1.  Field of the Invention
The present invention relates to a vibrating system for simulating, on a test bench, the load which would be applied from an actual road to a motor vehicle such as an automobile, a motorcycle, or the like, and also simulating three-dimensional vibrations as of an earthquake applied to such a motor vehicle, and a method of controlling such a vibrating system. 
2.  Description of the Background
Vibrating systems which are capable of simulating the loads applied from actual roads to completed motor vehicles on test benches are widely used for performance evaluation, durability tests, and various other purposes as they are highly effective in the development of automobiles, motorcycles, or the like.  Conventional vibrating systems for motorcycles are generally designed to vibrate the motorcycle through a wheel mounted on an axle.  These conventional vibrating systems, however, can only simulate the load which would be applied from a relatively flat road to a motorcycle.  When the motorcycle is vibrated at an increased rate, the tire of the wheel tends to jump from a vibrating table.  Therefore, it has been difficult for the conventional vibrating systems to simulate the loads from actual roads accurately. 
Heretofore, an early method of controlling a vibrating system vibrates a motor vehicle several times with noise characterized by a predetermined distribution of absolute values of a Fourier spectrum, and effects repeated corrective calculations for simulating actual running data based on a transfer function that has been determined from the data measured when the motor vehicle is vibrated. 
The prior method is also difficult to simulate the loads from actual roads with accuracy especially when the motor vehicle to be vibrated is a motorcycle or the like. 
More specifically, the transfer function used in the conventional control method is linearly approximated as measured with a certain vibration level.  When a motorcycle is vibrated, however, the motorcycle reacts with relatively strong nonlinearity due, for example, to the bottoming of a suspension thereof.  The linear approximation of the transfer function is therefore not suitable for use in motorcycle vibration tests.  Even with the corrective calculations being repeated based on the transfer function obtained from the vibratory measurements, therefore, it is difficult to simulate the roads applied from actual roads.  Specifically, as shown in FIG.  3 of the accompanying drawings, if a transfer function G1 that is determined from a target signal Ya when a vibrating signal Xa is applied is employed, then a vibrating signal Xc, which is wrong in reality, is determined as being necessary to obtain a target signal Y. When the vibrating signal Xc is applied, a target signal is produced using an actual transfer function G, in excess of the target signal Y. Therefore, if the target signal Y is of a value immediately prior to the bottoming of the suspension, then the motorcycle being tested will be damaged by the applied vibration.  That is, if an input vibrating signal applied to measure a transfer function is larger than the vibrating signal Xb, then the motorcycle to be vibrated will be subjected to an excessively large load. 
In known vibrating systems for motorcycles, it has been customary to place a weight, as heavy as an ordinary rider, fixedly on the rider's seat of the motorcycle.  According to a conventional method in which an equivalent weight of a rider is imposed on the motorcycle, the weight tends to move in unison with the rider's seat, making it impossible to simulate an actual rider whose moves vertically with a slight time lag with respect to the seat. 
One practice would be to vibrate the motorcycle with a weight simply placed on the seat.  However, the weight would move on the seat when the motorcycle is strongly vibrated, tending to shift the combined motorcycle and weight system laterally out of balance, and the weight might eventually slip off the seat. 
There is known a three-dimensional vibrating machine for exerting three dimensional vibrations to an object to observe its vibration-resistant capability.  The known three-dimensional vibrating machine comprises an X-axis linear actuator slidably mounted on a guide surface on a base by a static pressure bearing for linear sliding movement along an X-axis, a Y-axis linear actuator slidably mounted on a guide surface on the X-axis linear actuator by a static pressure bearing for linear sliding movement along a Y-axis, and a Z-axis linear actuator slidably mounted on a guide surface on the Y-axis linear actuator by a static pressure bearing for linear sliding movement along a Z-axis.  When the X-, Y-, and Z-axis linear actuators are driven, they apply vibrations to a vibrating table of the three-dimensional vibrating machine. 
There are also known a bidirectional vibratory testing apparatus for imposing vibrations only in horizontal and vertical directions through spherical bearings, and a three-dimensional vibratory testing apparatus comprising an X-axis movable frame and a Y-axis movable frame that are held in engagement with a Z-axis movable frame, for applying three-dimensional vibrations to a vibrating table. 
The three-dimensional vibrating machines are structurally complex as they require guides, spherical bearings, frames, and various other components.  Those three-dimensional vibrating machines which employ spherical bearings produce vibrations of limited amplitudes.  Those three-dimensional vibrating machines which employ guides and frames have difficulty in controlling vibrations in a high-frequency range because large inertial forces are produced as the vibrating table vibrates.  In the case where the Y- and Z-axis linear actuators are mounted on the X-axis linear actuator, the actuators are liable to swing back and forth on account of their acceleration, failing to achieve a high degree of control accuracy.