Dynamic control system for braking DC motors

Abstract: A system for dynamically controlling the braking of a pulse-driven dc motor. An inductor (19) is connected between a pulsed switch (35) and the motor's armature. A transistor (T1) is connected to be rendered conductive by a voltage induced across the inductor and to provide a path, in series with the inductor, for armature current to flow.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to control systems for direct current motors and, more particularly, to systems for controlling the braking of direct current motors. State of the Art When operating a direct current (dc) motor with a switched or "pulsed" voltage source, it is often desirable to control the rate at which the motor is slowed or braked. Braking control is desirable, for example, when pulse-driven dc motors are used to drive magnetic tapes in devices such as videotape record and playback machines. In such applications, it is often desirable to stop and reverse motor rotation as rapidly as possible but without damaging a tape being driven by the motor. In other words, it is often desirable to brake and reverse pulse-driven dc motors rapidly but smoothly. Although mechanical systems can be used for rapid frictional braking, those systems are difficult to precisely control and can lose their effectiveness because of wear. To overcome the shortcomings of mechanical systems for braking pulse-driven dc motors, it is known to use electrical braking techniques. Generally speaking, electrical braking techniques make use of the fact that a dc motor, because of its rotational momentum, operates as an electrical generator after its driving current is turned off; that is, in electrical braking, a motor is slowed by dissipating its generated electrical energy. More particularly, in conventional electrical braking, a dc motor's generated electrical energy is dissipated through a constant resistive electrical load (i.e., fixed resistance) connected external to the motor. Although electrical braking of pulse-driven dc motors has benefits, there are complications. One complication, for instance, is that switching circuits are required so that an electrical braking load can be disconnected when a dc motor is operated as a motor. In addition, known electrical braking systems do not consistently bring dc motors to rapid, smooth stops. The present invention, as described in the following, provides a system to effectively control electrical braking loads for pulse-driven dc motors to optimize energy dissipation while smoothly braking the motors. SU-A-989718 describes a motor circuit including a pulsed grounding switch which is externally controlled and which provides when conductive a path for dissipating the motor current. US-A-4287436 discloses a circuit which includes a pulsed grounding switch for a coil and a transistor switch which is rendered conductive when the grounding switch is non-conductive, the conduction of the transistor switch causing the grounding switch to be rendered conductive again. DE-A-2410926 discloses a circuit including a respective circuit for providing pulses for driving a motor in a forward direction and a reverse direction and also discloses smoothing inductors, whereby DE-A-2410926 discloses a combination of features corresponding to one of the alternatives in the pre-characterising part of claim 1. According to the invention, a control system for driving and braking a DC motor, comprises a first circuit constituting a source of pulses for driving the motor in one direction; a second circuit comprising either a pulsed switch operative to provide a connection to electrical ground or a second source operative to provide pulses for driving the motor in a direction reverse to the said one direction; and a current-smoothing inductor in series between the said second circuit and the armature of the motor; characterised by an active resistance element which is arranged to be rendered conductive by a transient voltage across the inductor when the second circuit is not operative as aforesaid and which provides a variably resistive current path, separate from the pulsed switch or the second source, in which current flowing in the inductor and the armature can dissipate during braking of the motor. