Motor control

Details Motor control Motor control

2000-09-06

2002-09-03

Masih, Karen (Department: 2837)

Electricity: motive power systems

Induction motor systems

C318S811000, C318S599000, C318S798000, C318S815000, C318S434000, C318S434000

Reexamination Certificate

active

06445155

ABSTRACT:

This invention relates to improvements in motor control strategies, and to motor control and drive circuits incorporating such improved strategies. It especially relates to improvements in motor control strategies for motors in electric power assisted steering systems, although wider applications are envisaged.
Electric motors are becoming increasingly common parts of everyday machines. One area of great interest is the use of an electric motor to apply an assistance torque to a portion of a steering column shaft in order to make the steering wheel of a vehicle easier to turn. By sensing the torque demanded by the driver as the steering wheel is turned, a motor drive signal can be produced which in turn drives a motor connected operatively to the steering column. The motor applies a torque of the same sense as the driver demand to the steering column.
To meet the demands for smooth torque and precise motor drive characteristics, it is known to provide an electric motor comprising a brushless rotor having a number of permanent magnets which rotates within a stator comprising a number (typically three) of phase windings. The phase windings are connected together in a delta or a star arrangement and can be controlled using pulse width modulated signals applied to switching devices in a bridge circuit.
In order to accurately control the motor a measurement of the current flowing in the motor, which enables motor torque to be determined, must be made. It has been proposed that a number of current sensing resistors provided in the motor drive circuit can be employed to take the motor current measurement. A single resistor is provided in series in each phase of the motor, the voltage developed across each resistor being proportional to the current flowing through the resistor. However, this necessitates the need for multiple current sensing circuits, one per resistor.
Alternatively one current sensing resistor could be eliminated since it is known that the algebraic sum of the currents flowing in all phases of the motor must equal zero. Therefore for an n phase motor, (n−i−) current sensing resistors and circuits would be needed.
In an alternative, it has been proposed that only a single sense resistor is used. This configuration reduces cost and component count and is not susceptible to measurement inaccuracies that can occur when two or more sense resistors are used, due to different component and sensing circuit tolerances associated with each current sensor. More accurate motor control can be achieved if measurements are made with precise timing, and offset voltages present in the current measuring circuitry are eliminated. Such a single sense resistor is typically placed in the circuit so as to measure the total instantaneous current flowing between a D.C. power supply and the bridge circuit and motor combination.
In accordance with a first aspect, the invention provides a method of monitoring the operation of a brushless motor comprising a number of phases each comprising one or more windings connected in a bridge circuit, the bridge circuit comprising a number of arms with one arm for each phase, each arm comprising a top switching device connected between the phase and a first supply voltage and a bottom switching device connected between the phase and a second, different, supply voltage, each device being switchable from an on state to an off state the method comprising the steps of: monitoring the current flowing into or out of the bridge circuit and motor using a current measurement means to produce an output signal indicative of the current;
measuring the output of the current measurement means when the instantaneous current flow through the current measurement means is known to be substantially zero;
and producing a modified output signal which is compensated for any differences between the actual measured output signal value and an ideal output signal value.
By switched on we mean that the switching device presents a low impedance to the arm and switched off we mean that it presents a high impedance to the arm.
By monitoring the output of the current measurement means corresponding to the instant when the current in the current measurement means is known to be zero, any offset or drift in the output signal from the current measurement means can be detected and compensated. If required, the output signal can then be reset to zero to produce the modified output value. This can be achieved by generating an offset value substantially equal in magnitude to the actual measured value. This can be added to or subtracted from the actual measured value to force the modified output signal to zero for zero current flow (or some other “ideal” value) by comparing the actual output to the ideal output. Indeed, the intention is that the compensating value can then be subtracted from any output signal value (even for non-zero currents) to compensate for any zero-offset.
By measuring the output of the current measurement means at instants when the current flowing through the current measurement means is known to be zero whilst the motor is running, the output may be compensated “on-line” whilst the motor is running. By this, we mean that, if desired, the measurements can be taken whilst the motor is operating in any one of its operating quadrants, i.e. motoring, braking etc. This provides increased flexibility over a system where compensation is made when the motor is switched off. For instance, a more regularly updated compensating value can be obtained without waiting for the motor to be switched off. This is especially useful where the motor forms a part of an electric power steering system, as the zero current measurements can be made periodically or randomly whilst the vehicle is being driven and, for example, the motor is producing torque whether rotating or otherwise.
The method may include a step of adding the offset value to the output signal from the current measurement means, or subtracting the offset value from the output signal value of the current measurement means to produce the modified output signal.
The method is especially suited to monitoring three phase motors although it may be adapted to control motors having more than three phases. The method may further include steps of controlling the motor by applying suitable signals to each switching device to vary the average voltage applied to each phase of the motor whilst also allowing the zero current measurement to be made.
The respective signal applied to each switching device may comprise a pulse width modulated signal. Preferably all the switching devices are modulated by respective pulse width modulated signals that have the same synchronised modulation cycle time.
Preferably, the signals are chosen so that during each or selected pulse width modulation cycles the instantaneous current into or out of the bridge circuit is zero at a first instant independent of the net current in the cycle. This enables a zero measurement to be made when the motor is running, regardless of the overall net motor current.
The preferred pulse width modulated signal applied to each switching device is defined by a single ON-OFF transition and a single OFF-ON transition or edge within each cycle.
The location of the ON-OFF edges within a or each cycle are preferably chosen so that all the top devices are switched on whilst all the bottom devices are switched off at the same time for at least a first instance within each period. This ensures that zero overall current flows into or out of the bridge circuit, and hence through the current measurement means during each period at that first instance. The current measurement means output at this instance can be monitored to produce the actual zero current measurement.
Alternatively, the edge positions are chosen so that at the first instance all the top switching devices are switched off whilst the bottom switching devices are switched on. This also ensures that zero current flows into or out of the bridge circuit, and hence through the current measurement means.
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