1. Field of the Invention
This invention relates to dynamometers which compensate for parasitic losses while simulating those forces (road load and inertial) which the vehicle would incur during actual operation on the road.
2. Description of the Prior Art
Test apparatus in the form of dynamometers is widely used for testing motor vehicles in place. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertia (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle. In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=A+BV+CV.sup.2 +DV.sup.3 +EW (1)
where A represents the effects of breakaway force, and B, C and D represent the effects of rolling friction and windage, V represents the vehicle velocity, E represents the grade of the slope and W represents the vehicle weight.
Dynamometers generally include a roll or pair of rolls for engaging the driven wheels of the vehicles. However, dynamometers can be coupled directly to a vehicle engine thereby eliminating the need for the roll or rolls. A power supplying and absorbing unit, typically a d.c. motor, or a power absorbing unit such as a friction brake, eddy current brake or hydrokinetic brake is coupled to the roll or rolls (or directly to the engine) for supplying power to and/or absorbing power from the roll(s) which in turn applies a retarding force to the surface of the vehicle wheels (e.g., tires) to simulate the road load forces. Inertial forces can be simulated by power supplying and absorbing units during both acceleration and deceleration but can be simulated by power absorbing units only during acceleration. Mechanical flywheels are generally used in conjunction with power supplying and/or absorbing units to simulate a part (or in some instances all) of the vehicle inertia.
Where mechanical flywheels are used to simulate all of the test vehicle's inertia, the power supplying and/or absorbing unit need only supply torque to the roll(s) and compensate for parasitic losses within the dynamometer and to satisfy the roll/wheel interface force dedicated by the road load equation. Where the unit is to simulate all or part of the vehicle's inertia in addition to road load and parasitic loss forces one of the following equations may be implemented: ##EQU1## where
Vd=the desired velocity of the driven roll(s),
F=the measured force at the wheel/roll interface,
P.sub.L =the parasitic losses associated with the dynamometer,
RL=the road load force, and
I=the desired vehicle inertia, or ##EQU2## where
where F.sub.d =the desired wheel/roll interference force, and
V=the measured velocity of the driven roll.
The rotational velocity of the roll is representative of V and can be accurately measured by coupling a speed encoder of the optical or magnetic pulse type to the dynamometer roll. However, there is no force measuring device which as a practical matter, can be placed between the rotating vehicle wheel and the roll. As a compromise, a force measuring device or transducer is generally placed either at the output of the power supplying and/or absorbing unit or between the flywheel assembly and the shaft connecting the flywheels to the roll. In either case, there are bearing friction and windage losses generated by the roll and/or flywheels and perhaps losses caused by the bending action of flexible belts which are not measured by the transducer. Such losses are commonly referred to as parasitic losses (PL) and must be compensated for in order to provide an accurate control signal for the power supplying and/or absorbing unit in the dynamometer as will be explained in more detail.
Basic equation (2) above can be implemented by integrating the measured force F (or torque) signal to compute the desired speed signal, comparing the computed speed signal with the measured speed signal to provide an error speed signal and controlling the power supplying/absorbing unit to reduce the error signal to zero as is explained in some detail in U.S. Pat. No. 4,101,116 assigned to the assignee of this application.
Equation (3) can be implemented by differentiating the measured speed signal V to provide a desired force (or torque) signal, comparing the desired force (or torque) signal with the measured force (or torque) signal to provide an error force signal and controlling the power supplying/absorbing unit to reduce the error signal to zero. A variation of the above techniques can also be used to provide an appropriate control signal to the power supplying/absorbing unit as is explained, for example, in an SAE Technical Paper Article No. 810749 (1981) entitled Feed-Forward Dynamometer Controller for High Speed Inertia Simulation by Severino D'Angleo and R. D. Gafford.
A parasitic loss profile or curve of the lost force at the roll surface (due to parasitic losses) versus roll speed for the roll can be computed by measuring the force required to maintain the roll or rolls at several selected (e.g., three) speeds. Such a loss profile can also be calculated by using the actual inertia of the roll system and allowing the roll (or rolls) to coast down from a high speed while measuring the change of roll speed at selected points on the speed curve. The parasitic loss profile can be represented by the equation: EQU PL=F+GV+HV.sup.2 ( 4)
where
PL=the parasitic loss torque to be supplied or absorbed by the power supplying/absorbing unit at a given speed and
F, G, and H are constants that define the magnitude and curvature of the profile.
Since the parasitic losses vary with environmental conditions such as temperature, it is prudent to measure the losses periodically e.g., several times during each day that the dynamometer is in operation. A power supplying and absorbing unit may provide the motoring power to operate the dynamometer periodically between testing intervals for this purpose. A separate motor must be used for this purpose where the unit for absorbing power from the roll(s) is incapable of supplying power e.g., an eddy current brake, friction brake or a hydrokinetic absorber. The use of a separate motor adds to the cost and complexity of the dynamometer.
A d.c. motor would appear to be ideally suited and has been widely used for accomplishing both tasks i.e., operating the dynamometer during vehicle tests and during parasitic loss measurements. However, d.c. motors of a sufficient size for accomplishing these tasks are very expensive. While a.c. motors, such as synchronous and induction motors, are less expensive (particularly induction motors), they are not as easily controlled with respect to speed and absorption characteristics. To change the speed of a synchronous motor, the frequency of the input stator voltage must be changed. While an induction motor can operate over a wide speed range with a fixed frequency input voltage, the motoring output torque will vary with the slip frequency. The motor will perform as a generator and thus absorb torque only when the rotor revolves at a speed greater than the rotating stator field i.e., greater than the synchronous speed. Systems which vary the synchronous frequency and magnitude of the stator voltage of induction motors are often referred to as vector drive systems. While such systems enable an induction motor to operate in a motoring and absorbing mode over a wide speed range, they require the conventional a.c. line voltage to be converted to d.c. and then back to a.c. at a different frequency and thus are complex and expensive.
It has been suggested that an induction motor could be used to produce a braking action for lowering loads in a crane hoist at a speed less than the synchronous speed by exciting the stator windings with direct current and consuming the power produced in a resistance. See Chapter 26 of the textbook Elements of Electrical Engineering by Arthur L. Cook and Clifford C. Carr, 6th Edition, published and copyrighted in 1924 by John Wiley and Sons, Inc., Library of Congress card no. 53-11045. However, the problems involved in virtually instantaneously changing the power absorption characteristics of a dynamometer with speed are not comparable to those involved in lowering a given weight at a safe speed as in a crane hoist.
There is a need for a less expensive dynamometer which is capable of simulating motor vehicle road load and/or inertia forces while providing parasitic loss compensation.