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a functional block diagram of one embodiment of a braking control system in accordance with the present invention; Figure 2 is a schematic diagram showing specific components of the braking control system of Figure 1; Figure 3 is a diagram of an alternate embodiment of a braking control system according to the present invention; Figure 4 is a diagram of yet another alternate embodiment of a braking control system according to the present invention; and FIGURES 5A and 5B are timing diagrams showing signals in the control systems of the preceding drawings. In the various drawings, common elements are given the same reference numbers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following figures, there are shown various embodiments of systems to control the braking of pulse-driven dc motors. In practice, the motors can be variable-magnetic-flux motors or constant-magnetic flux motors, also known as permanent-magnet dc motors. Typically, pulse-driven dc motors are capable of being driven in either direction, although bi-directionality is not required for the motor 1 in the system illustrated in FIGURE 1. Referring now to FIGURE 1, dc motor 1 has armature winding terminals 2 and 3, the latter of which is connected to ground through resistor 4. Motor 1 is driven by a pulse-modulated voltage source 5 which provides, at node 10, a train of generally rectangular voltage pulses. (FIGURE 5A shows an example of driving pulses having positive polarity for driving motor 1 in the forward direction.) When motor 1 is employed to drive a tape capstan in a videotape record and playback machine, for example, the pulse repetition rate is normally constant at a rate ranging from about 15,000 hertz to about 100,000 hertz (cycles per second). Normally, pulse-modulated source 5 can be controlled to selectively change the pulse duty cycle and repetition rate in response, for example, to feedback signals from motor 1. To provide such feedback signals, it is convenient to detect armature current through resistor 4, since the torque delivered by motor 1 is generally proportional to the armature current. As also shown in FIGURE 1, an inductor 11 is connected between nodes 10 and 12 or, equivalently, between node 10 and armature terminal 2. In practice, the value of inductor 11 depends upon the internal inductance of motor 1. To the extent that motor 1 has a high internal inductance, inductor 11 can have a small value. Also, a capacitor 13 is connected across armature terminals 2 and 3 external to motor 1. The braking control system in FIGURE 1 generally includes the combination of a pulsed grounding switch 14 connected between ground and a node 18, an inductor 19 coupled between nodes 18 and 12, and a tracking circuit 20. The tracking circuit 20 can be considered to have two ports, one of which is connected across inductor 19 and the other of which is connected between node 12 and ground. The purpose of tracking circuit 20, as will be explained in detail below, is to dissipate the energy of motor 1 smoothly and efficiently. Operation of the system of FIGURE 1 will now be described. Initially, it should be assumed that voltage source 5 is operating to produce uniform voltage pulses at a generally constant frequency to drive motor 1 in the forward rotational direction. Also, it should be initially assumed that pulsed grounding switch 14 is maintained open, thereby open-circuiting node 18 and rendering tracking circuit 20 non-conductive. Under such circumstances, armature current flows from source 5 to node 12 and then to ground through motor 1 and resistor 4. The magnitude of armature current under these conditions is a function of the voltage at node 12. In turn, the voltage at node 12 depends upon the driving voltage provided by forward voltage source 5 as well as upon the impedance values of inductor 11, capacitor 13, resistance 4, and the internal impedance and counter emf of motor 1. In practice, the above-mentioned impedances are chosen such that armature current is smoothed between the pulse duty cycles of forward voltage source 5, as shown in FIGURE 5B, so that motor 1 produces substantially "ripple-free" output torque and, hence, smooth driving action. Smooth driving action is highly desirable when, for example, magnetic tapes are driven in videotape recording and playback machines. At this juncture, it should be understood that the counter emf generated by rotation of motor 1 limits the armature current. Normally, the counter emf is proportional to the rotational speed of motor 1 and, thus, persists at node 12 after forward voltage source 5 is turned off. Further in operation of the system of FIGURE 1, several transient effects occur when forward voltage source 5 is turned off. One transient effect is that inductor 11 causes armature current to continue to flow for some period. (This transient effect is primarily responsible for the smoothing of armature current as shown in FIGURE 5.) In practice, transient current created by inductor 11 is dissipated in only a few milli-seconds. To operate the braking control system of FIGURE 1 after pulse-modulated voltage source 5 is turned off, pulsed grounding switch 14 is activated to periodically ground node 18. The frequency at which pulsed grounding switch 14 operates can be controlled, for example, by feedback of voltage or current through resistor 4. Whenever node 18 is grounded while counter emf persists at node 12, current flows to ground through inductor 19 and pulsed grounding switch 14. This current is accompanied by a voltage pulse which