Comparison of Optical and RT3000 Slip Angle by Vehicle Dynamics Group at Idiada, Spain
The Vehicle Dynamics department at Idiada spend a good proportion of their time testing vehicles. Or measuring Objective parameters of a vehicle chassis and suspension to be more specific. Tests can range from short, two-day affairs to several weeks if many vehicles are involved. Setting up the instruments on the vehicle often takes longer than the testing itself and anything that can reduce this is a major efficiency saving.
Enter the RT3000. Alexandre Català, Product Manager for the Vehicle Dynamics Department at Idiada, recognized that the time saving of installing the RT3000 is invaluable. But first he had to make sure that the RT3000 gave comparable results to existing instruments. “Being able to compare measurements made today with ones made in the past is often as important as making the measurement correctly in the first place. Otherwise you may believe you have made an improvement that is not there.” Idiada have some of the best facilities for testing cars and, therefore, some of the best facilities for testing instruments like the RT3000. It hardly ever rains, so the test tracks can be made flat rather than cambered for rainwater run-off.
There is a large 200 m square skidpan for circle tests, and two straight lead-in tracks allow left and right J-turns. The two wide straights, over 2 km long, have banked corners so the straight can be entered at speed. The RT3000 makes many very precise measurements.
Inertial and GPS Navigation Systems typically have extremely accurate acceleration, roll and pitch measurements; these are essential for navigation when there is no GPS. Heading, and therefore slip angle, is much harder for an Inertial Navigation System to compute accurately. It is only possible to measure heading and slip angle if the accelerations, pitch and roll are known accurately. So investigating the slip angle performance is a good place to start when assessing the overall performance of the RT3000.
Step steer response analysis: the graphs compare the slip angle of the Optical Sensor (red) to the Slip Angle of the RT3000 (black). The better bandwidth and lower time delay of RT3000 gives more realistic time response in transient manoeuvres like this. Additionally the evolution of the slip magnitude shows less suspicious disturbances with RT3000, optical systems have less stability specially when mounting height vary rapidly during the test evolution.
Connecting the RT3000 to Idiada’s B+S Multidata acquisition systems was easy. The RT3000 has a CAN bus output, and the acquisition system readily accepts CAN inputs. The existing instruments, optical slip sensor, yaw rate sensor, accelerometers and steering wheel angle, were already connected to the acquisition system and modification to acquire the RT3000 at the same time was simple.
The RT3000 makes its measurements in real-time. Without its real-time capability it could not be a serious contender to replace other existing instruments. The driver needs the speed measurement visible at all times since the car’s speedometer is not accurate enough for gathering Objective Measurements. The driver needs to be able to check the results of each test immediately; otherwise several hours of testing can be wasted if the system is not working correctly.
When comparing to other sensors on the vehicle (like steering angle) having the data collected in one database also helps reduce the time required to analyse the data; and real-time helps this process too. Because the RT3000 provides high-accuracy measurements in real-time with very low latency it is ideal for aiding the driver during the test and viewing the results during and immediately after the test.
The installation of the RT3000 can take less than 10 minutes when using the optional mounting pole, and swapping between cars when benchmarking takes a minute. Compare this to attaching the optical slip sensor rigidly; mounting several laser ride-height sensors for roll and pitch; mounting the yaw-rate sensor and lateral accelerometer; these can take hours to install and calibrate. The RT3000 is small too, so it can be mounted very close the desired measurement point; the outputs can be displaced to a remote, virtual measurement point in case it can’t be mounted exactly where you want it. The tests conducted by Idiada included Frequency Analysis and Step Steer input and steady-state circles. Some of the data from these tests is presented here, comparing the RT3000 output to the existing sensors. In conclusion the RT3000 performs almost identically to the conventional sensors, but shows several distinct advantages. If we focus on the most critical signal, vehicle slip, the steady state data shows an almost perfect correlation to the Optical Sensor, confirming the amplitude of the RT3000 is correct. In transient tests, the RT3000 delivers lower-latency, wider bandwidth and lower noise, compared to the Optical Slip Sensors.
The lateral acceleration and yaw rate sensors give an approximation of vehicle slip and the RT3000 matches the pattern of this too, giving a non-approximated slip angle measurement with similar latency and bandwidth. Overall the RT3000 is a more reliable and accurate system that can be used for all measurements of vehicle dynamics. It has advantages in both accuracy and installation time; it delivers better performance and requires less engineering time. On the steady-state circle test the RT3000 and the Optical Sensor results are almost identical.
The graph compares the amplitude of the steering input to the slip angle for different input frequency sinusoids. The optical sensor is shown in red and the RT3000 is blue. The green shows an approximation to Slip Angle calculated using a yaw rate sensor and lateral accelerometer; this approximation is often used to combat the bandwidth problems of the Optical sensor. The solid lines show the response at the front of the vehicle and the dotted lines show the response at the rear. The RT3000 shows very similar gain response to the calculated channel, confirming that the bandwidth is excellent. It is reasonable to assume that the RT3000 is more accurate because it matches the Optical in the steady-state case and the calculated values are only an approximation.