,\: ~!~ ..... '~:i...... '.-,: '',.. )" ........ "',,.,
120 o
:180 o
Fig. 12.
~'J