ESP-EPB at PSA
The new Citroën C4 Picasso features an electric parking brake enabling automation of this function and the addition of new functions. The system is controlled by two ECUs. The first, which is linked to the system itself, controls the braking force. The second is the ESP ECU; besides its traditional functions, this ECU also manages the control logic of various functions associated with the electric parking brake. The process of validating the system as a whole was optimised using a HIL test bench for two networked ECUs. This test stand has been developed by IPG, the specialist in this area.
The 2006 Paris Motor Show revealed a new competitor in the 7-seater compact monospace segment: the Citroën C4 Picasso. In what is a hotly contested market, the C4 boasts a number of important new plus points: a spacious interior with excellent visibility and outstanding modular design. The vehicle also features several innovative driving aids such as the parking space measurement system, a Lane Departure Warning System (LDWS), the automatic parking brake and the hill-start assist. These latter two features, which are bundled under the name Easy Move, are useful add-on benefits of the Electric Parking Brake (EPB).
A complex EPB provides a new driving aid
The Easy Move delivers real driving comfort, with the parking brake engaging automatically whenever the engine is turned off. The brake is also released automatically as soon as there is sufficient engine torque to move the vehicle forward. A dashboard-mounted console allows the driver to pilot the system manually if the car has to be halted momentarily without the engine being turned off. With security in mind, the electric parking brake can only be manually released with the driver's foot on the brake or the accelerator. Drivers will also appreciate the hill-start and manoeuvering aid. Many drivers feel apprehensive about hill starts, and will be reassured to know that the parking brake remains on for up to two seconds after the brake pedal is released. This gives the driver enough time to switch from brake to accelerator without having to worry about an uncontrolled downhill movement. The uphill/downhill start function takes effect on slopes of 3% and over.
The EPB comprises an electric motor located under the left front seat, which pulls the cables attached to the device that activates the rear brake discs. Its ECU controls the amount of force to apply, depending on the slope, to immobilise the vehicle. A braking force readjusting program monitors breaking efficacy at all times, e.g. whilst loading a vehicle on a slope, or in the event of extreme temperature variations.
Test bench for networked ECUs facilitates full validation
Whilst the EPB ECU controls the electric motor, the Easy Move function is managed by the ESP ECU. This process involves a number of parameters: position of the clutch pedal (with manual gearbox) or the gear selected by the driver (automatic or automated gearboxes), the slope, the vehicle speed and the amount of braking force. Being highly complex, the system necessitated a special operating validation procedure which was carried out by one of the PSA Peugeot Citroën Fine-tuning, Validation, Integration (MPVI) units.
The MPVI units throughout France are tasked with validating electronic systems positioned outside the vehicle interior (e.g. ESP, engine controller, suspension). The tests are as much about validating the individual systems themselves as they are about verifying how they operate together. Thus the systems which have already been validated individually are verified to ensure that they will communicate perfectly between themselves once installed in the vehicle.
The MPVI unit at the Belchamp site in France was entrusted with validating the Easy Move system on the new Citroën C4 Picasso. The unit has used IPG benches for many years - a choice dictated by the quality of the HIL (Hardware in the Loop) product in terms of suspension systems - and currently operates five IPG benches. These benches are used for all validation work on separate systems, and also to validate functions involving several ECUs, these being tested as a sub-system. That is the type of validation procedure used for the Easy Move function in the Citroën C4 Picasso.
Laurent Marcuzzi, Head of the MPVI unit at the Belchamp site, showed us the test bench that ensures compliance with the schedule of specifications: "Meeting the requirements of this validation procedure involved designing a bench for networked ECUs that is capable of simulating the global vehicle environment associated with the sub-system." The IPG team therefore transformed one of the benches into a test stand for multiple ECUs. It simulates the presence of all the sensors or other ECUs, and recreates all the real-life situations drivers might encounter. This same bench is also used to validate each ECU on an individual basis prior to the validation tests of the ECU network. Laurent Marcuzzi adds: "We tried to achieve a totally representative device by incorporating as many real elements as possible." Another unit, ILDR (Suspension Systems and Vehicle Dynamics Engineering) uses this bench to conduct performance and safety tests. For example, the ILDR analyses the way in which the system behaves if a sensor fails at high speed whilst cornering, which can be done in complete safety with the help of the bench. Hence IPG has succeeded in designing a bench that is as realistic as possible. All the vehicle cables used in the final roll-out model were fitted, for example, as well as real brake callipers and the rear axle.
Substantial time savings
The CarMaker/HIL software suite installed on IPG test benches allows automation of the test scenarios and the bench operating modes. Hakim Ben Salah, who manages the test benches for MPVI Belchamp explained one of the tool's major advantages: "We have established that this IPG test tool, CarMaker/HIL for Easy Move, has delivered time savings of 1100%." And indeed, if all the tests carried out using the bench were conducted manually, it would take 160 hours per iteration. Thanks to the IPG bench, these tests can be completed in a single night. This huge time-saving obviously translates into a cost savings in terms of the large number of engineering hours otherwise required. But time-savings and reduced validation costs are not the only advantages: the fact that the tests are both rapid and automated means testing can be achieved with virtually 100% coverage at all types of software iterations. Hence the product quality is excellent. Laurent Marcuzzi : "We test software related to safety functions such as the ESP. It is essential to ensure that tests are of the highest possible quality; with that in mind, we have every confidence in the products and services supplied by our partner, IPG."
