1. Field of the Invention
The present invention relates generally to systems and methods of controlling electric motors. More particularly, the present invention relates to an electronic motor controller that dynamically adjusts a motor resistance parameter used in the control of an electric motor.
2. Description of the Related Art
Electric motors are used in a wide variety of applications. When electric motors are used to drive a vehicle, they are typically controlled by an electric motor speed controller. Generally, a design goal for such motor speed controllers is to control the operational characteristics of the motor so that the motor consistently operates smoothly and safely in a manner that provides a comfortable feel for the operator.
In order to properly control an electric motor, many motor controllers store a set of motor parameters that characterize the electric motor being controlled. The motor controller uses these motor parameters to translate user input into control signals for the electric motor. One motor parameter that is frequently used by the controller is the resistance of the motor. Knowing the resistance of the motor may be useful in a number of control applications. For instance, the motor resistance may be combined with the current and voltage to estimate the speed of the motor—which is particularly useful in vehicles that don't have speedometers to directly measure their speed.
In some motor speed controllers, the resistance of the motor used by the motor controller is a set value. The set resistance value may be a resistance value provided by the motor manufacturer (typically available on a data sheet) or the vehicle manufacturer (typically determined experimentally). However, this set resistance value does not always accurately represent the actual resistance of the motor. For instance, the actual motor resistance may fluctuate during operation based on operating characteristics such as on the temperature of the motor. It is generally known that a direct relationship exists between the resistance and temperature of the motor. As the motor increases in temperature during operation, the resistance of the motor will also increase. Similarly, as the motor decreases in temperature, the resistance of the motor will also decrease. Factors that may lead to temperature fluctuations in the motor include the load applied to the motor. For example, the motor temperature may increase as the vehicle travels up a hill and decrease when the vehicle remains stationary or is lightly used. For these reasons, the resistance of the motor may fluctuate during operation. In some applications the operational resistance fluctuations can be quite dramatic. For example, in small, low cost, brushed permanent magnet DC motors often used in electric scooter and electric wheelchair applications, the actual resistance of the motor can vary dramatically during operation—which can have a very significant impact on performance when motor resistance is used as motor speed control parameter.
In order to address this issue, attempts have been made to estimate or calculate the resistance of the motor and to adjust the motor resistance parameter accordingly. One way to estimate the operational variations in resistance is to measure the actual motor temperature. This generally works satisfactorily because the relationship between motor resistance and temperature is generally known. However, the need to measure motor temperatures adds costs and complexity to both the motor and controller design. For example, the additional sensors required to measure motor temperature are not desirable since they represent potential points for failure and added cost.
Of course, the problem of speed control may be addressed in motor controllers through a variety of other methods as well. For example, speedometers may be used to measure the actual vehicle speed. However, these options are generally more costly and/or require additional hardware. Therefore, it would be beneficial if a cost sensitive mechanism were created to account for motor or vehicle operating characteristics that vary over time or otherwise occur after initial production of the vehicle. Similarly, it would be desirable if a cost sensitive mechanism were capable of dynamically adjusting the motor resistance parameter stored in the motor controller in response to measurements received from the motor during operation.