Slip-Sliding Away

Iain Morrison, Test & Validation Engineer, April 3, 2007

To ensure that the Tesla Roadster is as safe as possible in extreme conditions, we have just finished putting one of our Engineering Prototypes through an extensive test schedule at the Continental Proving Ground in Arvidsjaur, Sweden. The proving ground is in a beautiful location on and around a frozen lake about 60 miles from the Arctic Circle.

Sunset in Arvidsjaur, Sweden

Initially we were slightly nervous about driving on water, as it were, but the locals convinced us that the minimum 80cm of ice was ample support for the weight of the Tesla Roadster. And besides, there were plenty of other prototype cars around that were considerably more portly than the diminutive Tesla Roadster (to say nothing of its occupants – a 7 pound weight gain is considered normal during a winter test trip).

The testing was focused on the following two safety systems, both of which act to stabilize the car:

  • Anti-lock braking system (ABS)
  • Traction Control - using the sophisticated electronics from Tesla Motors

Our development partners at Continental are supplying our ABS system, which is calibrated specially for our car and subjected to a vigorous testing program. Winter testing is only one part of the development program; the car had previously been put through its paces at one of Continental’s other proving grounds in Germany. The proving ground at Arvidsjaur, however, offers a range of different surfaces on which to test every worst case eventuality – snow, polished ice, and "split µ" (or split surface) in various configurations, such as hills, circles, and checkerboard layouts.

The Greek letter µ (pronounced moo in Norfolk, or mew in the rest of the world) is the symbol used in physics for the co-efficient of friction. The lower the number, the more slippery the surface. "Split-µ" surfaces, where one side of the car is on a low friction surface (in our tests, ice) and the other side is on a high friction surface (in our tests, asphalt), can cause particular difficulties. The wheels will behave differently depending on the surface they are reacting against. Under braking, the wheels on the ice surface will lock earlier than the wheels on the asphalt, tending to spin the car. ABS will stabilize the car in this situation by controlling the brake pressure to each wheel individually, preventing any wheel from locking. Check out the video below of our tests braking on a split-µ corner. The split-screen video shows the car on the left with ABS on and the right with ABS off. Without stabilization from ABS (right screen), the car spins out when the driver applies the brakes.
Watch movieABS testing (Free QuickTime player required to view video.)

Whilst in Sweden we also took the time to refine our traction control system. The exact algorithm is another part of our "secret sauce." Suffice to say, it works by controlling motor torque in response to wheel slip and ensures the car stays stable. Wheel speed sensors form a part of the ABS system and they send individual wheel speeds to one of our processors 100 times per second. These speeds are constantly analyzed in order to determine if any wheel is slipping, and, if so, the motor torque is limited until the slip is brought back under control. This system works not only when the throttle is applied but also under regenerative braking.

Regenerative braking works by reversing the direction of the current to flow into the battery, effectively turning the motor into an electrical generator. This negative motor torque is used to slow the car and feels similar to engine braking on a regular gasoline vehicle. Not only does this minimize brake wear, it also gives some lovely charge back to the battery. One potential downside: If too much negative torque is applied, the car’s wheels could lock and the car could become unstable, so our traction control system is tuned to take this into account and ensure that we can safely maximize the level of regen without upsetting the car’s balance. The regen control is always on, but the traction control can be switched off by a button on the center console, and that’s when some real fun can be had… Time for a disclaimer here: The video below shows driving with winter tires on a closed track and nothing to smack into. Please don’t try this at home.
Watch movie Testing with traction control turned off (Free QuickTime player required to view video.)

Sliding around is not just done for kicks, though. An important part of the car's development was tuning the throttle response – i.e. how much torque you get from the motor for a certain pedal position. This has been set up very carefully to allow the best control of the car over the full motor speed range – this sort of drifting just isn’t possible without very good throttle response. (That said, with only five hours of daylight you do need something to lighten the workload!)

Testing the Tesla Roadster

Beyond the dynamic vehicle development, we also gained lots of useful learning on how the car behaves in real-world, extreme-cold conditions. This supports our ongoing climatic chamber development (see the Blowing Hot and Cold blog). This chamber can replicate cold temperatures but cannot simulate snow ingress, for example. Among the (non-automotive) interesting discoveries we made was that -17° C (1.4 ° F) is the temperature below which your nostrils start feeling crunchy, though I guess readers in New York state will already know this!

Other cold weather testing completed includes that of our Tire Pressure Monitoring System (TPMS ). This system constantly monitors tire pressures to warn you if they are outside of spec. As pressure varies with temperature, it is important to calibrate the system for all operating conditions – including extreme temperature changes, such as driving from a hot garage into the cold outside – otherwise the system could throw up spurious warnings.

As with all our development activity, the learning we get at this stage is funnelled into the next level of prototypes and ultimately the finished car. Although we are now thoroughly satisfied with the low-µ performance of car, we are continuing refinement of the traction control and regen systems at the Lotus test track here in the U.K. to ensure that it performs equally well on grippier surfaces. (See map of our facility in Hethel, Norfolk)

However, due to the famous unpredictability of the British weather, we don’t always have a dry, grippy track. In fact, conditions so far this year have occasionally been better suited to testing boats! The solution to this is to go somewhere nice and sunny like IDIADA near Barcelona, Spain. Final ABS / Traction Control testing is scheduled to take place there in early May.