On the level: Automotive electronics

The automotive industry is the centre of much attention at the moment. A lot of this attention is focused around emissions, with legislation placing increasingly stringent requirements on manufacturers to reduce the levels of CO2 produced by their engines. But the recent scrappage scheme has seen the industry get something of a new lease of life as owners of older vehicles have taken the opportunity to replace their cars with something more efficient and certainly with more features. Features have always been an important part of the car selection process. Once, this could have been as simple as whether the car was supplied with electric windows or not. Today, potential buyers are faced with a range of options that can, depending upon the amount of cash available, boggle the mind. However, one thing remains a constant through all levels of the cars on offer – comfort. With the exception of those seeking a racing experience on the road, ride comfort is an increasing important aspect of automotive design. But providing a comfortable ride – particularly in higher end sport oriented models – can often be at the expense of handling performance. And those who are contemplating paying a lot of money for a sports type model don't want to have their car handling like a boat. But it's not just on road performance that is focusing the attention of car designers; some models – such as the Range Rover – are intended for off road use. Technology is being brought to bear in these models to make the car a lot more stable when being driven across the equivalent of a ploughed field. The particular problem here is known as 'head toss', where different undulations on either side of the car cause occupants' heads to be thrown about. The problem can be exacerbated by a combination of stiff stabiliser bars and soft springs. Ride comfort isn't, of course, a new problem and designers have come up with a range of solutions. Some semi active control systems have used complex electromechanical valves to adjust suspension performance while the vehicle is in motion. But these not only feature moving parts – and therefore introduce reliability problems – but also add weight, something manufacturers are keen to avoid. BWI Group, whose origins lie in automotive technology specialist Delphi, has been developing its MagneRide suspension control system and has recently introduced the third generation of the technology. MagneRide – introduced in the US in 2002 and in Europe in 2006 – is now specified on vehicles from around the world. One of the major claims made by BWI for MagneRide is its relative simplicity: the system features no moving parts, except for the suspension itself. Instead, the system uses dampers whose response can be changed dynamically. Sensors that monitor body and wheel motion feed information back to a control unit. This analyses the data and then issues control signals to the car's dampers. Each damper is equipped with an electromagnetic control system that can vary the properties of the damper fluid in response to the control system's commands. The result is a continuously variable damping system which is, according to BWI, simple, cost effective and reliable. Mike Zimmerman, manager of controlled suspension systems development for BWI, explained: "The system is based on a magneto rheological fluid in which iron particles are suspended. When a magnetic field is applied, the fluid reacts and the particles line up to resist flow through a small gap in the actuator. Fine control can be applied because the level of resistance varies with the flux applied." The fluid retains the same viscosity, no matter whether the electromagnetic force is applied or not. When the force is applied and the particles line up, the shear force of the fluid changes dramatically and the alignment needs to be sheared if the fluid is to move. Zimmerman says MagneRide offers designers the ability to vary the suspension settings infinitely. Surprisingly, MagneRide is a relatively small control system. It features two sizes of damper; with 36mm and 46mm diameter pistons, as well as multiple rod diameters. "The pistons are just 30mm in length," Zimmerman continued, "and the gaps through which the fluid flows vary from 0.5mm to 1.2mm, depending upon the piston size." With generation three of the MagneRide system, BWI has introduced a tapered piston, which Zimmerman said has a larger gap on the rebound side. "The compression to rebound ratio for most car dampers will offer the same ratio of force control to compression. It's essentially a mechanical system, but it needs to be controlled." Zimmerman noted that valve based systems need to include collars to open and close the valves. "With MagneRide, we can do the whole thing in a device 30mm long, which helps designers to reach suspension travel goals." Control is a relatively simple affair. "There are four position sensors," Zimmerman explained, "and an electronic control unit (ecu). The four position sensors determine wheel position and allow the relative velocities of the wheels to be calculated. Their inputs also allow the ecu to work out whether the body is heaving – moving upwards or downwards – rolling or pitching. Based on that, the ecu will send instructions to the dampers on each wheel at a rate of 1kHz." According to Zimmerman, development of the third generation ecu has concentrated on two areas: control frequency; and the drive circuitry. "The ecu has a pwm control frequency of 30kHz. We've chosen this frequency in order to take the pwm out of the audible noise range. Meanwhile, developments to the drive circuitry have allowed us to get commands to the dampers more quickly. In previous systems, we could get current to the dampers quickly, but there was a natural delay between when the current turned off and when the magnetic field dissipated. Now, we can provide quicker current off performance." Providing this higher level of capability has required a change of microcontroller in the ecu. Olivier Raynauld, manager of forward engineering controlled suspensions at BWI, said: "Previous generations of the ecu were designed around the 16bit ST10; now, the ecu takes advantage of the power of a 32bit Infineon TriCore device." Faster response at the suspension is accomplished using a two wire approach and by using two smaller coils instead of the single larger coil used in previous systems. Essentially, the two wire system allows the ecu to issue a 'defluxing' signal, which improves system response by doing away with the need to wait for the magnetic flux to dissipate naturally. Optimised control algorithms help the system provide better performance. With this dual coil architecture, the suspension system can have a higher dynamic range. BWI describes this as providing a 'softer soft' and a 'harder hard'. It also improves system response at low body velocities, where it has been harder to control movement using conventional suspension techniques. Raynauld added: "By developing the new coil technology in generation three of MagneRide, we have moved from single to dual coils. This allows the system's range to be widened and provides additional speed of control. What it means is the system can become more or less flexible more quickly." Where vehicles are equipped with the MagneRide system, they are completely reliant upon the ecu for suspension. So how does the system 'fail safe'? Zimmerman explained: "The ecu has a watchdog circuit and a back up current source. If everything fails, the ecu can output a constant current and this can be set according to the manufacturer's requirements." But the magneto rheological approach isn't limited to suspension systems, BWI has developed a system that can be applied to engine mounts. The system is being used in some Porsche models. The engine mounts used in previous Porsches have been hydraulic in nature. The problem with this approach is that peak damping is only provided at one frequency and amplitude. Raynauld explained: "It is possible to get what's known as 'power hop' at the rear wheels and passive engine mounts can be too stiff to deal with this. With the magneto rheological system, we can take care of the compromises effectively." The reason for this is that there are some instances where designers would want the engine to move and other instances where engine movement is not wanted. The system allows compression to be stiffened if necessary and rebound managed in order to improve the car's stability. The work on developing MagneRide is part of a broader investigation into how vehicle dynamics can be improved by sharing data with other onboard systems. A global chassis control system would see a supervisory computer managing an array of subsystem controllers. Potential candidates for this system would include antilock braking, electronic stability control, engine torque, roll control, dampers and electronic steering. BWI gives the example of a severe emergency turn while braking. In such an instance, the MagneRide system would be able to optimise the vehicle's dynamic behaviour rapidly, helping the driver to maintain control. Raynauld commented: "The main barrier to this level of integration is the need for a standard high speed communications protocol implemented by the manufacturers of each system. We are participating in various consortia, including FlexRay, to develop the infrastructure that will support safe and reliable collaboration between the many chassis subsystems in today's vehicles."