Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)32

Improvement in Robustness ofHigh-speed Hole Position Measurement Systemfor Construction Machines

YUSUKE TANIZAWA*1

HIDEAKI YAMAMTO*2

When the boom of a construction machine is processed by an opposed horizontal boringmachine, the hole position is measured using a touch sensor. However, this measuring time is abottleneck in the enhancement of productivity and therefore the improvement of the measurement
speed is desirable. Mitsubishi Heavy Industries, Ltd. (MHI) developed a contactless hole position
measurement system and realized the enhancement of productivity through a reduction of theprocess time. This measurement system offers an optimum processing algorithm for workpieces
that have various disturbance factors at customer machining sites to realize high robustness. This
paper presents specific issues in improving the robustness of a contactless measurement systemand their solutions, as well as the automatic hole position offset function that can eliminate manualintervention.

|1. Introduction
In recent years, demand for construction machines is drastically increasing in emerging
countries, and needs for opposed horizontal boring machines used for processing construction
machine components are expanding significantly. Accordingly, MHI faces a pressing need todifferentiate its opposed horizontal boring machines by providing high-value-added functions inorder to increase their market share. We focused on the enhancement of productivity through areduction of the cycle time as one of the high-value-added functions. Construction machinecomponents as shown in Figure 1 are assembled by welding the parts that are made by sheet metal
working and casting, and then machined. The hole boring position needs to be measured before themachining process because of welding distortion and errors in mounting the support tools, and atouch sensor is typically used for this measurement. The time required for this measurement canaccount for as much as one third of the entire process time, and a reduction of measuring time issought after. MHI developed a contactless hole position measurement system using cameras andlaser length-measuring devices as a function for the reduction of measuring time.

Figure 1 Opposed horizontal boring machine and example of construction
machine components*1Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*2Chief Staff Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)33|2. Summary of contactless measurement system
This measurement system is mounted on the saddle of an opposed horizontal boring machine
equipped with two columns. Figure 2 shows the configuration of this system. The system consistsof cameras for the measurement of the hole center position and laser length-measuring devices forthe measurement of the hole height. The system measures the hole position of a workpiece on thetable from both sides.

Figure 2 Configuration of measurement system

Figure 3 compares the conventional method and the developed system. The conventional
method uses a touch sensor to measure the center of a hole and requires the sensor to contact themeasurement point at a very slow speed, and therefore approaching time is required. On the otherhand, the developed measurement system performs contactless measurement and can reduce
measuring time significantly as a result. Overviews of the hole center position measurement and thehole height measurement are described below.
(1) Hole center position measurement
For hole center position measurement, the camera is moved to the hole position to take apicture. Then the edge of the hole is detected by image processing and the hole center positionis pinpointed by calculating the circular shape.
(2) Hole height measurement
The hole height is calculated based on the measurement of the two edge points on the end
face of the cylindrical article, and the machining allowance in the Z axis direction and the error
in setup change are checked.
Actual workpiece measurement is affected by various noise factors and the edge detection
accuracy for the hole center position measurement deteriorates as a result. For this reason, it isdifficult to ensure accuracy just through standard image processing. Issues in image processing ofan actual workpiece and their solutions are described in the next section.

Figure 3 Comparison of measurement methods

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)34|3. Efforts to ensure high robustness
3.1

Characteristics of workpiece to be measured

Figure 4 shows example conditions of workpiece end faces to be measured. Although themeasurement of a workpiece is performed on a single workpiece, it has high-brightness milledsections with a high-reflectance and low-brightness cast surface sections with a low reflectance.
Accordingly, the brightness of acquired images varies significantly even when the images are taken
under the same conditions. In addition, some holes have a pattern or markings on their end face andothers have a tapered or rounded angle between the end face and the inner cylinder. As a result, toachieve the targeted result, it is necessary to correctly capture the characteristics of images andperform image processing accordingly. The specific efforts we made for this improvement ofrobustness are described below in chapters 3.2 and 3.3.

