Abstract:
A method and an apparatus for operating a test bench, e.g. a roller test bench, for carrying out simulation drives of a vehicle placed on the test bench. At least one vehicle wheel contacts at least one roller of the roller test bench. To determine the roller setpoint speed, a reaction torque of the roller in relation to the vehicle wheel is measured on the roller of the roller test bench.

Description:
The following disclosure is based on German Patent Application No. 10309247.1, filed on Mar. 3, 2003, which is incorporated into this application by reference. 
   FIELD OF AND BACKGROUND OF THE INVENTION 
   The invention relates to a method for operating a test bench to carry out simulation drives of a vehicle placed on the test bench. The invention further relates to a test bench configured, e.g., to carry out this method. 
   Test benches for driving simulation are known in the art in several variants (e.g. roller test benches, conveyor belts). Roller test benches are configured as a counterpart to a tire, and are fashioned as one or as a plurality of rotatable rollers or cylinders (single rollers, double rollers). These rollers receive the tire and follow a motion imparted to the tire or produce such a motion. Likewise, conveyor belts or running, closed belts stretched across two or more rotatable cylinders are configured as a counterpart to a tire. These belts receive the tire and follow an imparted motion or produce a motion. As used in the present application, the term ‘roller’ and the like encompass rollers, cylinders, belts, etc. 
   In a first drive modus, in particular, a plurality of drives are mechanically intercoupled with belts using rollers. During acceleration or deceleration, the individual drives are mechanically interlinked via couplings. If the system is accelerated by means of a vehicle, the speed change depends on the magnitude of the forces on the surface of the wheels and on the mechanically installed mass or the moments of inertia of all the mechanical parts that are to be set in motion. 
   In another drive modus, a unit (i.e., a drive train that can receive a wheel) is driven by an electric motor. If there are several respective units, there is no mechanical link between the units, except in service operating modes. One unit or two units (1 axle) is/are designated as the master. It is possible to control the master drive to a constant torque (positive or negative for load relief or loading), but the drive is not speed-controlled. The actual speed of the master drive is measured and provided as a setpoint value to the slave drives, if any. Under these conditions, it is possible that the rpm&#39;s and the speeds of the four sets of rollers differ from one another, because there is no mechanical link between the individual rollers and drives. 
   OBJECTS OF THE INVENTION 
   One object of the invention is to provide a method for operating a test bench, e.g. a roller test bench, that offers particularly realistic insights into the response behavior of a motor vehicle. A further object of the invention is to provide a test bench to be used with such a method. 
   SUMMARY OF THE INVENTION 
   According to one formulation of the invention, these and other objects are attained by a method for operating a test bench to carry out simulation drives of a vehicle placed on the test bench, which method includes: contacting at least one wheel of the vehicle to at least one roller of the test bench; measuring a reaction torque of the roller in relation to the vehicle wheel on the roller of the test bench; and determining a setpoint speed of the roller from the measured reaction torque. According to a further formulation, the invention is directed to a test bench that includes at least one roller configured to indirectly or directly contact at least one wheel of a vehicle; means for measuring a reaction torque of the roller in relation to the vehicle wheel; and means for determining a setpoint speed of the roller from the measured reaction torque. 
   For instance, in a method according to the invention for operating a roller test bench to carry out driving simulations, an associated roller reaction torque in relation to the wheels of the vehicle (e.g., an automobile or a motor cycle) is measured on at least one of the rollers of the roller test bench. This reaction torque, which is determined preferably taking into account the inevitable losses, in the form of friction or acceleration, for the test bench itself, is converted into a reaction force using the radius of the corresponding roller (reaction force F=reaction torque M/radius r). This determines a reaction force for the roller of the roller test bench. This determination of a reaction force and the other embodiments of the described method can be used analogously for test benches equipped with conveyor belts. 
   The reaction forces of all the rollers can now be added to obtain a total roller reaction force. The individual reaction force, or the total reaction force obtained from the individual reaction forces added together, and the vehicle mass m are then used to determine the acceleration to which the vehicle would be exposed in actual on-road traffic (acceleration a=reaction force F/vehicle mass m). 
   The setpoint speed is now corrected, e.g. in a feedback control circuit, using the determined acceleration, and the individually driven rollers are correspondingly accelerated or decelerated until the setpoint speed is reached or until the reaction forces no longer prompt any acceleration or deceleration. 
   Once the reaction torques and thus also the reaction forces are available and determined, the determined setpoint speed can now be continuously brought near the actual speed by accelerating or decelerating the individual rollers. This is accomplished by a feedback control circuit, which is known per se. 
   With the method according to the invention, other forces (e.g., gradient resistance, drag) aside from the reaction forces occurring on the individual rollers can be included when the reaction forces are added. An increased or correspondingly decreased acceleration is thereby determined, such that an increased or decreased setpoint speed is determined. As a result the simulation is made more realistic. 
   Preferably, the precise vehicle mass m is taken into account when the setpoint speed is determined. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail with reference to a switching example for controlling the roller test bench depicted in the single drawing Figure. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The control system illustrated by way of example consists of four secondary control loops  20 ,  30 ,  40 ,  50  (for 4 rollers for a motor vehicle with 4 wheels), which are respectively used to control the speed of each individual roller  26 ,  36 ,  46 ,  56 . Superimposed thereon is a control loop  10  that enables self-controlled operation, such that acceleration (stepping on the gas) or deceleration (braking) of the vehicle causes the roller speed to be adjusted. 
   The four secondary control loops  20 ,  30 ,  40   50  are identified in the drawing figure by dash-dotted lines. The function is best described as follows, with the explanations starting at v_setpoint of secondary control loop  20  (the description of the other secondary control loops is similar):
     1. The setpoint speed v_setpoint is constrained to stay between the maximum speed v_max and the minimum speed v_min. Typical values could be, for example 200 km/h and 30 km/h (or e.g. 125 mph and 20 mph).   2. The setpoint value is compared with the actual value—v_actual—and the difference is formed in adder  21 .   3. The controller  22  calculates a suitable torque M_brake to which M_addition is added in adder  23 . M_addition compensates the acceleration—M_ACCEL.—of the roller  26  and the friction—M_FRICTION—in the test bench. Initially, these are still digital computational values, not physical quantities.   4. The torque for all quadrants is limited to its maximum value, defined by the performance data of the converters and the motors, or by the given technology.   5. The resulting value is fed as a specified value to the converter  24  that supplies the motor  25 . The motor  25  very accurately produces an actual, physically measurable torque —M_measured—. This torque can be made reproducible by calibration enabling feedback to a works standard.   6. The roller  26 , with its own moment of inertia and the vehicle as a load, responds to the torque. The resulting speed (actual value) is measured—as v_actual—.   

