Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Indication or control of braking – acceleration – or deceleration
Reexamination Certificate
2000-05-31
2002-05-07
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Indication or control of braking, acceleration, or deceleration
C701S079000, C318S112000, C318S161000
Reexamination Certificate
active
06385522
ABSTRACT:
This application is a national stage filing under 35 U.S.C. §371 and priority is hereby claimed on International Application No. PCT/GB98/02039, Filed Jul. 10, 1998, which International Application was published in English as No. WO 99/02363.
The present invention relates in general to brake control in apparatus comprising an electric motor arranged to drive a wheel. In particular, although not exclusively, the present invention relates to electric vehicles, and is especially relevant to brushless motor-driven wheelchairs controlled by the user solely by means of a joystick.
The joystick is usually the only means by which the user can control an electric wheelchair. The wheelchair is often fitted with a static brake, that is a brake which ideally is to be applied only when the wheelchair is stationary, to hold the chair and prevent undesired further movement after coming to rest on level or, more importantly, sloping ground.
In the case of wheelchairs in which the wheels are driven directly by brushless dc motors, the static brake is particularly important because of the absence of gearing which would increase rolling resistance and assist in holding the chair when the motor windings were not excited. It is important that the motor controller connected to the joystick can operate the static brakes satisfactorily as the user normally has no direct control over them. Ideally, the static brake should come on immediately-and automatically after the chair has been dynamically slowed by the motors and stopped. Good brake timing prevents rolling backwards or forwards after slowing down on gradients. It also reduces wear on the brakes.
A problem is that on a brushless system often there is no speed feedback of sufficient quality to tell the motor controller when to apply the static brakes (ie. tell the controller that the wheels are revolving slowly enough).
Ideally one would like to monitor the back EMF of the motor, which would of course fall to zero when the motor stopped. However, when the motor is slowing down the voltage measured across its terminals is equal to the back EMF plus IR (where I is the current flowing through the motor and R is the resistance of the motor). Thus, if one tries to calculate the back EMF from this measured voltage for use as a type of speed feedback there are three sources of error: the measurement of the voltage across the motor; the measurement of the current flowing through the motor windings; and the assumed resistance of the motor. The resistance of the motor can vary with temperature, and IR becomes large compared to the back EMF at low speeds when dynamically braking. Accordingly, speed feedback values will become increasingly inaccurate at low speeds. Unfortunately, this is at exactly the point one would like an accurate measure of the speed for the correct timing of static braking.
To add quality speed feedback sensors would be expensive, and so, in the past, other indirect techniques have been employed to guess at the wheelchair's velocity and apply the static brake at an appropriate time.
One of these techniques has been to apply the static brake a fixed time interval after the motor control voltage (set according to signals from the joystick, and which will also be referred to as the demand voltage) has fallen to zero. A disadvantage of this technique is, however, that a fixed time interval suitable for use when the chair is stopping on level ground may be too long when the vehicle is stopping on an uphill slope, resulting in rollback, and too short when the vehicle is stopping on a downhill slope, resulting in the static brake being applied when the chair is travelling at inappropriately high speed, causing wear on the brakes and possible skidding.
It is known to set the fixed time interval according to parameters such as the weight of the user and the weight of the chair, but the problem of ensuring correct static brake application on a variety of slopes remains.
It is an object of embodiments of the present invention to provide apparatus comprising an electric motor arranged to drive a wheel with improved brake control.
It is a further object of embodiments of the present invention to provide an electric vehicle with improved brake control.
It is a further object of embodiments of the present invention to provide an improved brake control method.
According to a first aspect of the present invention there is provided brake control apparatus comprising:
an electric motor arranged to drive a wheel;
brake means operable to apply a braking force to inhibit the rotation of said wheel;
input means operable to generate an input signal indicative of a desired angular velocity of said wheel;
current monitoring means for generating a monitoring signal indicative of a current flowing through said electric motor; and
control means, for controlling the supply of power to said electric motor in response to said input signal by generating a control voltage, and for controlling said brake means such that after setting said control voltage to zero said brake means apply said braking force at a braking time determined according to said monitoring signal.
By determining the braking time according to the monitoring signal, which in turn is determined by the motor current, the control means is able to control the brake means to apply the braking force to the wheel at an appropriate time when the wheel is stopping in a variety of load configurations.
The apparatus may be comprised in an electric vehicle, providing the advantage that the control means is able to control the brake means to apply the braking force to the wheel at an appropriate time when the vehicle is stopping on a variety of slopes, thus reducing wear on the braking means and rollback or skidding.
Advantageously the braking time may be determined according to the magnitude of the monitoring signal when the control voltage is set to zero. In the case of electric vehicles, for a range of slopes, the motor regeneration current at this time has been found to be a good indication of the time that the vehicle would take to come to a halt under dynamic braking on the particular slope, after the zero demand point. This current has also been found to be largely independent of the vehicle speed at the start of the stopping process.
The control means may comprise a microprocessor, and the control voltage and braking time may be calculated according to the input signals and monitoring signal respectively.
The control unit may be operable to control the brake means to apply the braking force a delay time interval after setting the control voltage to zero, this delay time interval being determined by the monitoring signal.
The delay time interval may be determined according to the magnitude of the monitoring signal substantially at the time when the control voltage is set to zero, and this delay time interval may be increased as the magnitude of the monitoring signal at this time increases over at least a range of values.
Again, in the case of electric vehicles, the motor regeneration current at this time has been found to be a good indication of the time that the vehicle would take to come to a halt under dynamic braking on a particular slope, and by increasing the delay time interval with increasing monitoring signal the control means is able to operate the brake at a time appropriate to the slope.
In embodiments of the present invention where the control unit comprises a microprocessor, the delay time interval may be calculated according to the monitoring signal, and advantageously may be calculated according to an algorithm which includes the step of calculating a quantity which is proportional to the magnitude of the motor current when the control voltage is set to zero, i.e. at the time the control voltage substantially reaches zero in response to the input signal having reached a value indicative of the user wishing to stop the rotation of the wheel.
Of course, the delay time interval may also be dependent on the sign of the monitoring signal.
The algorithm may include the step of calculating a
Elliott Industries Limited
Nguyen Tan
Tran Dalena
Westman Nickolas E.
Westman Champlin & Kelly
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