Dunlop releases accurate circumference...
Dunlop releases accurate circumference data for its AMA-spec tires — shown here are the profiles for the Daytona SportBike tires. This data can be used in a traction control or data acquisition system to manipulate the wheel speed signals for more accurate results.
A set of 50-page patents assigned to Kawasaki (Slip Suppression Control System for Vehicle) describe a comprehensive traction control system that detects slip using wheel speed sensors and controls traction using the rear brake along with the engine’s ignition, injection and secondary butterflies. Using various threshold values for slip, the system modulates power to hold slip at a calculated value, using a multitude of maps to optimize the threshold values and control power.
Using RPM, speed and throttle position, the Kawasaki system determines an initial slip threshold value at which the traction control system activates. This threshold is higher than the target value for optimum slip, so that the chance of falsely detecting a loss-of-traction event is minimized. The system immediately applies the rear brake, closes the secondary throttle butterflies and retards ignition timing to a set value. When slip falls below another threshold, indicating traction is returning, the rear brake is released. By advancing and retarding the ignition timing at a set rate, the system attempts to keep slip at a second threshold value — timing is retarded as slip goes above the threshold, and advanced as slip goes below the threshold. Once ignition timing is advanced to its normal state, the secondary butterflies take over in a similar manner, modulating power to keep slip at the second threshold. This order — brake, timing, and then throttle butterflies — represents responsiveness to power reduction, according to the patent, and cutting and then applying power in this manner keeps good drivability. The attached diagram from the patents graphically shows a traction event and the various responses.
This graph, taken from our...
This graph, taken from our Harley-Davidson XR1200 project bike at Infineon Raceway, shows the disparity between actual speed and measured wheel speed. A slip signal compares the two speeds and can be used to determine wheelspin, but the tire’s changing circumference as the bike leans makes this very difficult.
The three patents outline various alternatives and variables for operation. As described above, in one example the ignition timing advances and retards at a set rate to control slip. In a second example, ignition timing is advanced and retarded in steps as slip goes above or below a number of predetermined thresholds. Or, ignition timing may be changed at a set rate, but a number of “overshoot” thresholds retard timing to corresponding set values. In another example, the slip thresholds change dependent on how much power is being cut over time, as traction is gradually restored. Feedforward operation, with control based on the rate of change of RPM or rear wheel speed, may also be incorporated.
A multitude of maps are used in the Kawasaki system to enhance performance. The slip thresholds change based on the gear selected, engine rpm and vehicle speed. Compensation for lean angle includes accounting for changing tire circumference as well as more or less slip required at certain lean angles. The system even takes into account how the rear tire deforms with acceleration, with a compensation map for each gear that looks much like a thrust chart. To improve drivability, the second slip threshold, which the system works to maintain over time, changes with throttle position or a rider-selected switch. This allows the rider, through the use of the throttle (or TC level) to control the amount of spin in a given situation. While some parts of the system described in these patents — such as the rear brake control — are obviously not found in the ZX-10R’s elaborate traction control system, certainly other aspects are included.
The traction control system...
The traction control system described in a Piaggio patent includes a map generated when a loss of traction occurs. The throttle position (θ) and slip (λ) at that time are noted (as θhold and λhold respectively). Proportionally more or less slip is allowed, based on a maximum permissible slip value (λmax) at full throttle and no slip at closed throttle, giving the rider continued control ver the slip at each occurrence.
The Kawasaki system is notable for its comprehensiveness and multitude of maps to account for a number of scenarios, but a patent (System and Method for Controlling Traction in a Two-Wheeled Vehicle) assigned to Piaggio, the parent company of Aprilia, goes one step further. The patent describes a traction control system that, when a traction loss occurs, generates its own maps based on the circumstances and works from those maps. The patent states that a closed-loop system (using feedback from wheel speed sensors) is slow to respond but able to account for variables such as road conditions, tire wear and different tracks. A feedforward or rate-of-change system, on the other hand, is quicker and more instantaneous in its response but can only be used in known circumstances, such as with a specific tire or at a certain racetrack. The Piaggio system combines both types of systems in a unique manner.
The system monitors front and rear wheel speeds along with lean angle, RPM, gear, speed and throttle position, and takes into account changing tire circumference “in an inventive manner”. When slip goes above a minimum value (as determined by a map based on throttle position, speed and lean angle) or if the throttle is opened rapidly, the system activates and makes a note of the throttle position and amount of slip. From maps based on the recorded parameters, a reference slip is noted that represents the maximum amount of slip allowed at full throttle in the referenced conditions. A map is generated that allows proportionally more or less slip as the throttle is opened or closed from the values noted at activation. This map is intended to give the rider control over the slip based on throttle position.
This chart, from one of the...
This chart, from one of the Kawasaki patents, shows how ignition timing, the sub-throttle valve and the rear brake are used to control slip. The various threshold values are based on a multitude of variables and maps.
The system calculates how much power should be increased or reduced at any given moment based on the closed-loop format as well as the rate-of-change format. These values are mixed, with more of one or the other being used depending on speed, gear, lean angle or the position of a rider selected switch. The mixer gives the system the benefits of both types of systems, allowing quick reaction as well as adaptability to various scenarios. Aprilia’s Performance Ride Control system as found on its RSV4 models no doubt employs much of the system described in the patent, and — as we found in our test of the SE model in our last issue — works extremely well at controlling slip. SR