With automotive display cluster being the main means to convey the status and information of a vehicle system and drive conditions, it is of utmost importance to ensure reliable functional testing for these cluster devices. This article describes the test coverage for a typical automotive cluster and how these tasks are done through system simulation techniques.
All vehicles are equipped with a panel to display to the driver status and information of the vehicle system and drive conditions. This cluster assembly (also known as dashboard) usually includes a speedometer, tachometer, fuel gauge, temperature gauge, odometer, and a set of telltale warning lamps. In addition, most modern vehicles are also capable of on-board diagnostics enabled by embedded systems connected through communication networks such as controller area network (CAN) and K-line.
All drivers rely on the dashboard for every vital piece of information: When the low fuel indicator lights up, it’s about time to visit the gas station; if the brake warning light remains on even though the handbrake is released, this could indicate insufficient brake fluid and it may be unsafe to use the vehicle. Therefore, a dashboard with guaranteed performance is important to provide a better and safer driving experience.
In the automotive industry, rather than using standard test commands, real loads and real stimuli are "must haves" during the product testing stages as the actual functions of a vehicle depend on these. With these loads and stimuli, clusters need to be correctly tested.
For a novice automotive electronics test engineer, this may well be very challenging as every section on a cluster calls for different sets of inputs or outputs. The door indicator on the dashboard gets input from car doors and has to respond correctly on whether to notify the driver on the status of the doors; the tachometer will be able to display the rate of rotation of the engine’s crankshaft with an engine running.
It might end up that the entire vehicle has to be placed on the production floor just for cluster testing. In manufacturing, production floor space equates to premium real estate, and cost must be well-managed, thus making the above approach unrealistic. In addition, to guarantee the accuracy for every manufacturing test performed, well-calibrated equipment must be used to achieve accurate test results. What should be the design of a practical test system for automotive cluster testing in production?
In manufacturing, most of the end-of-line functional test systems would resemble the standard Agilent TS5000 series system which houses industry-standard equipment. A typical test system will include:
1. Power supply to represent vehicle battery
2. Multimeter for voltage and current measurement
3. Frequency generator to source square waves of various frequencies
4. Switch/load box and plug-in cards (relay cards)
The instruments shown above are the main trunk of a tester in an automotive cluster testing. They are used for powering up, stimulating, switching, and performing measurements by the module.
Input and output tests
Functional testing of an electronic cluster can be briefly categorized under input and output tests. As explained in the example on the previous page, every task on the automotive clusters is performed when hard-wired input is stimulated, and no input commands are involved. Input testing for a cluster involves applying simulated conditions to the input pins and analyzing the cluster’s corresponding response; while the output testing verifies if the cluster output behavior is in accordance to the microcontroller setting in the cluster assembly.
By applying resistors of various values to the temperature or fuel input pins, one can simulate varying temperature or fuel levels to verify the behavior of temperature and fuel gauges in response to these changes. Unlike the temperature and fuel gauges, speedometer and tachometer require frequency inputs. One can verify the accuracy of the values shown by the speedometer and tachometer by applying square waves of different frequencies equivalent to specific speed or RPM to these input pins.
Most of the telltale warning lamp tests involve the use of sensors such as those at the car door and trunk lid; where the sensor input can easily be simulated by connecting the sensor pins to a ground or battery supply, depending on the cluster design.
Under these simulated conditions, the response of the cluster is often accessed electrically through voltage and current measurement of telltale lamps or the respective test points. In order to verify if the indicated values of the speedometer or tachometer corresponding to the simulated inputs are within the specified limits, it usually requires the tester to retrieve information from the cluster microcontroller via the communication network. For example, as the load value present at the fuel input changes, the ADC values of the respective microcontroller analog port should vary accordingly. By retrieving this information from the microcontroller, the tester can gauge the accuracy of the cluster reporting capability.
All functional testers must have the means to connect the cluster input pins to external stimulus such as the various load values. Normally, these will be routed via relay cards or multiplexer cards.
A common scenario observed in manufacturing line is that all loads are housed in a load box separately from the tester. Cable festoons will be running between the load box and relay cards. This increases the tendency for mistakes and debugging the tester will be extremely tedious with thousands of cables running in a system.
Flexibility is gained by mounting external loads in the tester with the correct relay switching. This is a plus point for using the Agilent TS5000 series test system with the switch/load box (the mainframe) and plug-in cards. Most of the external loads are can be mounted onto the multiple channel plug-in cards themselves, and be connected to the respective cluster pins through relay switching. This will in turn simplify the tester cablings, hence reducing the risk of human error in cable handling.
Modern test systems are usually designed to test more than one kind of cluster. This reduces the tester idle time and saves cost and production floor space. Most general purpose relay boxes in the market are usually able to accommodate a small number of relays, making it impossible to have only one relay box in the system to support all cluster variants. In addition, the current limitation for each channel on the relay box is another blocking point for general purpose relay box.
Taking into account the different requirements for different cluster variants, using a switch/load box that is capable of housing up to 21 plug-in cards may be an option, as each can be individually configured to suit the different requirements. Each slot-in card is able to with-stand different current values depending on applications, thus increasing the flexibility of the tester to accommodate more cluster family testing on a single tester.
Performing sensor input/output tests in automotive cluster is a must in dashboard functional testing to guarantee the accurate indication for the driver on the vehicle state. In addition, it is vital to trace dashboard malfunctions at an early stage before production of a vehicle begins. This significantly reduces the chances of bad clusters being installed and thus helps to increase the vehicle production yield.