This application relates to a method of monitoring the position of a component driven by a DC brushed electric motor, which counts a periodic back electromotive force.
DC motors are utilized in various industries to drive any number of components between different positions. As one volume application, DC motors are utilized in automotive applications to move windows, etc. For a number of reasons, the position of the component driven by the electric motor is desirably detected. The end of travel position must be detected such that current flow to the motor can be stopped. In motors utilized in automotive application, the motor current to the windows is monitored. The motor current would increase at the end of travel position of a window, for example. Motor current is a function of torque, which would increase when the window reaches it end of travel position.
Unfortunately, motor current and torque can vary greatly based upon age of the component and motor, and based upon environmental issues such as heat or dust. The use of the known motor controls may result in a false stopping of current because higher torque levels are reached prematurely.
Further, the use of electric motors in some applications, and in particular for driving valves for water, can result in heating of the components. The flow of hot water, as an example, could result in changing torque, such that the motor current is read inaccurately as to end of travel position. In addition, these known controls would not provide good indication of the motor reaching some intermediate position for an application, which would desirably be able to stop the component at a plurality of positions.
DC motors may utilize a brush, or may be brushless. In brushless motors, a technique for determining position is known wherein a back EMF force is counted. However, this method has not been utilized in brushed DC motors.
In a disclosed embodiment of this invention, a back EMF current generated by a brushed electric motor is identified. Applicant has discovered that with each rotation of the motor rotor, as the brushes and commutator move out of engagement, a “ripple” or back EMF force is induced into the motor current. This small ripple can be isolated, or filtered out, of the overall current signal, and counted. By counting the number of occasions of this periodic “ripple,” the present invention is able to provide a very accurate indication of then number of rotations of the rotor, and hence the position of the component. This method is relatively insensitive to environmental changes. Also, the present invention is capable of stopping a motor precisely at any number of intermediate locations.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
An electric motor-driven water valve system 20 is illustrated in
Applicant has discovered that as the brushes 34 move out of contact with the commutators 36, a small ripple R is induced in a main motor current supply signal as shown in
Applicant filters the main current supply signal, and isolates this back EMF force into a series of square waves such as shown in
The present invention thus provides a very simple way of identifying the position of a brushed DC electric motor driven component. The method can be included into off-the-shelf existing motors, and is relatively insensitive to environmental changes.
Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.