EP0241531A1 - Device for automatic control of direct current motors - Google Patents

Abstract

Device for automatic control of direct current motors, characterized in that an armature current control unit (10) is mechanically fixed to the rotor of the motor so as to rotate at the same time as the latter and receives a voltage of positive and negative continuous power supply via two sliding rings (18 and 20) acting in concert with two brushes (22 and 24) connected in turn to the direct current power source, while organs information transmission systems located outside the motor provide the control unit (10) with the information necessary for controlling the armature current.

Description

DEVICE POR AUTOMATIC CONTROL OF DIRECT CURRENT MOTORS

This invention relates to a device designed to provide automatic control of a DC motor speed or torque.

In existing low-power DC motors the magnetic field is generally obtained by means of either a magnet or two or more windings, while the armature usually consists of three to five field poles in which the direct current flowing through the windings is switched by a commutator comprising two stationary brushes that slide on a rotating segmented commutator to automatically control the switching sequence.

DC motors of this type are known to have several drawbacks, the main one being that temporary switching shorts occur on the segmented commutator and on brushes while the motor is running, and these shorts reduce the motor life in addition to generating switching noise.

An additional serious drawback in such motors is that changing the operation parameters, such as rotation speed and torque is difficult to do. That is why these motors are generally run at constant speed and several types of speed controls have been designed.

One such control is based on a switch operated by centrifugal force to open as soon as a preset speed is exceeded and close as soon as speed drops below that value. This type control constantly switches from one state to the other and generates switching noise as well as random stresses and vibrations during operation which may prove detrimental in certain applications.

Other recently developed control types use electronic control devices of both the open loop and closed loop type or feedback type.

In order to increase the motor life and eliminate the brush switching noise, brushless motors have been developed recently, where the stationary magnetic field is switched while the magnet rotates integral with the shaft axis.

These motors, however, have lower power than conventional brush motors, though being in the same size.

The main object of this invention is, therefore, to provide a device for automatic control of a DC motor whereby no commutator switching is used though operating a switched rotating armature motor in order to obtain unaltered power output without increasing the motor size.

Another object of this invention is to provide a highly accurate most reliable control device able to operate under heavy operating conditions.

One more object of this invention is to provide a multipurpose control device, easy to operate to obtain different control modes.

One further object of this invention is to provide a control device of simplified and low-cost construction suitable for mass production.

The DC motor automatic control device according to this invention is characterized in that one armature current control unit is mechanically attached to the motor rotor so as to rotate along with it and receives positive and negative DC supply voltage through two continuous slip rings cooperating with two brushes which are in turn connected to the DC power source, while appropriate information transmission media outside the motor provide the control unit with the information required to accomplish armature current control.

Several advantages are, therefore, offered by the device according to this invention, the main one being that no switching of power supply current on the segmented commutator, as employed in motors of past technology, is required.

Another advantage lies in the considerable increase of motor life under the same stress conditions .

An additional advantage is the ability to change the motor operating parameters, as required, through the action of information transmission media, and extend this ability to the point of providing step-by-step mode of operation.

One further advantage is that closed loop control can be provided in the motor by simply comparing within the control unit the motor output parameters, e.g. position and speed, with the externally provided information.

In the following, the present invention will be further clarified by the description of one practical embodiment of the DC motor automatic control device, a description made out in a purely illustrative and not limitative way, with reference to the accompanying drawings, in which: figure 1 is a schematic perspective view showing the position of the control device according to this invention; figure 2 is a bloch diagram of the control device circuitry according to this invention; figure 3 is a circuit diagram of one embodiment of the power circuit; figure 4 is a time-phase diagram of the rotor winding supply currents; and figure 5 is a circuit diagram of a second embodiment of the power circuit.

Referring to the accompanying drawings, and in particular to figure 1, it can be seen that the present preferred embodiment of the automatic DC motor control device according to this invention is comprised of a control unit 10, which is mechanically attached to the motor rotor to rotate along with it and is connected to windings 12, 14, 16 which produce magnetic fields on the rotor field poles.