   The depicted superimposed control loop  10  (above the dash-dotted lines) is active only in the self-controlled mode—v_controlled—(also: disconnected mode with switch position to v_test), which will be described in greater detail below. The v_controlled—mode is activated by the switch  1 , which toggles between v_controlled and v_test. 
   In the position shown, the—v_controlled—mode is active. As a result, the torques of all rollers are added in adder  11  and the acceleration that the vehicle would experience under road conditions is simulated. The speed v_setpoint is then corrected by acceleration means  2  and is fed to the secondary control loops  20 ,  30 ,  40 ,  50  (within the dash-dotted lines). In detail, the system goes through the following steps:
     1. The one or more vehicle tires attempt to change the absolute rotational speed because of an external influence J 1 , J 2 , J 3 , J 4  (actuation of the gas pedal or the brake).   2. On the surface of the counterpart to the tire, a force is transmitted, and the speed controller  22 ,  32 ,  42 ,  52  subsequently detects a deviation between the setpoint and the actual value.   3. The speed controller  22 ,  32 ,  42 ,  52  tries to counteract this deviation (responds with a torque) and provides a correcting variable in the form of a torque (the sign—positive or negative—is a function of the sign of the deviation between the setpoint and the actual value).   4. The measured torque—M_measured—is converted into a force using constant factors (in the specific case of the roller test bench using the radius of the rollers).   5. The axles to be taken into account are selected. Normally, these are all the axles.   6. The forces of all the axles are calculated by multipliers  16 ,  17 ,  18 ,  19  for each roller  26 ,  36 ,  46 ,  56 , respectively, and are added in adder  11 .   7. The total force F that accelerates or decelerates the vehicle is divided by the vehicle mass m in divider  12 . The result is the acceleration that the vehicle would experience on the road if it were exposed to the calculated total force.   8. Multiplication with the sampling time T in multiplier  13  of the control loop gives the speed increment, i.e., within the sampling time, the speed of the vehicle on the road would change by this amount.   9. The speed increment is added to the current setpoint speed in adder  14 .   10. The new setpoint speed is used in the next sampling step. The secondary control loops are used to attempt to reach this new setpoint speed on all four rollers  26 ,  26 ,  46 ,  56 .   

