Electricity: motive power systems – Positional servo systems – Pulse-width modulated power input to motor
Patent
1998-03-17
2000-02-29
Sircus, Brian
Electricity: motive power systems
Positional servo systems
Pulse-width modulated power input to motor
318656, 318810, 388801, 388812, G05B 1128
Patent
active
060313501
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to means for driving and controlling an electric motor. Particularly, but not exclusively the invention relates to means for driving and controlling a linear electromagnetic motor, which comprises an integral position measurement system providing information on the output movement produced and enabling servo position control. A motor of this type is described in a prior GB Patent Application 9517603.8.
The control electronics required by such motors must provide power drive to the device, in order for it to produce mechanical output power, and must simultaneously provide precision excitation signals for the integral position measurement system. In addition a means of extracting and signal conditioning the resultant position signal and for implementing the servo control loop is required. The performance. efficiency, and cost of manufacture of systems employing such actuators is critically dependent on the concept and design of the associated drive and control electronics.
FIG. 1 of the accompanying drawing shows a linear motor and integral position measurement system of the type to which this invention is particularly relevant, and FIGS. 2a, 2b and 2c show the equivalent circuit or electrical schematic representations of the motor. The figures show the motor to consist basically of two or four inductive circuit elements which are connected in a half bridge according to FIGS. 2a and 2b or full bridge configuration as in FIG. 2c. DC or quasi-static DC drive voltage is applied across the bridge in combination with a superimposed steady state AC excitation signal to provide energization for the position measurement function. The resultant position signal appears as a single ended output at the node of the half bridge circuit or as a differential output at the mid point of the full bridge configuration. Imbalance of the bridge is bought about by movement of the magnet and pole piece assembly--which creates differential incremental changes in the inductance of the windings. As described in GB Patent application 9517603.8, interconnection of the windings is arranged such that the forces produced on the magnet assembly by current flow in the coils contribute in unison, whilst the incremental changes in inductance are lumped according to their sign in the relevant arms of the half bridge or full bridge circuits.
The present invention sets out to provide a combined power drive, transducer excitation and signal conditioning system and position control loop, which performs simultaneously to maxirnise the overall electromechanical efficiency of the system and to realise the capabilities of the integral position measurement of the actuator.
According to the invention there is provided an electronic system, comprising a power amplifier with antiphase outputs for driving the actuator/transducer assembly, a pre-amplifier for receiving output signals from the actuator/transducer assembly, an input command signal, and an excitation and signal conditioning circuit which functions to combine the outputs of the pre-amplifier, the input command signal, and a periodic excitation signal to provide input signals to the power amplifier for driving the actuator/transducer assembly.
The excitation and signal conditioning circuit may comprises an error conditioning and converter stage, which provides input to the power amplifier and a transducer excitation function generator, which provides periodic signals for combination with the input signals to the power amplifier.
Preferably the excitation and signal conditioning circuit includes a stage which transforms the input command signal supplied to the system to a suitable form for feeding a summation circuit, which derives a position error signal; and that the pre-amplifier supplies the actuator/transducer output signal to a second input of the summation circuit. The power amplifier may be a switching power amplifier.
The input command signal may be converted to a proportional AC signal before being compared with a position feedback signal, alternatively the
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