Figure 4 Example conditions of workpiece end faces to be measured

3.2

Clamp filtering

Figure 5 summarizes issues to be resolved in this section. Normally the boundary between
the end face and the inner cylinder is detected as the edge. In cases where the hole to be measured
has a tapered part and the end face is a high-brightness milled surface, however, the boundarybetween the end face and the tapered part may be partially detected as the edge, and the accuracy
deteriorates as a result.

Figure 5 Example workpiece of issue 1

To resolve this issue, a threshold process that adjusts the brightness of an end face to theoptimum value (hereinafter referred to as clamp filtering) was developed. Clamp filtering classifies
an image into three forms and then determines the optimum end face brightness adjusting method
respectively. In the case of the image example in Figure 4, "End face: Milled surface (highbrightness + tapered)" is classified as form 1, "End face: Milled surface (high brightness +scratches and patterns)" is classified as form 2, and "End face: Sheet metal work (middlebrightness)" and "End face: Cast surface (low brightness + scratches and patterns)" are classified asform 3.

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35

Figure 6 shows the details of this clamp filtering. For the image of form 1, a process that
performs thresholding of the end face so that the boundary between the end face and the tapered
part is not detected as the edge is effective. For the image of form 2, a process that performs
thresholding so that the brightness of the end face is reduced to the extent where the scratches andpatterns disappear is effective. If the same thresholding as that for form 1 is performed, theboundary between the end face and the inner cylinder cannot be detected as the edge and a changeof the circular shape results, which is inappropriate. For the image of form 3 in which thebrightness difference between the end face and the inner cylinder is small and the same
thresholding as that for form 1 or form 2 cannot determine the boundary between the end face andthe inner cylinder as the edge, a process for reducing only the brightness of a high-brightness
picture element that is manifestly believed to be noise is effective. As stated above, the edges on aworkpiece with various surface characteristics can be detected correctly by proper preprocessing.

Figure 6

3.3

Details of clamp filtering

Donut filtering

Figure 7 summarizes the issues to be resolved in this section. When the difference inbrightness between the end face and the inner cylinder is small and therefore the image is not clear,or the angle between the end face and the inner cylinder is rounded, it is necessary to detect a weakedge with a small brightness change on the boundary between the end face and the inner cylinder.
In this case, however, there is the problem where a large amount of irrelevant noise existing on theend face and the inner cylinder is detected as an edge. In particular, when a tool mark or a scratch
exists on the end face or the inner cylinder, it is susceptible to being detected as an edge andtherefore the correct circular shape cannot be detected.

Figure 7 Example workpiece of issue 2

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)36

For the reduction of such noise, we devised a process that defines a donut-shaped range inthe calculating operation and uses repeated calculation for limiting the range where edge detection
is performed (hereinafter referred to as donut filtering). This process narrows down the periphery of
the boundary between the end face and the inner cylinder where a proper edge exists. For thepurpose of noise reduction, it is favorable to make the donut width narrower, but the boundary
between the end face and the inner cylinder may fall outside the donut filter range if the width is
too narrow. Thus, repeated calculation is used to gradually make the donut narrower.
Figure 8 shows the specific processes of donut filtering. An edge is detected only in the edge
detection range defined by the donut filter, and the radius and the center position of a circle
acquired based on the detected edge are calculated. Then a donut filter narrower than the previous
one is defined based on the calculated radius and center position, and an edge is detected again. By
performing this repeated operation, an edge can be detected only from the periphery of theboundary between the end face and the inner cylinder even when the image contains a large amountof noise components.

