Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-06-08
2001-10-16
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C324S207160, C324S207130, C318S254100
Reexamination Certificate
active
06304014
ABSTRACT:
The present invention relates to an apparatus for and a method of controlling electric motors. The invention has particular, although not exclusive, relevance to the electronic commutation of brushless motors.
Existing servo-controlled motors sense the motor's current position, speed and/or torque and use this in a feedback control loop to control the current applied to the stator and/or rotor coils in order to obtain/maintain a desired position/speed and/or output torque. Brushless motors also need to monitor the relative position of the rotor and stator in order to control the commutation of the drive current into the stator and/or rotor coils. To date, a separate position sensor has been used to perform these two control functions. In particular, most systems employ a plurality of Hall effect sensors spaced apart around the stator (typically three spaced apart by 60°) for controlling the commutation and use either an optical encoder or a resolver for providing position/velocity/acceleration feedback for controlling position, speed and/or output torque of the motor by controlling the amount of drive current applied to the stator and/or rotor coils.
The Hall effect sensors, whilst being relatively inexpensive and reliable, suffer from a number of problems. These include that the output signals can be noisy because they are susceptible to electromagnetic interference and their output characteristics change with temperature. Furthermore, since a plurality of individual Hall effect sensors are used, they must be accurately aligned relative to the stator in order to obtain the correct phase relationship between the commutation signals. In any event, errors in the commutation signals will be inevitable because each of the Hall effect sensors will have a slightly different characteristic to the others.
With regard to the resolvers, they suffer from the problem of a poor response time to changes in, for example, the desired motor speed or motor load. This is because they sense position using iron-cored pick-up coils which have an “inertia” due to magnetic inertia of the iron core. This inertia increases their response time to changes in motor speed. Additionally, the coil which is mounted with the rotor is relatively heavy and must be securely attached and balanced on the rotor so that it does not cause mechanical imbalance at high speeds.
With regard to optical sensors, both incremental and absolute optical sensors are available. An absolute optical sensor could be used to control both the commutation and the drive to the motor. However, absolute optical position sensors are expensive and therefore servo motors which employ optical sensors usually employ an incremental optical sensor for providing position/velocity/acceleration feedback to a controller which controls position/speed and/or output torque of the motor; and separate Hall effect sensors for controlling the commutation. A further problem with the optical system is that they need to be isolated from the environment so that they do not become contaminated with dirt.
One aim of the present invention is to provide a brushless motor which employs an alternative type of position sensor.
According to one aspect, the present invention provides an electric motor comprising: a stator and an element movable relative to said stator; a plurality of energising coils; means for switching a supply current to one or more of said energising coils; a position sensor for sensing the position of said movable element, the position sensor comprising a plurality of circuits each having at least two loops connected in series and wound in opposite sense; and a field generator for generating a field, which generator, is adapted, in use, to be electromagnetically coupled to each of said circuits such that, in response to an electromagnetic field generated by said field generator, a plurality of output signals are generated respectively dependent upon the electromagnetic coupling between the field generator and the respective said circuits; and means for processing said plurality of output signals to generate a plurality of control signals for controlling said switching means.
In one embodiment, the output signals vary periodically with said relative position and out of phase with respect to each other. Preferably, means are provided for varying the phase of said signals so that the position of the first and second circuits and of said field generator relative to said rotor do not have to be accurately aligned during manufacture.
In another embodiment, the output signals from the position sensor are used in a feedback control loop to control the position, speed and/or output torque of the motor.
According to another aspect, the present invention provides a position sensor comprising first and second members which are adapted, in use, to be movable relative to each other; said first member comprising a first circuit having at least two loops connected in opposite sense and a second circuit; said second member comprising a field generator having a first portion which is adapted, in use, to have a substantially constant electromagnetic coupling with said second circuit and a second portion having at least two loops connected in series and being arranged so that signals generated in said at least two loops by a common electromagnetic field oppose each other; wherein the loops of the second portion are adapted in use to be electromagnetically coupled to said loops of the first circuit, which coupling varies as a function of the relative position of said first and second members, such that in response to an input driving signal applied to one of said first and second circuits, said field generator generates an electromagnetic field which in turn generates an output signal in the other one of said first and second circuits, which output signal varies as a function of the relative position of said first and second members.
A further aspect of the present invention provides a similar system to the first aspect except instead of using the output from the position sensor to control the commutation of the drive current to the energising coils, the output signals from the position sensor are used by a controller to control, for example, the motor position, speed and/or output torque.
According to another aspect, the present invention provides an electric motor comprising: a stator and an element movable relative to the stator; a plurality of energising coils; means for switching a supply current to one or more of the energising coils; means for sensing the position of said movable element relative to said stator and for generating a plurality of output signals which periodically vary with said relative position and out of phase with respect to each other; and phase shift means for shifting the phase of said plurality of periodically varying output signals to generate a plurality of control signals for controlling said switching means.
The present invention also provides a circuit for use in a position sensor comprising at least two series connected alternate sense loops of conductor and wherein the loops are arranged to cross each other to define a plurality of sub-loops, adjacent ones of which have opposite sense.
According to another aspect, the present invention provides a circuit for use in a position sensor comprising a first portion having at least one loop and a second portion connected in series with the first portion, the second portion comprising a number of loops connected in series and being arranged so that signals generated in adjacent loops by a common magnetic field oppose each other.
Preferably in this circuit, the first portion comprises a plurality of concentric loops connected in series so that signals generated in at least two of the loops by a common electromagnetic field oppose each other. In this way, both the first and second portions can be made relatively immune to electromagnetic interference.
According to a further aspect, the present invention provides a circuit for use in a position sensor comprising a plurality of series con
Burwell Malcolm
Dames Andrew N.
Ely David T. E.
England James M. C.
Foote Geoffrey
Mullins Burton S.
Nixon & Vanderhye P.C.
Synaptics (UK) Limited
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