Robert Bosch České Budějovice is one of the most modern and developed branches of the Robert Bosch company. The branch features a R&D department and a long-term testing department. Its main focus is developing and manufacturing components for passenger cars for many established automotive manufacturers.
One of these manufactured components, the AdBlue pump (Figure 1), is a diesel exhaust fluid standardized as ISO 22241. Car manufacturers use diesel exhaust fluid to limit the NOx concentration in diesel exhaust emissions. This helps them keep limits of the European norm Euro IV and higher. The pump must inject the AdBlue liquid under the pressure of 4,5-8,5 bar to the exhaust pipe close to the catalytic converter.
Every manufactured pump goes through multiple pressure tests before being shipped to the customer. When we started looking for a solution for automated testing, we already had a PLC that controlled the I/O valves and communicated with our manufacturing execution system (MES). Based on a positive experience on another project, we decided to use NI PXI hardware and NI software to build our test system. The main advantages of the NI PXI platform include a robust industrial form factor, the ability to add new modules to modify the measurement, and simple programming. One reason we chose the NI platform is high-speed acquisition (10 kS/s or higher), which we could not do with the current PLC system.
As mentioned, we used the PLC to control the valves and pumps. It acts as a master that sends a request to the PXI system. This request contains information about what type of test should be carried out and some configuration parameters. The PXI then carries out the test and sends back the measured data and test result that is saved into a database. Figure 2 shows the system architecture. The PXI system contains a built-in controller and two additional PXI-6281 multifunction modules. One module handles PWM generation and the other one indirectly measures the current using a shunt resistor.
During the test, we need to measure the pressure in the outlet of the AdBlue pump and control the I/O valves using PWM. Once the tester gets a TCP message from the PLC with test configuration information, it can start the test sequence.
As the pump does not have a dedicated pressure sensor, we must use an indirect method of pressure measurement. The indirect method uses measurement of the current that flows through the winding of the main magnet. On this current curve, we can identify two inflection points (Figure 3). The first one is caused by the valve starting to open. The second one is caused by the valve reaching the final position. From these inflection points, we can calculate the level of pressure in the pump and compare it with required values.
We originally wrote the calculation of pressure from the current curve using The MathWorks, Inc. MATLAB® software. We wanted to use the code we had, so we used a structure in LabVIEW called MathScript Node. We used this to import the current .m files and call them from LabVIEW on a station with no MATLAB installed.
When we started code development, we were coding everything in LabVIEW. With support from local NI representatives, we discovered an easier means of test management with TestStand software. We attended some NI trainings to learn how to use the tools. We managed to create an architecture that contains a user interface (Figure 5), test execution framework, and test modules dedicated for each step of the test. Figure 5 shows the user interface created in LabVIEW. It communicates with the TestStand engine and can read results from the currently running test sequence and control the test.
We chose TestStand as the test execution framework because it saves time during the development phase. We do not need to develop the parts of the test framework that are the same for every test, such as test steps execution, logging, and reporting. We can easily configure these features in TestStand. It is also beneficial for standardization because the test framework is the same for every test.
As mentioned, the last parts of the system are test modules. We wrote the test modules using LabVIEW. Each of the test modules is responsible for one type of measurement on the pump.
Currently we have 12 testers running on the production line (Figure 6). However, we are planning a tester for a new generation of the pump that will be more complicated. This test requires communication with a pressure sensor through the SENT protocol, which is a robust serial communication protocol commonly used for lower cost sensors in the automotive industry. With the need to test both the messages sent and the physical layer of SENT communication, we are evaluating PXI as the main test controller that could also read the data from the SENT sensors and make the test of the physical layer of the SENT communication. Using the NI PXI platform saved us time on development of the test framework and empowered us to build a reconfigurable test station. We gained valuable experience during the first PXI tester that we can use in future projects.
Robert Bosch, spol. s r.o.