It is becoming necessary to rethink the actual train infrastructure: travel time must be reduced to compete with airlines, existing tracks must be shared with freight trains, and land or budget constraints often prohibit the construction of dedicated high-speed tracks. The only solution is tilt technology. The need for tilting control systems was discussed in the November 1996 issue of Popular Mechanics magazine, in an article entitled "American Flyer", as being a solution to improve passenger comfort during train rides. As stated in the magazine, advanced tilting systems could reduce the lateral force felt at the passenger level from 15 lbs. to 7 lbs.
A "tilting system" is a combination of electronic and hydraulic components that control a railway car's longitudinal roll motion mechanism. It is used in passenger trains in order to increase passenger comfort, that is affected by centrifugal acceleration in curves. Centrifugal acceleration is a serious limiting factor to the maximum cruising speed of a passenger train.
The maximum speed allowed in curves is limited by three factors: the maximum tilt angle of the car (usually between 5.degree. and 9.degree.), the maximum steady state residual lateral acceleration and the forces applied to the tracks by the non-tilting locomotive, which is almost two times heavier than a passenger car. The dynamic wheel/rail forces are identical for both a tilting and a non-tilting car at a given speed. All forces vary with the square of the speed.
Railroad curves are generally designed in order to compensate for a proportion of the centrifugal acceleration by means of track super-elevation (or cant angle) that will force the car body to tilt along its roll axis. Properly oriented, this tilt angle creates a gravitational component vector opposite to the centrifugal force felt by the passengers in curves. The super-elevation angle of a track is always determined according to the maximal forces that the inner rail can tolerate when the heaviest vehicle allowed to roll on the said track is immobilized in the curve. On conventional tracks, the presence of heavy freight trains is the source of limitation for the maximal super-elevation.
Considering this design criteria, one can demonstrate that most passenger railway corridors in North America and Europe presently lack the proper amount of curve super-elevation that would allow the operation of high-speed trains without seriously compromising passenger comfort. Since modifications to conventional tracks are too costly and since speed and passenger comfort are the key to the survival of the passenger train industry, the solution resides in tilting systems.
Passenger cars equipped with an active roll motion mechanism, also called a "tilting system" can overcome this cant deficiency problem by giving the proper amount of roll to the car body in order to compensate for the lack of curve super-elevation. Passenger comfort is then improved and high-speed operation becomes possible on most of existing railway corridors.
Tilting the body of a rail passenger car during curve negotiation offers the possibility of increasing the speed of a trainset in a curve without exceeding the maximum allowed steady state lateral acceleration felt by the passengers. Typically, the centrifugal acceleration must be lower than 1 m/sec.sup.2 (i.e. lower than 0.1 g). This tilting feature reduces the overall traveling time without requiring track modification. Moreover, an effective tilting system greatly improves the passenger ride comfort during curve entry and exit by minimizing the transient accelerations.
Usually, the tilting mechanism only cancels 70% of the centrifugal force. A March 1993 article in Popular Mechanics magazine entitled "Bullet Train for America" explains the effect of the tilting system on the passenger: "Standing up, a rider notices the floor push gently against the left foot, as the view out the window pitches skyward". The reason why the centrifugal acceleration is not compensated 100% is because neural signals from the eye would clash with those from the inner ear of the passengers, which senses no change at all, and would cause motion sickness.
The tilting system is activated by the locomotive engineer before the train undertakes a run. A cab indicator informs the engineer of the tilting system status. When the system is activated, the locomotive engineer can operate the train at higher speeds. If the tilting system is deactivated, the train engineer must return to conventional speed in all curves for passenger comfort purposes. The difference between tilting and conventional speeds in high-speed curves is typically 35 km/h.
The main problem associated with taking an input signal from an inertial sensor device located on the train bogie is the high frequency component that will be present because of the contacts with the rails. The train bounces from one side to the other on the tracks, this produces noise in the sensor signal. It is then hard to identify the real inertial signal that is produced when entering or exiting a curve. A filter must then be used to remove the undesirable frequencies from the input signal, otherwise the system behavior would be uncomfortable, by reacting to track defect. In addition, high frequencies in the input signal would shorten the life cycle of costly hydraulic components used in the tilting system by introducing unnecessary demand. The filtering delay is then determined according to the worst conditions found on the tracks where the tilting system will operate. This usually leads to the selection of a long delay filter.
Tilting of the car is accomplished by a servo-valve controlling the hydraulic mechanism, which in turn tilts the car. The tilting control system responds to the output of the long delay filter. The outward acceleration felt by the passengers is compounded by the acceleration of the tilt system, i.e. the outward acceleration due to the curve is added to the inward acceleration due to the compensating tilting. The delay introduced by the filter will lead the passengers to experience a discomfort twice in a curve: at entry and exit. The reaction time of the control system is therefore critical.
The delay in tilt compensation causes lateral acceleration during both curve entry and exit. Each curve represents a potential discomfort source for the passengers. The acceleration due to the tilting of the car is added to lateral acceleration and makes the situation even worse. A possible solution would be to implement costly tilt mechanisms such as the one described in EP patent publication no. 0 808 758 A1, published on May 15, 1997.