Electricity: motive power systems – Positional servo systems – Pulse-width modulated power input to motor
Patent
1996-10-29
1998-01-13
Ip, Paul
Electricity: motive power systems
Positional servo systems
Pulse-width modulated power input to motor
318139, 318254, 318269, 388806, H02P 706, H02P 308
Patent
active
057083436
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a control device for a direct current electric motor suited to be used for PWM control of a direct current electric motor such as an auxiliary motor for driving a bicycle.
BACKGROUND ART
A bicycle provided with an auxiliary motor has been known. In this kind of bicycle, treading force on the pedals is detected, and an auxiliary which is suitable to the treading force is generated by the motor, so that the wheels are driven by a resultant force of the auxiliary torque and the treading force. In order that the rider comfortably pedal, the auxiliary torque supplementing to the treading force to the pedals should be exactly generated.
Usually, the auxiliary torque is adjusted by a PWM (pulse width modulation) control to the electric current supplied to the motor. In this case, a control signal is supplied to a switching element, for example, a gate of an FET, whereby a pulse current, having a duty factor calculated by a control unit such as a micro computer, is provided to the motor. A relay is interposed between the FET and the motor for stopping the drive system when the main switch is turned off or when the FET is broken.
In the auxiliary motor-equipped bicycle, the rate of the auxiliary torque to the pedaling torque is controlled to be gradually reduced according to the velocity of the bicycle when the velocity is above a first threshold velocity Vth1 (for example, 23 km/h) . Also, the ratio is controlled to be zero above a second threshold velocity Vth2 (for example, 26 km/h). For this purpose, when the velocity is increased to the first threshold velocity Vth1 or higher, the duty factor of the pulse for the PWM control is gradually reduced according to the excess of the velocity, and when the velocity reaches the second threshold velocity Vth2 or higher, the duty factor becomes zero, so that no auxiliary torque is generated by the motor. In other words, the PWM control for the motor is interrupted.
As the above-mentioned relay, a normally open type relay is used. After the main switch of a circuit is turned off, the current supply to the exciting coil of the normally open type relay is stopped, whereby the normally open type relay breaks the circuit. After the main switch is turned off, when the circuit voltage is lowered to be a certain value, the contact points of the relay are separated from each other. Until the contact points of the relay is separated, an induced voltage of the motor is applied backward between the gate and the source of the FET, so as to cause a failure of the FET.
As described above, when the velocity reaches the second threshold velocity Vth2, the auxiliary torque is controlled to be zero. Then, the duty factor of the current supplied to the motor becomes zero, and the FET is maintained to be off. However, although the current supply to the motor is stopped, the motor does not stop to revolve immediately because of the inertia, so as to generate electric energy. Consequently, an induced voltage is generated between the terminals of the motor, so as to cause a failure of the FET as in the above-mentioned case.
Therefore, some countermeasures are needed: for example, a sort of FET, of which the rated voltage between the gate and the source is high, is employed. Alternatively, a Zener diode is interposed between the gate and the source, so that the uppermost limit of the voltage between the gate and the source is restricted.
However, Zener diodes and FETs, which can be used under high voltage, are usually expensive., Therefore, the production cost is extremely increased. In order to lower the cost, it is necessary to protect the FET by countermeasures other than the above.
DISCLOSURE OF INVENTION
It is, therefore, an object of the invention to propose a control device for a direct current electric motor, by which when the main switch is accidentally turned off or the like, a switching element such as an FET can be protected without increasing the production cost.
In order to resolve the above problems, according to the inventio
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Hara Nobuo
Ogata Hiroaki
Suenari Masaya
Ip Paul
Yamaha Hatsudoki Kabushiki Kaisha
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