Control unit 10 receives positive and negative DC supply voltage through two slip rings 18 and 20 and two brushes 22 and 24 which are in turn connected to the DC power source that supplies the power required for operation of the motor.

Control unit 10 is also connected, through six leads 26, 28, 30, 32, 34, and 36 to the six segments 38, 40, 42, 44, 46, and 48 of a segmented commutator 50 that come into contact with brush 52. Brush 52 is in turn connected to a control signal generator 54 and serves to provide control unit 10 with the desired control mode information, as will be better described later.

Referring to the block diagram of figure 2, control unit 10 includes a decoding circuit 56 that detects the desired speed information, the present rotation revolutions per minute information, and the desired rotation timing and direction information.

The desired speed information is sent, through lead 58, to oscillator circuit 60 whose frequency is present on lead 62 and can be varied according to the information conveyed by the desired speed signal in the form of one voltage value.

The present rotation revolutions per minute information, which is dependent upon the actual motor speed, is sent from decoding circuit 56, through lead 64, to a frequency comparator 66, where it is compared with the oscillator 60 frequency to generate, over lead 68, a frequency error signal representing the difference between the desired and actual motor speed.

This error signal is processed in a speed control circuit 70 and converted into a supply voltage error which is sent, through a lead 72, to a power circuit 74 which also receives the phase timing signals supplied by decoding circuit 56 through leads 76, 78, 80, 82, 84, and 86. Power circuit 74 supplies in turn the three windings 12, 14, and 16. Referring to figure 3, power circuit 74 includes six power transistors 88, 90, 92, 94, 96, and 98 which are connected and driven in such a way that the corrected voltage current supplied by control circuit 70 will alternately flow through windings 12, 14, and 16, as better shown in figure 4, the current flow time and direction being according to the desired rotation timing and direction information.

In the diagram of figure 4, the time axis is subdivided into 60-degree fractions of the motor rotation period and the ordinates show the phase relationship between current flows through windings 12, 14, and 16, respectively.

Referring bach to figure 2, a desired speed and desired rotation speed voltage is set in control signal generator 54 and applied to brush 52. The voltage on brush 52 is applied to the lead, out of the six 26, 28, 30, 32, 34,and 36, which comes into contact with brush 52 through the six-segment commutator 50. Hence, decoding circuit 56 will receive the following information while the motor is running: the desired speed information in the form of a voltage on the live lead, the desired rotation direction information in the form of a voltage polarity on the live lead, and the present rotor position information in the form of switching pulses between leads 26, 28, 30, 32, 34, and 36. As previously mentioned, the control voltage amplitude generates a signal on lead 58 and the switching frequency generates a speed signal on lead 64. The detected switching times together with the desired rotation direction data, is used by decoding circuit 56 to generate saturation and cutoff signals for transistors 88, 90, 92, 94, 96, and 98, and the speed error signal present on lead 68 is used to cause a change in power circuit 74 supply voltage and hence a change in motor speed i.e. acceleration or deceleration.

The power required will obviously be supplied through brushes 22 and 24 and slip rings 18 and 20; continuous slip rings are, however, employed as mentioned previously which do not cause mechanical switching of armature currents.

Switching of the control signal supplied by generator 54 does not cause any switching noise since no significant power is involved in the transmission of information contained in the control signal.

The information conveyed by the control signal genera ted by generator 54 may be affected by parameters other than its amplitude: for instance the information may be conveyed by a square wave signal and may be affected by its mark-to-space ratio or its frequency.

In another version, the present rotation rate information and the rotor winding power supply timing information can be generated by Hall effect sensors incorporated in the rotor and sequentially operated by either the motor stationary magnetic field or magnets placed on the stator. In that casen the desired speed and rotation direction information can be introduced by superimposing it to the supply voltage through brushes 22, 24.

