Development of (partially) automatic test stands

In the manufacturing of almost all goods, it is necessary to check their function and appearance during the production stage to ensure quality. Products with excessive defects and high return rates can result in immense costs for the producer and subsequently damage the reputation of the product and the producer among customers.

A quality assurance plan is usually drawn up during the development stage to specify which parts or components need to be tested and to which extent, so that as few defective end products as possible reach the customer. While more economical random testing is sufficient for the majority of products, safety-relevant parts, of which failure can lead to costly damage to property or even personal injury, are usually subjected to full testing (100% inspection). Full testing is, especially in medical device manufacturing, often necessary.

With increasing production quantities, a large number of tests quickly becomes necessary. This is even more the case when full testing is required.

Should these tests be performed by employees using standard measuring equipment, it will result in a large investment of time and manpower, resulting in increased manufacturing costs. These can significantly harm the productivity of any manufacturing operation. In addition, staff may need to be specially trained for complex testing, or additional specialists may have to be hired for this purpose.

For these reasons, it often makes sense to partially automate component testing. The full- or semi-automatic test stands used for this purpose make product testing faster and more efficient. Test stands allow large quantities of parts to be tested in a short time and with few employees. Additionally, test equipment operating errors can be avoided, and complex tests can be carried out by employees with low technical know-how (for example, in production in low-income countries).

Our following example uses an already developed, semi-automatic flow test stand for medical differential pressure flow sensors. We explain what needs to be taken into account when developing automatic test equipment, what information is required for the development, and how best to proceed.

What information is needed?

When beginning the planning of test stands, you need to gather some information in advance. Decisions on the type of test stand and the degree of automation must also be made or, if necessary, discussed with the responsible persons in your production and management departments.

Scope of the test

The most important thing before development begins is to know what is to be tested and how the tests are to be carried out.

Our example: Different flows are generated and passed through the flow sensor. The differential pressure is determined for each flow in order to display the function of the flow in a diagram. In addition, the sensors are leak-tested.

Test requirements (benchmark)

The minimum requirement (value) above which the test is considered to be passed (PASS) is determined by means of tests, a tolerance calculation, or based on a pre-defined standard.

Our example: A test is considered passed if the generated differential pressure curve is within a defined tolerance to the idealized TARGET curve.

Test result format

In general, a component test in production should show whether a part or assembly (UUT, Unit Under Test) meets the requirements (PASS) or not (FAIL). Defective UUTs are either scrapped or reworked. In special cases, for example, if the product is sold in different quantities, a qualitative test result (measured value) may also be required. This allows UUTs to be sorted according to their quality and processed further accordingly.

Our example: Software automatically compares the generated curve with the TARGET curve and displays the result (PASS/FAIL) on a screen.

Traceability

For products with safety relevance and products in the medical field, it is mandatory that products can be traced back deep into production. This is done by means of serial numbers to identify further products from the same batch in the event of a fault and thus prevent further damage. When traceability is required, the test results must be stored in such a way that they can also be assigned to a serial number. The degree of traceability of a product is usually defined in the quality management plan.

Our example: Tracing the production of flow sensors back to the batch is required. The batch corresponds to one day’s production. For this reason, all test results of each day are stored and can thus be assigned to the batch later.

Test stand automation

Considering the test quantity and the available resources, which degree of automation of the test stand should be decided. Should the machine test the UUTs completely autonomously or together with a worker? The decisive factors for or against a high degree of automation are shown in the following table:

Our example: The test stand for the flow sensors was developed to make ongoing production faster and more efficient. The choice fell on a semi-automated test stand as the resources (especially development time) were scarce and the entire production process was only partially automated. This significantly simplified and accelerated the testing process. To cope with increasing production rates later, several copies of the test stand were put into operation.

What should be given special attention?

In addition to the usual considerations that come with any development, some areas of test stands should receive special attention.

Composition of the project team

Only very few test stands are purely mechanical and therefore in most cases at least the electronics and often software teams are involved. For this reason, all required competencies should be clarified in advance and the relevant people included in the project team.

Our example: Only standard electronic components were installed in the flow test stand, so no special electronics skills were required. The project team consisted of only one specialist from the areas of mechanics and software.

Human-machine interface (HMI)

It is important to clarify the interface between the human and the test stand, especially with semi-automated test stands. This includes how the UUT is connected to the test stand, how the test is started and runs, as well as how the result is communicated to the worker. A good solution can significantly reduce turnaround time or result in only one operator having to be assigned to two test stands.

Our example: In the test stand under consideration, the existing production was analyzed, and care was taken to ensure that as few steps as possible had to be performed by the operators. An attempt was also made to make the handling ergonomically comfortable.

Required components

Specific test equipment is often required for test stands to automate complex tests. The availability and price of the test stands must be clarified early in the project to reduce the risk of unpleasant surprises in terms of production costs or exceeding project schedules. In the case of core components that are difficult to procure, it should be considered whether it is worthwhile to purchase spare parts in advance. This means that if these components fail, the test stand can be repaired quickly and put back into operation. The use of common standard components makes this obsolete.

Our example: Since several units of the test stand are used in parallel and a defect, therefore, does not last the entire production, no spare parts were ordered. In addition, only electronic standard parts were installed, which can be easily replaced.

Arrangement of components

The arrangement of the components can sometimes be very decisive, especially in the case of space constraints or if the test stand is to be integrated into an existing production line. That is why it is important to know these boundary conditions well. This means that no adjustments need to be made to the arrangement later as there is less space than expected or the production direction runs from right to left, contrary to the assumption.

Our example: The production direction and the space available for flow sensor production were discussed with the production manager and the test stand was subsequently realized within these specifications.

How should we proceed? Which (development) tools are helpful?

To start a project on the colloquial green field, it is highly important to get an overview of the task ahead. A list of requirements usually exists, but the first step towards implementation is often difficult. Suitable development tools can help to simplify this.

Plan / Flowchart

It makes sense to first think quite abstractly about how a test procedure should run, especially for complex test stands. If the test stand consists of several autonomous subcomponents, information flows among them can also be mapped.

System architecture / black box

Based on this abstract concept, the system architecture can be developed. It shows the information and material flows within the test stand, as well as between the test stand and the environment.

In many respects, the development of an automatic testing machine is not significantly different from the common development of products and devices. At the beginning of a project, however, the initial situation is often much more open. While in usual development projects many defaults (design, function mode, …) usually strongly limit the range of solutions, with test automation only a few abstract requirements (what is to be tested, space requirements, number of units, …) are formulated. As a result, there is an abundance of possible solutions and an almost unlimited number of implementation variants.

For this reason, it is extremely important to familiarize yourself with the specifications and requirements at the beginning of the project in order to design the most suitable concept for the application. This enables many potential difficulties to be circumvented or mitigated at an early stage of the project.