develops across inductor 19 because of the changing magnetic field as the current through the inductor begins to drop. The transient induced voltage biases tracking circuit 20 for operation in its conductive region. Then, during periodic intervals when node 18 is not grounded, generated armature current flows to ground via inductor 19 and tracking circuit 20. As the generated current flows through tracking circuit 20, the energy of motor 1 is dissipated by the resistance of the tracking circuit and by the motor winding resistance, thereby slowing the motor. FIGURE 2 is similar to FIGURE 1 but shows specific components to provide the functions ascribed to the functional blocks in FIGURE 1. Thus, in FIGURE 2, motor 1 is driven in the forward direction by an ac voltage source 25 coupled to a full-wave rectifier 26. The illustrated rectifier 26 comprises a primary winding 27A magnetically coupled to a secondary winding 27B whose center-tap is grounded and whose two terminals are connected to node 10 via diodes 28 and 29, respectively. Together, forward voltage source 25 and rectifier 26 in FIGURE 2 are equivalent to the forward voltage source 5 of FIGURE 1 and cooperatively operate to produce rectified dc voltage at node 10 such as shown in FIGURE 5A. The braking control system in FIGURE 2 includes a pulsed actuator 33 which is connected to operate a switch 35. In the illustrated embodiment, switch 35 is a field effect transistor (FET) which has its gate connected to actuator 33 and its electrodes connected between node 18 and ground. Together, pulsed actuator 33 and FET switch 35 are equivalent to pulsed grounding switch 14 of FIGURE 1. Also in the braking control system of FIGURE 2, tracking circuit 20 includes a PNP transistor T₁ whose collector is grounded, whose emitter is connected to node 18 via a diode 23, and whose base is connected to node 12 via a resistor 22. In practice, a capacitor 24 is also connected between the base of transistor T₁ and ground. Under steady state conditions in the forward driving direction, the system of FIGURE 2 operates in the manner previously described for the system of FIGURE 1. Under these conditions, nodes 12 and 18 have equal voltage and, therefore, transistor T₁ is non-conductive. When braking is to begin in the system of FIGURE 2, forward voltage source 25 is deactivated and pulsed actuator 33 is operated to periodically close FET switch 35, thereby periodically grounding node 18. Whenever node 18 grounded while motor 1 is rotating in the forward direction, counter emf appears at node 12 and generated current flows to ground through the switch and inductor 19. Thereafter, whenever FET switch 35 is periodically reopened, induced voltage develops across inductor 19. The induced voltage, in conjunction with counter emf at node 12 and the voltage drop across diode 23, causes transistor T₁ to operate in its active, non-saturated region. That is, under the stated conditions, transistor T₁ becomes resistively conductive and generated armature current flows to ground via inductor 19, diode 23, and the emitter-collector path of transistor T₁. In practice, the armature current flowing through transistor T₁ is generally constant. Therefore, since motor torque is proportional to the armature current, the braking torque is generally constant as rotational momentum of motor 1 dissipates during braking. The transient response of tracking circuit 20 in FIGURE 2 depends upon the internal inductance of motor 1 and upon the values of capacitor 24 and resistor 22 at the base of transistor T₁. In practice, the values of capacitor 24 and resistor 22 are chosen to avoid either large voltage or current transients when the braking system begins operation. Also, in practice, it is sometimes desirable that resistor 22 be variable to increase its resistance as forward rotation of motor 1 slows. Automatic resistance increases can be achieved by implementing resistor 22 as an element having a positive temperature coefficient. For instance, one suitable element having a positive temperature coefficient is a small wattage incandescent lamp. In that case, the resistance at the base of transistor T₁ will normally increase as flows current through resistor 22, and this will effectively increase the resistance of the emitter-collector path through the transistor as motor 1 slows to a stop. Referring now to FIGURE 3, a system is illustrated which controls braking both during forward and reverse rotation of a bi-directional, pulse-driven dc motor 1A. In this embodiment, energy for driving motor 1A in the forward direction is provided by a dc voltage source 45, and energy for driving the motor in the reverse direction is provided by an opposite-polarity dc voltage source 45A. The system in FIGURE 3 further includes pulsed switches connected to voltage sources 45 and 45A. More specifically, the electrodes of a FET switch 46 are connected between forward voltage source 45 