It is obvious that the third brush would not be used in this version and a more compact motor would result.

Moreover, if in the version being discussed the motor should be called to run at constant speed and only one rotation direction throughout its lifetime, the desired speed and rotation direction can be generated locally at the control unit 10 input instead of being supplied by external sources. The only external information required will be the present rotation rate and timing which would be transmitted through the Hall effect sensors, as mentioned previously.

It should also be emphasized that a motor including a device according to this invention can be easily operated in the step-by-step mode: the only provision needed would consist in having the motor stopped upon the occurrence of an appropriately selected value of one of the information parameters that control the armature currents. The stop signal, as decoded in control unit 10, would cause the supply voltage to be applied to one or more coils forming one phase of the armature winding, thus immediately stojj ping the motor. The step-by-step mode of operation would thus be obtained by sequentially supplying the windings.

Accordingly, figure 5 shows a power circuit 100 that permits separate supply of each armature winding. Here, twelve transistors 102, 104, 106, 108,110, 112, 114, 116, 118, 120, 122, and 124 are four-by-four connected in a bridge configuration, and one rotor winding is connected to the cross branches of each bridge. Separate supply of each bridge could be provided to achieve maximum flexibility of operation. The circuit shown in figure 5 permits changing the armature current timing and each winding can be supplied during time periods corresponding to angles smaller or greater than 60 degrees, indipendently of the other windings supply, according to the desired type of control.

It should also be noted that it is possible to incorporate a highly accurate and low-cost closed loop negative feedack device by simply applying to the control unit 10 input the motor output parameters, i.e. position and speed.

It is obvious that other numerous and different changes and modifications can be performed by the skilled in the art on the embodiments of the present invention hereinbefore described, without departing from its scope.

It is intended, therefore, that all these changes and modifications are encompassed in the field of this invention.

Claims

1. A device for automatic control of direct current motors, wherein one armature current control unit is mechanically attached to the motor rotor so as rotate along with it and receives positive and negative DC supply voltage through two slip rings (18 and 20) cooperating with two brushes (22 and 24) which are in turn connected to the DC power source, while appropriate information transmission media outside the motor provide the control unit (10) with the information required to accomplish armature current control.

2. A device according to claim 1 wherein said armature current control unit (10) is comprised of: a decoding circuit (56) that receives the information needed to control the armature currents and detects from it the desired speed information, the present rotationrevolutions per minute information, and the desired rotation timing and direction information; an oscillator circuit (60) that processes the desired speed information and generates a signal whose frequency is correlated to the desired speed information; a frequency comparator ( 66) which compares said oscillator (60) output with the present rotation revolutions per minute and generates a speed error signal; a speed control circuit (70) which outputs a voltage corrected according to said speed error signal; and a power circuit (74) which receives the desired timing and rotation direction information supplied by decoding circuit (56) and the corrected voltage supplied by speed control circuit (70) and controls the currents flowing through the armature windings.

3. A device according to claim 2 wherein said power circuit includes a number of power transistors connected in a bridge configuration whereby current will flow through the armature windings in both directions and according to a present time sequence.

4. A device according to any one of the previous claims wherein said transmission media include a segmented commutator (50), whose segments are electrically connected to as many control inputs of said control unit (10), said segmented commutator cooperating with one brush (52) which is in turn connected to a control signal generator (54).

5. A device according to any one of the previous claims wherein the motor is stopped upon the occurrence of an appropriately selected value of one of the information parameters that control the armature currents and the stop signal, as decoded in control unit 10, causes the supplying of only one armature winding phase, thus providing step-by-step mode of operation of the motor.

6. A device according to any one of claims 1 through 3 wherein said transmission media include one or more Hall effect sensors, integral with the rotor, and sequentially operated by either the stationary magnetic fields or appropriate magnets placed on the stator, and connected to said control unit (10) to provide it with present rotation direction information and timing information.

7. A device for automatic control of direct current motors, substantially as described previously with reference to the accompanying draaings.