Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2002-06-25
2004-07-13
Nappi, Robert (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S685000, C388S800000, C388S811000, C388S901000
Reexamination Certificate
active
06762575
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the control of DC motors, and particularly, but not exclusively, to a method and apparatus providing electronic rotor pointing with high angular resolution using a sensorless permanent magnet DC motor.
There are numerous techniques in the prior art for controlling the positioning of a rotor of a permanent magnet (PM) DC motor. These techniques can be generalized into two major categories. The first category generally includes those techniques in which a PM DC motor is designed to move at a precise speed in synchronism with, or locked to, the waveform of the driving voltage or current which energizes the windings. PM stepper motors, which have a permanent magnet in the form of a rotor magnetized in alternate polarity “stripes” parallel to the rotor shaft, are used with this type of technique. The step size (angular resolution) of such a motor is entirely a function of the angular “width” of these magnetized stripes, and an angular resolution of 7.5 degrees is common in most inexpensive motors. However, costs increase greatly if such motors are to provide high resolution in the 2 to 5 arc minute range. Additionally, PM stepper motors move in steps by sending pulse trains of varying polarity to multiple windings. The frequency of the pulses and the phasing between the pulses applied to the various windings determines the speed and direction of motor motion, respectively. As a result, precise control over rotor positioning is provided, but at the cost of control circuitry complexity.
The second category of PM DC motor rotor positioning techniques includes those in which sensors are external to, or built into, the motor. Typically, such position sensors include Hall effect sensors and optical encoders. With Hall effect sensors, resolution is limited by the number, the positioning accuracy, and the gain tolerance of the sensors. With optical encoders, high resolution is provide at a higher cost. Optical encoders require electronics for decoding and accumulation. Such a system requires initialization, and over time may also require alignment and adjustment. Accommodating either Hall effect sensors or optical encoders also increases the size of the resulting device.
Conventional laser levels typically use the rotor of a sensorless permanent magnet DC motor to rotate a prism. The prism reflects a beam of laser light used in leveling operations. The operator controls such a sensorless DC motor using open loop control (no feedback) and therefore, moves the beam at a desired speed by increasing or decreasing the DC voltage applied to the motor. In order to position a beam in a desired direction with such an arrangement, the operator typically jogs the DC motor (applies a succession of voltage pulses) to point the laser beam at a small, distant object. However, with open loop control, rotor pointing by jogging the motor is inaccurate, often unrepeatable, and can be frustrating to the operator due to the lack of reliable beam pointing.
For example, stopping the laser beam on a two-inch wide object 100 feet away requires a DC motor speed of about one revolution per minute given an average human reaction time of 100 ms and a typical 6:1 drive ratio. However, rotating a DC motor at such slow speeds is problematic, since a motor's resolution changes with temperature and at different angular positions due to unavoidable variations in the manufacturing process and wear patterns of a motor's bearings. These variations make fine positioning operations at such a slow speed difficult with open loop control of a PM DC motor. However, due to space and cost considerations, using a stepper motor and/or position sensors in a laser level for closed loop rotor positioning control of the DC motor to provide acceptable resolution is not economical.
Therefore, a method and apparatus are needed for providing electronic rotor pointing to a sensorless permanent magnet DC motor, which take into account friction, temperature, bearing manufacturing, and wear pattern variations to provide accurate, repeatable, and reliable resolution position control.
SUMMARY OF THE INVENTION
The present invention is a sensorless control method and circuit which uses back electromagnetic force (EMF) as a feedback control to position a beam of electromagnetic energy within a few arc-minutes of a desired angular position. The system moves a sensorless PM DC motor in fine angular increments by applying short, high current drive pulses, which overcome static friction and induce movement of the rotor. A measurement of the rotor's angular distance moved due to the drive pulses is obtained by integration of a sampled back EMF voltage generated by the motor in a time window following each pulse. This measurement of the rotor's angular distance moved per pulse is used to control the pulse width or “on” time of the next drive pulse applied to the motor, thereby resulting in accurate, repeatable, and reliable fine positioning operation of about 2 to 5 arc minutes.
Sensorless control systems, such as the type provided by the present invention, possess a number of advantages. Although the present invention is not limited to specific advantages or functionality, it is noted these advantages include reduced component and sensor costs, reduced tooling and manufacture costs, improved reliability, and invariance to changes in the operating environment and noise reduction.
In one aspect of the invention, one embodiment comprises a method for providing improved angular resolution to a sensorless permanent magnet DC motor for rotor pointing. The method comprising supplying to the motor a pulse width modulated (PWM) motor drive pulse having an “on” time of a pulse width, and providing a sampling delay which prevents sampling of inductive stored energy of the motor. The method further includes providing after the sampling delay, a sampling window for sampling back EMF of the motor, and changing the pulse width of the “on” time of the motor drive pulse based on sampled back EMF in order to adjust rotor speed, thereby maintaining a set angular distance in a set time-period.
Another embodiment of the invention comprising a method of providing improved angular resolution at rotational speeds below about 1 rpm to a rotor of a sensorless permanent magnet DC motor used to move and position a beam of electromagnetic energy. The method comprises supplying to the motor a pulse width modulated (PWM) motor drive pulse having an “on” time of a pulse width which produces rotor motion, and an “off” time. The method further includes providing after a sampling delay, a sampling window for sampling back EMF of the motor during the “off” time, and changing the pulse width of the “on” time of the motor drive pulse. The “on” time is based on the sampled back EMF and its pulse width is varied to adjust rotor speed and maintain a set angular distance in a set time-period. The sampling delay prevents sampling of inductive stored energy of the motor after expiration of the “on” time.
In another aspect of the present invention, one embodiment provides a motor controller for driving and providing close loop control of a sensorless permanent magnet DC motor with improved angular resolution for rotor pointing. The motor controller comprises motor drive logic adapted to drive the motor with a series of motor drive pulses. Each of the motor drive pulses has an “on” time pulse width and an “off” time pulse width. Pulse width control logic is adapted to set the “on” time pulse width. The motor controller further includes feedback sample logic adapted to measure back EMF generated by the motor. The feedback sample logic provides a sampling delay which prevents sampling of inductive stored energy of the motor after expiration of the “on” time pulse width, and a sampling window for sampling the back EMF of the motor during the “off” time pulse width. The pulse width control logic is adapted to vary the “on” time pulse width of the motor drive pulse based on sampled back EMF in order to adjust rotor sp
Dinsmore & Shohl LLP
McCloud Renata D
Nappi Robert
Trimble Navigation Limited
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