Electricity: motor control systems – Closed loop speed control system for dc motor with commutator – Armature control by digital or combined analog and digital...
Reexamination Certificate
1997-06-06
2001-06-05
Masih, Karen (Department: 2837)
Electricity: motor control systems
Closed loop speed control system for dc motor with commutator
Armature control by digital or combined analog and digital...
C388S904000, C318S587000
Reexamination Certificate
active
06243532
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a speed control device for an electric motor fed by a direct voltage source, comprising a controlled switch means by of which the motor voltage is alternately switched on and off under the control of a modulated switching pulse sequence.
BACKGROUND OF THE INVENTION
Such a speed control device can be utilized, for example, for an additional heating apparatus of a combustion engine of a vehicle, with the combustion engine driving a blower for supplying the amount of air necessary for the fuel-air mixture. For obtaining an optimum combustion of the heater fuel, the ratio X between air and fuel of the fuel mixture should be kept constant. This means that a constant fuel amount per unit of time and a constant amount of air per unit of time have to be supplied for producing the fuel-air mixture. In order to keep constant the supplied amount of air per unit of time, the combustion engine or blower motor must be kept at a constant speed. However, this presupposes a constant battery voltage of the vehicle battery powering the additional heating apparatus of the vehicle.
In practical applications, the battery voltage of a vehicle may show relatively large fluctuations. In case of a vehicle battery having a nominal voltage of 24 V, as used for example for lorries/strucks or busses, a fluctuation width or range of the battery voltage between 18 V and 32 V is assumed. This results in a correspondingly high fluctuation of the speed of the blower motor and in a corresponding fluctuation of the amount of air supplied per unit of time for preparing the fuel-air mixture.
For obtaining a constant blower speed despite these high fluctuations in battery voltage, it is common practice to keep the speed of the blower motor at a constant set value by means of a speed regulating circuit. Such a regulating circuit necessitates a sensor disposed on the electric motor or on the blower impeller driven by the latter and serving for measuring the actual speed value. To this end, optical sensors or magnetic sensors are usually employed. Both sensor types increase the manufacturing costs and entail problems. Optical sensors are subject to the problem of soiling. Magnetic sensors are subject to the problem that they must be disposed as close as possible to a magnetic generator.
SUMMARY AND OBJECTS OF THE INVENTION
It is the object of the invention to be able, also without such sensors and with as little cost expenditure as possible, to keep constant the speed of an electric motor fed by a direct voltage source, also in case of greater fluctuations of the direct voltage. This object is met with respect to the speed control device of the type indicated at the outset in that the switching frequency is higher than the frequency corresponding to the motor time constant of the electric motor and in that the switched motor voltage is regulated to a voltage set value corresponding to a desired motor speed by influencing the modulated switching pulse sequence.
The modulation of the switching pulse sequence preferably is a pulse width modulation, but may also be a pulse frequency modulation or a pulse amplitude modulation.
The fact that the switching frequency is to be higher than the frequency corresponding to the motor time constant of the electric motor means in the present context that the motor winding inductance is not to have an effect on the switching frequency, i.e. that during the switching-off intervals of the pulse width modulated switching pulse sequence no, or virtually no, discharging of the magnetic energy is to take place that was stored in the motor winding inductance during the switching-on time.
With such a high switching clock frequency, the motor winding inductance cannot yet have an effect on the motor terminals. During the switching-off intervals of the switching pulse sequence, it is not yet possible that a generator voltage builds up that is caused by the magnetic discharge on the motor winding inductance. It is thus possible to directly measure the rectangular signal of the controlled switch. With these high switching or clock frequencies, the electric motor behaves as if a direct voltage were applied to the motor terminals that corresponds to the average value of the clocked, i.e. alternately switched on and off direct voltage. The electric motor is not subject to higher loads as if a direct voltage were applied that corresponds to this average value.
Thus, according to the invention, no speed regulation takes place, but instead a speed control with the aid of a regulated motor voltage takes place. As the speed of an electric motor is proportional to the motor voltage thereof, a regulated motor voltage has the effect of a speed regulation. One thus obtains a constant motor speed that is independent of the battery voltage.
The lengths and cross-sections of the electrical lines connecting the electric motor to the vehicle battery are at the discretion of the vehicle manufacturer and may vary from vehicle type to vehicle type. Corresponding differences are thus present in the electric resistances between the terminals of the vehicle battery and the motor terminals. Due to the fact that, in the speed control device according to the invention, the motor voltage is measured at the motor terminals, or at least very close to the motor terminals, the influence of differently dimensioned resistances of the electrical connecting lines between the vehicle battery and the electric motor is eliminated by the speed control device. In addition thereto, tolerance factors, aging etc. of the circuit components of the speed control device are compensated.
In a preferred embodiment of the speed control device according to the invention, the voltage regulation is effected with the aid of a regulating circuit having an integration means through which the clocked motor voltage is integrated for forming an average voltage serving as actual value, a comparator means comparing the actual value to a predetermined set value, and a regulating means effecting, depending on the result of the comparison, a pulse width modulation of the switching pulse sequence fed to the switch means.
The comparator means and the regulating means preferably are composed with a microprocessor to which the set value is supplied in the form of a digital signal. In this case, the integration means has an analog to digital converter associated therewith which converts the analog average voltage value formed by the integration means into a digital value.
In the speed control device according to the invention, the pulse duty factor of the switching pulses supplied to the switch means preferably is regulated such that the average voltage value of the clocked directed voltage supplied to the electric motor corresponds to the predetermined voltage set value. In case pulse frequency modulation or pulse amplitude modulation is employed, the frequency or the amplitude of the switching pulses, with constant switching-on time or pulse width, is regulated such that the electric motor is fed with a clocked direct voltage having an average voltage value corresponding to the set value.
By providing a corresponding predetermined set value, the average motor voltage may be varied continuously or in steps, depending on the operating condition of the additional heating apparatus of the vehicle. For example, it is advantageous for a soft start of the blower motor to predetermine a set value for a voltage curve increasing in ramp-like manner. It is also possible to preset a lower motor voltage and thus a lower speed during the ignition operation of the burner of the heating apparatus, since during ignition of the burner, less air is as a rule desired for the fuel-air mixture. After ignition, the amount of air delivered by the blower per unit of time may then be slowly increased by a continuously increasing the set value. It is also possible to control the heating power by correspondingly presetting the set value.
For blower motors as employed in usual additional heating apparatus of vehicles, a switching frequency is
Steiert Edwin
Wacker Heinrich
J. Eberspacher GmbH
Masih Karen
McGlew and Tuttle , P.C.
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