Automation Design in Six Steps
Most manufacturing floors include test stations. These stations contain test equipment that exercises the UUT (unit under test) over the range of specified input parameters, and checks for expected performance. These tests not only validate completed units, they can also identify trends in quality that can in turn uncover problems in product components as well as discrepancies in the manufacturing process.
For startup lines or small-run products, testing is often done manually. The test technician assembles a set of equipment including generators, oscilloscopes, and flow meters, and proceeds through a list of test steps, filling in measured values along the way. As product lines become more mature or small-run products become large-run products, the testing time often becomes a significant portion of the per-unit cost. Automation reduces the unit testing time. Manufacturers often find that they can realize significant increases in profitability by automating their test stations.
1. Layout Diagram
The first step in automation is to create a layout diagram. Engineers can start by making a testing diagram of the current existing setup, and follow by creating a parts and equipment list. Engineers must be careful to identify any equipment that cannot be automated, in other words, does not have a control interface. Most modern equipment does have such an interface. However, any that cannot be automated must be flagged on the diagram, and a replacement ordered. This is a critical initial step, because of long equipment lead times. For critical schedules, the engineer can lease interim equipment.
2. Design the Automation Interface
An essential part of the layout diagram is the interface specification. Reducing connection complexity increases the reliability of the test setup. To reduce the complexity of the connections, a common connection interface is preferable. For example, if the common interface is GPIB, then most of the connection cables will be GPIB cables. This allows the designer to connect the cables in series or parallel, whichever best suits the physical setup of the station.
One critical part of this effort is the automation interface for the UUT. An important part of “design for manufacturability” is the test interface for the product unit. Although the units do not necessarily require a standard testing interface such as GPIB, a standard computer interface such as parallel bus, serial bus, or USB is very handy and flexible. There are many interface chips for each of these interfaces that the design engineer can easily use to connect to the UUT circuitry, particularly if there is a main processor.
A key consideration for the UUT interface is that it should be a simple, reliable connection. In an automated setup, the UUT connection time is often the lion’s share of the testing budget. The interface should be a one-step connection if possible, with hardware that can reliably withstand many reconnections. If possible, setting up a hot-swap interface is the best strategy. This avoids the delays of the power-up time. Surge protectors are a key component for protecting UUT components during a hot swap.
3. Choose the Connection Components
Modern engineers have several excellent alternatives to hardwired connections. The following is a description of each method:
- Hardwire- this makes the most reliable connection. Insulated, double-twist cables are the least susceptible to ambient interference. However, hardwiring the UUT interface can become unreliable after many connections. In addition, complex test setups cause a “cable farm” effect, which can reduce the efficiency of the station. Cables also limit the physical separation possible between the test equipment and the UUT.
- Ethernet – many modern setups bypass testing interfaces like GPIB in favor of Ethernet interfaces. The connection components are less expensive and more readily available. The interface to a computer is standard. The transmission speeds are high and the cables can be longer. New product setups are definite candidates for Ethernet connections over standard test interfaces.
- Wireless – wireless connections are an excellent way to avoid a cable farm. They also make changing the equipment very flexible. If the UUT connection can be made wireless, the per-unit testing time can drop dramatically. Engineers can turn almost any interface into a wireless interface with readily available conversion components. A combination of Ethernet and wireless technologies make a truly powerful and flexible automated system.
During this process, the engineer should remember that the best automation gains are almost always associated with the UUT. Any change in the design that shortens the UUT handling time will usually bring significant benefits.
4. Choose the Controller
Simple test setups can be driven by stand-alone automation controllers. However, the most flexible modern systems are designed with a standard personal computer at the core. This allows quick changes to the test routine, offers the ability to log and save information easily, and often affords the lowest cost. Because most computers are online, computer control also allows test technicians to monitor the stations remotely through the Internet.
5. Choose the Management Software
Some complex, unique test systems, such as those that test telephony components, have specialty test software. For more general setups, designers can take advantage of automation software designed especially for testing. Several of these packages are offered by major test equipment manufacturers, in order to support the use of their hardware products. However, engineers can realize the most powerful and flexible designs by using a standard design software such as C# or Java. To do this, engineers must choose equipment with an interface API (application programming interface) that drives the equipment. Most modern equipment does have an API. If the engineer adopts this strategy, then he or she generally finds it easy to design the API driver for the UUT itself.
Using a general software interface also allows engineers to take advantage of full product management software. This software integrates the performance requirements, the test procedure, and the compliance results to maintain a complete design and manufacturing system for the lifetime of the product.
6. Test the Automation
Before the automation setup can become a standard, it must be validated against the original manual setup. This is often done by designating a set of three or more “golden” UUTs, which have been qualified by the original, manual setup. In the next step, the technician tests the golden units with the new automated setup. Results are validated across the entire cross-section of the test procedure, allowing the automated setup to become the new standard.
By following these steps, engineers can modernize test setups, reduce per unit test times, and increase the operational profitability of their product lines.