Application of FMEA to assessing the quality of measurement modules

1. Introduction

The knowledge about the application of a product quality control station when operations are robotized is limited. It is related to technical problems which occur at the stage of implementation of control systems in serial and mass production. Large industrial concerns are not willing to reveal the technical details of quality control organization, so as not to provide competitors with useful tips in the struggle for a customer. The solutions, which are known and have been published, are typical and as a rule just being phased out from technological lines [1, 3].

Numerous examples of the application of various inspecting and inspecting-sorting devices, which objectively control quality parameters, can be found after a careful analysis of the most recent publications this paper, however, focuses only on the inspecting-sorting devices used in mass production [2].

The application of a system to a production line depends to a great extent on factors determined by economic possibilities as well as technical conditions which must be satisfied for the smooth operation of the control process. In the case of modern control systems, a complete analysis of the process subject to quality control is required. The data collected, after being statistically elaborated and verified in accordance with the quality assumptions, provide information about the quality of the process and reasons for defects which arose in the process [3, 4].

2. Project of simulation stand arrangement – conceptual assumptions

In order to check the cam-gate system in product quality control in practice (small-sized rollers) a test stand simulating the partial operation of the inspecting-sorting automatic machine was designed. The operation of the stand was limited to practical simulation of operation of the measuring module cooperating with the control and transporting system [6].

The fundamental assumption was to compare the results of quality parameters measurements taken during operation of the inspecting – sorting device with the results obtained during measurements taken in laboratory conditions with the use of a micrometer screw.

Performance of the following tasks was included in the project [5, 6]:

– Providing the roller to the control zone,

– Measurement of quality parameters (cam-gate system),

– Classification into sorting groups (cam-gate system),

– Transferring the roller from the control zone to a position that facilitates putting the detail into the sorting device.

The above mentioned tasks were carried out in an automatic cycle.

Fig. 1 illustrates the arrangement of the simulation module of the inspecting-sorting device under consideration.

The principle of operation of the module is as follows: the roller under control (2) fixed in V-blocks (3) is transferred by means of a linear servo-motor (7) to the control zone. Upon taking measurements with the CCD camera (4) using the source of light (1), the detail was classified and passed through to the sorting device after a previous shift to the initial position. The source of light (1) together with the CCD camera (4) were located coaxially in the body frame (6) [7, 9].

Fig. 1Scheme of the arrangement of the simulation module

Fig. 2The results of experiment for CCD camera (dashed line) and micrometer (solid line)

3. Analysis of the possibilities of applying the cam-gate system

A series of check measurements was performed in order to analyze the possibility of applying the cam-gate system in the measuring module of the inspecting-sorting automatic machine.

The measurements included checking the dimensions of sample diameters (rollers $Ø=$10.5$\pm$0.1 [mm], in the amount of $n=$30 pieces) by means of the cam-gate control system mounted on the simulation stand.

4. Advantages and disadvantages of the modules employed in inspecting-sorting automatic machines

In order to perform FMEA, the basic failure modes following from the technique of making measurements and constructional solutions of the inspecting-sorting device were described. Depending on the technique of measurement, efficiency and the material of which the element being inspected is made, inspecting-sorting automatic machines may have various typical constructional solutions [8].

The structure of characteristic modules described in affects the final result of measurements and operational properties of the in inspecting-sorting device.

A thorough analysis makes it possible to find a “weak” link in the structure and allows remedy actions not defining, however, how to perform them. While making a comparison, advantages and disadvantages of the systems employed in inspecting systems with the use of the CCD camera and other sensors (pneumatic ones) were specified.

In the linear method of product flow, products are transported on a conveyor belt, in front of which there are cameras and sources of light. The method used in the machine building industry and glass making industry is characterized by the following advantages: high speed of transport; good insight into the product being inspected; possibilities of product concentration.

Besides the advantages, the method has also got drawbacks which include: possible jamming of products; noticeable vibrations of the conveyor belt; slide of the transport system; safety hazards at large linear speeds; the device is large and occupies a lot of space.

Another method is the carousel method, which consists in placing products on the circumference of a circle, which while being moved in front of the inspecting stations are rotated at a constant speed along the axis of symmetry. The advantages of the method include: little space required; secure fixture of the product; defined sequence of products; accurately set position; relatively low vibrations of the mechanical system; favorable position of the element being inspected in the case of undesired gravity centre.

