Electricity: motive power systems – Positional servo systems – With particular motor control system responsive to the...
Patent
1994-08-09
1996-06-25
Ro, Bentsu
Electricity: motive power systems
Positional servo systems
With particular motor control system responsive to the...
318701000, 318260000, 318442000, 318599000, 318606000, H02P 800
Patent
active
055303323
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a drive circuit for a stepper motor.
BACKGROUND OF THE INVENTION
The background to the invention lies in a different field. In our British Patent Application published under no. 2 236 887 (which is incorporated herein by reference) there are disclosed rotatable mechanical coin escrow mechanisms. These are intended to be driven by an electric motor which rotates the drum of the mechanism by an integral number of complete rotations so as to cause coins selectively to be delivered to a cashbox, or returned to a user. The number of coins present in the escrow mechanism when it is to be driven varies greatly, and so therefore does the inertia of the mechanism, this being rather small when only the empty plastic drum has to be rotated but two or more times as great when the drum is carrying its maximum number of coins. The amount of friction to be overcome, both in the bearings of the drum and between the coins and the drum, also varies considerably.
A feasible, and conventional, system for rotating the drum would be a small high-speed DC electric motor driving the drum through a large reduction ratio gearbox, in conjunction with an indexing system such as an optically-sensed tooth wheel, for the purpose of detecting absolute drum position.
Low cost is an essential requirement in the applications for which such escrow mechanisms are intended, such as pay telephones, and a further requirement in that context is for the drive power to be derived from the telephone line.
Cheap, high-speed (say 3000 rpm) DC electric motors do not have sophisticated and therefore relatively expensive very low friction bearings, and relatively high frictional losses are also characteristic of cheap high reduction ratio gearboxes. It is therefore very difficult to produce cheap drive systems of the above kind which will have a sufficiently low power consumption (typically less than 100 mW) to permit their operation from telephone lines.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a low-cost, low power consumption, electric drive system.
The present invention involves the realisation that a stepper motor could beneficially be used to drive such a coin escrow because its inherently low rotational speed means that friction losses in its own bearings are small, and also that a gearbox of much smaller reduction ratio can be used so that friction losses in the gearbox can also be reduced.
Various drive circuits for stepper motors are known, for controlling the application of the necessary drive pulses to the motor windings, but these have normally been designed with the object of achieving a desired output performance from the motor irrespective of power consumption. In the context envisaged by the present invention, although certain output performance criteria need to be met, nevertheless an overriding requirement is for the input power to be minimised to the greatest extent possible.
One performance criterion is that the motor should be able to rotate the drum of the escrow mechanism through each complete rotation in an acceptably short period of time, for example 400 ms, while consuming as little power as possible, and for this speed of rotation to be achieved within acceptable power limits even when the inertia and friction of a load in which both these factors may vary are at their maxima.
According to a first aspect of the present invention, there is provided a drive circuit for a stepper motor intended to drive a load, which circuit is arranged in use to operate in a chopper drive mode during a motor acceleration phase, characterised in that said circuit is arranged in use to operate in an L/R (non-chopped) drive mode during a subsequent cruise phase.
According to a second aspect of the present invention there is provided apparatus for driving a stepper motor intended to drive a load, characterised in that said apparatus comprises means for determining, before driving the load, the expected load to be driven and means for selecting a level of power i
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Mars Incorporated
Ro Bentsu
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