Figure 8

Details of donut filtering|4. Automatic hole position offset function
4.1

Function overview

If the distance between the centers of holes for which the positions have been measured
deviates from the prescribed value, machining without any countermeasures will result in a
defective product. For this reason, the machining position was typically offset manually while
ensuring the machining allowance of the holes. To handle this, we developed a function that
performs automatic offset to the optimum machining position based on the hole measurement
results, the hole positions prescribed by the drawing, and the allowable distance between centers.
Thus, manual intervention can be eliminated, and as a result, the time is shortened and the risk ofhuman error is significantly reduced.
Figure 9 shows an example of the automatic hole position offset function. It presents a casewhere the measurement of a workpiece as shown in Figure 9 results in the distance between centers
AD of 6005 mm, which deviates from the prescribed value of 6 000 +/-3 mm. In this case,offsetting the machining positions of the A boss and the B boss inwards by 1 mm can result in a
distance between centers of 6003 mm, which means that the workpiece is acceptable. If multiple

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)37

permissible errors are exceeded simultaneously, however, no local optimization should beperformed. To eliminate the need for thickness deviation processing, it is desirable to make theoffset value as small as possible. The details of the automatic offset function are described in thenext section.

Figure 94.2

Automatic hole position offset function

Function details

This system calculates the hole center positions (0Xa, 0Ya), (0Xb, 0Yb), (0Xc, 0Yc), and (0Xd,0Yd) where the error of the distance between centers is within the prescribed value and the offset
amount is minimized by performing a repeated calculation using a weighting coefficient andsolving the minimization problem of the multivariable function. The following paragraphs describe
the example of a workpiece with four holes shown in Figure 9.
For the distance between centers to be managed, the error amounts of distance between
centers (ΔAB, ΔABV, ΔAD, ΔCDH, ΔCDV) are defined based on the offset hole center positions.
Then the offset amounts (Δ∅A～Δ∅D) are defined based on the measurement results and the offset
hole center positions. The evaluation amount S is the sum total of each of the squared error
amounts and squared offset amounts multiplied by the respective weighting coefficient. S allows
simultaneous adjustment of the error of the distance between centers and the offset amount.
222S = (W1 ∗ ΔAD 2 ) + (W2 ∗ ΔAB 2 ) + (W3 ∗ ΔABV ) + (W4 ∗ ΔCDH ) + (W5 ∗ ΔCDV )+ (WA ∗ ΔφA2 ) + (WB ∗ ΔφB 2 ) + (WC ∗ ΔφC 2 ) + (WD ∗ ΔφD 2 )W1～W5 : Weighting coefficient of error of distance between centers
WA～WD : Weighting coefficient of offset amount
When the minimization problem is solved with the weighting coefficient of a certain term ofS increased, the term becomes dominant in comparison to the amount of S and therefore a solutionthat reduces the term results. This time, the purpose is to obtain a solution with which the error ofthe distance between centers is within the prescribed value and the offset amount is minimized, andtherefore the weighting coefficient is set so that the offset amount becomes dominant incomparison to S. That is to say, the repeated calculation is started with the initial value of theweighting coefficient set to 0 (zero) for the error of distance between centers and 1 (one) for theoffset amount. When the minimization problem is solved with the weighting coefficients set asabove, the result indicates that it is unnecessary to move the position from the measured position
because S becomes minimized when all of ∆∅A~∆∅D are zero. In cases where there is asection that deviates from the allowable value, solving the minimization problem of S withthe weighting coefficient of the corresponding term increased from the above initial value results inthe contribution of the term to the amount of S, and therefore a solution with which the error of thedistance between centers in the section decreases and the offset amount of the hole that configures
the section changes accordingly can be obtained.

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In this way, the offset amount can be adjusted gradually so that the error of the distance
between centers becomes smaller by repeating calculation of the minimization problem of theevaluation amount S while increasing the weighting coefficient little by little, and a solution withwhich the error of the distance between centers is within the prescribed value and the offset amountis minimized can eventually be obtained.

|5. Conclusion
As described in this paper, as a method to reduce the cycle time and enhance productivity,
MHI developed a contactless hole position measurement system using cameras and laser length
measuring devices that reduces the measuring time significantly from twelve minutes in theconventional method to two minutes and thirty seconds. In addition, we resolved specific issues forhigher added value and then established a high-robustness algorithm. We also researched the needsof our customers and developed an automatic hole position offset function that can eliminate
manual intervention and developed a further high-value-added measurement system. We will work
hard to gain the top share in the global large machine tool market by not only understanding
customer needs directly from processing sites, but also by offering high-value-added systems forthe enhancement of customer productivity.