and node 10, and the base of FET switch 46 is connected to a pulsed actuator 47. Similarly, the electrodes of a FET switch 46A are connected between reverse voltage source 45A and node 18, and the base of the switch 41A is connected to a pulsed actuator 47A. Additionally, the system of FIGURE 3 includes first and second tracking circuits 20 and 20A, respectively. Second tracking circuit 20A in FIGURE 3 can be considered to have two ports, one of which is connected across inductor 11 and the other of which is connected between node 12 and ground. More specifically, in the illustrated embodiment, tracking circuit 20A includes an NPN transistor T₂ whose collector is connected to ground, whose emitter is connected to node 10 via diode 23A, and whose base is coupled to node 12 by resistor 22A. A capacitor 24A is connected between the base of transistor T₂ and ground. Operation of the system of FIGURE 3 will now be described. When motor 1A is driven in the forward direction, pulsed actuator 47 opens and closes FET switch 46, thereby providing pulsed forward voltage to motor 1A via node 10 and dc source 45. (Again, examples of the forward driving pulses are shown in FIGURE 5A.) At all times while motor 1A is being driven in the forward direction, pulsed actuator 47A holds FET switch 46A open, thereby open circuiting node 18 and rendering tracking circuit 20 non-conductive. Then, during intervals when switch 46 is periodically opened, transistor T₂ is driven to saturation by the voltage at node 12 in conjunction with the induced voltage across inductor 11 and the potential drop across diode 23A. When saturated, transistor T₂ allows armature current to flywheel through its collector-emitter path. When forward rotation of motor 1A is to be braked in the system of FIGURE 3, pulsed actuator 47 holds switch 46 open, thereby open-circuiting node 10 and rendering tracking circuit 20A non-conductive. Then, pulsed actuator 47A is operated to periodically open and close switch 46A, thereby periodically reversing voltage at node 18. Whenever switch 46A is opened after node 18 has been reversed, tracking circuit 20 operates in the manner described in conjunction with the embodiment in FIGURE 2. That is, during such intervals, voltage induced by inductor 19 and the counter emf at node 12 drives transistor T₁ to conduct in its active but non-saturated region, thus allowing armature current to flow to ground through the collector-emitter path and, thus, dissipating forward rotational momentum of motor 1A. After forward rotation of motor 1A stops in the system of FIGURE 3, the motor can be driven in the reverse direction. During such times, pulsed actuator 47 continues to hold switch 46 open, thereby open circuiting forward voltage source 45. Then, to drive motor 1A in the reverse direction, pulsed actuator 47A periodically opens and closes FET switch 46A, thereby providing pulsed driving voltage to node 18 from reverse voltage source 45A. When switch 46A is opened under conditions where motor 1A is being driven in the reverse direction, the voltage induced across inductor 19 in conjunction with counter emf at node 12 and the potential drop across diode 23, causes transistor T₁ to saturate. Consequently, when motor 1A is driven in the reverse direction, reverse motor current flywheels through the emitter-collector path of transistor T₁. When reverse rotation of motor 1A is to be braked in the system of FIGURE 3, switch 46A is held open by pulsed actuator 47A, thereby open circuiting node 18 and rendering tracking circuit 20 non-conductive. Then, switch 46 is periodically opened and closed. Under such conditions, voltage transients are induced by inductor 11 which cause transistor T₂ to operate in its active but unsaturated region, thereby allowing generated current to flow from ground through the emitter-collector path of the transistor. Thus, when motor 1A is braked in the reverse direction, tracking circuit 20A dissipates the motor's reverse rotational momentum in a manner analogous to the way in which tracking circuit 20 dissipates forward rotational momentum during forward braking. Also, as to the system in FIGURE 3, it should be appreciated that resistors 22 and 22A can be variable. Again, automatic variations in the resistance can be accomplished by using small wattage incandescent lamps as the resistors. FIGURE 4 shows an alternative embodiment of a system to control braking both during forward and reverse rotation of bi-directional motor 1A. In this embodiment, motor 1A is driven in the forward direction by ac voltage source 25 coupled to a fullwave rectifier 26. As indicated by the reference numerals, those devices are essentially the same as the forward voltage source and rectifier in FIGURE 2. Further in the system of FIGURE 4, motor 1A is driven in the reverse direction by an opposite-polarity ac voltage source 25A. This latter voltage source is essentially the same as voltage source 