   The superimposed control loop  10  (above the dash-dotted lines) can be disconnected. For this purpose, the switch  1  is switched from the—v_controlled—position (as shown) to the v_test position. 
   In the disconnected mode—_test—, the speed of the four rollers  26 ,  36 ,  46 ,  56  is controlled according to the setpoint speed. A higher-level computer specifies a driving cycle with changing setpoint speeds. There is no feedback of the brake or acceleration forces to the setpoint speed. Operation in the disconnected mode is intended for tests that are known per se, namely the static brake test, the brake test at different speeds and the ABS test. 
   The following tests can be conducted using the disconnected mode: 
   
       
       1. Starting from a Dead Stop (with Front Wheel Drive)
 
A vehicle with front wheel drive is located on the test bench. The driver puts the vehicle in first gear, engages the clutch and steps on the gas. This causes an accelerating torque to be applied to the front wheels. The setpoint speed is initially 0; the actual speed increases. The front electric motors build up a torque to maintain the setpoint speed 0. Within a sampling period the torque on the two front rollers is converted into the drive force with which the vehicle would be accelerated on the road. The ensuing acceleration is calculated and the setpoint speed is increased accordingly. The acceleration that the vehicle would experience under the same conditions on the road ensues.
 
       2. Braking from an Average Speed (with Rear Wheel Drive)
 
A vehicle with rear wheel drive is located on the test bench. The speed is 50 km/h; the driver has disengaged the clutch. When the driver actuates the brake, the actual speed initially decreases. The electric motors must apply an accelerating torque to the rollers to reach the setpoint speed again. In parallel therewith the braking forces of all four wheels are added. The sum of the four forces is used to calculate the deceleration that the vehicle would experience on the road. The setpoint speed is correspondingly reduced and approaches the actual speed. Thus, the control keeps the difference between the setpoint speed and the actual speed small by adjusting the setpoint speed to the actual speed through feedback. The deceleration that the vehicle would actually experience on the road is thereby simulated.
 
       3. Simulation of Uphill Travel
 
Uphill travel is simulated in that, in corrected operation, the force parallel to the inclined plane Ft is deducted from the drive force of the vehicle. At a constant speed on a flat road the engine does not need to apply an accelerating torque, assuming that one initially neglects rolling friction and drag. Once uphill travel starts, a positive force is added in the superimposed control loop such that the total drive force applied to the vehicle is negative from a physical perspective. As a result, a negative acceleration occurs, the setpoint speed is reduced, and the vehicle slows down. To maintain the speed, the driver must step on the gas. If he does not do that, the vehicle slows down as a function of the gradient.
 
     
  
   The above examples are illustrative of the possibilities of a realistic simulation using the described method for operating a roller test bench. 
   The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.