The disadvantages include: slow operation; small concentration of products; danger of scratching the base of products being inspected.

A CCD camera is not always necessary to perform the analysis and in some cases its use is even not possible. It follows from objective reasons, most frequently economic ones, and from the organization of quality control procedures in an enterprise. In the event of statistical quality control (where there is no need to check 100 % of products) inspecting all quality features in a continuous manner does not seem optimal [9].

The use of other sensors, e.g., pneumatic ones, for inspecting quality parameters can suffice, however it limits the capacity of inspection. In such a case there is no technical possibility of inspecting such defects as discolorations and insertions. The advantages offered by pneumatic sensors employed in the measuring system of an inspecting-sorting automatic machine include: simple structure; secure operation; spark safety; small sensitivity to vibrations of the external system; possibility of application in various conditions of the environment; relatively low cost.

The main disadvantages of pneumatic sensors employed in the measuring system of an inspecting-sorting automatic machine include: low speed of operation; relatively not too flexible application; no possibility of measuring inclusions and scratches of the surface; necessity of supplying air of strictly defined parameters to the system.

The use of a CCD camera in the measuring system of an inspecting-sorting automatic machine offers the following advantages: short time required for identification of quality parameters; large flexibility of the inspecting system; relatively large efficiency of inspection (up to 28 pieces per second); possibility of identifying inclusions and discolorations; high accuracy of measurement (up to do 0.001 [mm]); compatibility of the system with other units of the controlling and technological systems.

The method, however, has the following drawbacks: complicated structure of the measuring system (optics, mechanics, IT, automation); sensitivity of the system to external vibrations; sensitivity to changeable conditions of the environment (pollution, oil emulsions, reflections); relatively high cost (software and the CCD camera).

5. FMEA for selected modules

Table 1 and Table 3 illustrates FMEA for a module of the inspecting-sorting automatic machine, in which the CCD camera was used for measurements (Fig. 3). The potential failure mode (defect) of the system, i.e., incorrect readings of parameters, affects directly manufacturing of products which are defective in terms of quality. It follows from Table 3 that the following causes affect most significantly the readings of quality parameters:

– Incorrect description of technical conditions for making the measurements (duration of measurement, accuracy, lighting, efficiency),

– Delayed reaction of the measuring system to changes in the ambient temperature and in the temperature of the element being inspected,

– Faulty construction of the system connecting the optical elements and the source of light.

Table 2 illustrates FMEA for the module of the inspecting-sorting automatic machine, in which pneumatic sensors of MHAR type were employed for measurements (Fig. 4).

Fig. 3Measurement scheme – CCD camera

Fig. 4Measurement scheme – pneumatic sensor (Mahr)

The causes of failure modes indicate that the following factors affect the correct readings of quality parameters:

– Untightness of the system due to fractured tubes or couplings,

– Wear of mechanical elements which keep the prescribed pressure (membranes, springs),

– Vibrations of the mechanical system due to a faulty construction of elements.

6. Discussion of results

Table 2 illustrates FMEA for a module of the inspecting-sorting automatic machine, in which the CCD camera (Fig. 1) was used for measurements [4].

The potential failure mode (defect) of the system, i.e., incorrect readings of parameters affects directly manufacturing of products which are defective in terms of quality. It follows from Table 2 that the following causes affect most significantly the readings of quality parameters:

– Incorrect description of technical conditions for making the measurements (duration of measurement, accuracy, lighting, efficiency);

– Delayed reaction of the measuring system to changes in the ambient temperature and the temperature of the element being inspected;

– Faulty construction of the system connecting the optical elements and the source of light.

7. Conclusions

The analysis of the failure modes of inspecting-measuring systems used in inspecting-sorting devices leads to the following conclusions:

– The number of factors affecting directly the quality of operation of the CCD based measuring system is much larger than that in which pneumatic sensors are employ;

– The number of factors affecting directly the quality of operation of the CCD based measuring system is much larger than that in which pneumatic sensors are employed;

– The number of factors affecting directly the quality of a system is not an indicator that determines the possibilities of applying a given system. The main and decisive factors are flexibility, accuracy and efficiency of the measuring system applied in inspecting-sorting automatic machines.