25 except that it produces opposite polarity voltage. Further, the system of FIGURE 4 includes first and second tracking circuit 51 and 51A, respectively. Like the previously described tracking circuits, first tracking circuit 51 can be considered to have two ports. More specifically, in this embodiment, first tracking circuit 51 includes NPN transistor T₂ whose collector is connected to ground, whose base is coupled to node 12 by resistor 22A, and whose emitter is connected, via diode 23A, to the center tap of secondary coil 27B of rectifier 26. Similarly, second tracking circuit 51A includes a PNP transistor T₁ whose collector is connected to ground, whose base is coupled to node 12 by resistor 22, and whose emitter is connected to the center tap of secondary coil 47B of rectifier 46. In operation, the system of FIGURE 4 operates in substantially the same manner as the system of FIGURE 3, except that the voltage sources 25 and 25A provide alternating current rather than direct current. This distinction is not critical, however, because rectifiers 26 and 26A produce rectified voltage pulses at their outputs. While the invention has been described with reference to certain preferred embodiments, it will be evident to those skilled in the art that many variations and adaptations thereof may be made without departing from the scope of the following claims. For example, workers skilled in the art will recognize that the frequencies and duty cycles of the driving voltages, as well as the frequencies and duty cycles of the pulsed grounding circuits, can be controlled by voltage or current feedback from motor 1. In particular, the duty cycles of the pulsed actuators can be increased with decreasing motor speed to, thereby, increase the voltages induced by inductors 19.

Claims: A control system for driving and braking a DC motor, comprising: a first circuit (25; 25,46) constituting a source of pulses for driving the motor in one direction; a second circuit comprising either a pulsed switch (35) operative to provide a connection to electrical ground or a second source (45A,46A; 25A) operative to provide pulses for driving the motor in a direction reverse to the said one direction; and a current-smoothing inductor (19) in series between the said second circuit and the armature (2,3) of the motor; characterised by an active resistance element (T1) which is arranged to be rendered conductive by a transient voltage across the inductor (19) when the second circuit is not operative as aforesaid and which provides a variably resistive current path, separate from the pulsed switch or second source, in which current flowing in the inductor and the armature can dissipate during braking of the motor. A control system according to claim 1 wherein the active resistance element (T1) comprises a transistor. A control system according to claim 2 wherein the transistor (T1) comprises a bi-polar transistor having its emitter coupled to a first node between the said second circuit and the inductor and its base coupled to a second node between the inductor and the armature of the motor. A control system according to claim 3 wherein a resistor (22) is connected between the base of the transistor and the second node. A control system according to claim 4 wherein the resistor (22) has a positive temperature coefficient of resistance. A control system according to any of claims 3 to 5 wherein a diode (23) is connected in series between the active resistance element and the said first node. A control system according to any foregoing claim wherein a second inductor (11) is coupled between said first circuit and the armature and a second active resistance element (72) is coupled to the second inductor (11) and the first circuit (25; 45,46), the second active resistance element being non-conductive to resist the passage of pulses from the first circuit but arranged to be rendered conductive by a transient voltage across the second inductor (11) when the said first circuit is cut off and which has a variably resistive path in which current flowing in the second inductor and the armature can dissipate. A control system according to any foregoing claim wherein either or both of the said first and second circuits comprises a respective pulsed switch operative to make a connection to a reference voltage. A control system according to any of claims 1 to 7 wherein the first circuit comprises a transformer and rectifier. A control system according to claim 9 wherein the second circuit comprises a transformer and rectifier. A control system according to claim 9 or claim 10, wherein the or each rectifier circuit comprises a balanced circuit and the respective active resistance element is coupled to a centre tap of the secondary winding of the respective transformer.

Inventors: CLARK HAROLD V, CLARK, HAROLD V.
Applicants: AMPEX, AMPEX CORPORATION
Application year: 1989
Main classification: H02P3
IPC codes: H02P3