STEPPING MOTOR CONTROLLER

Abstract: A control system for a stepping motor includes first and second pairs (W1, W2) of complementary windings. The windings are coupled via respective switching devices (T3-T6) to ground, a low voltage source is coupled via diodes (CR6, CR7) to the respective pairs (W1, W2) of windings, and a high voltage source is coupled via further switching devices (T1, T2) to the respective pairs of windings (W1, W2). In operation, to provide a stepping operation, the switching devices (e.g. T3, T4) of one of the pairs of windings (e.g. W1) are operated to switch the low voltage source and initiate step operation, the associated further switch (e.g. T2) being operated to disable the high voltage source for a predetermined time period until the stepping motor is approaching its new desired position at which time the high voltage source is applied to strongly hold the stepping motor rotor in its new position and minimize oscillations.

Description: STEPPING MOTOR CONTROLLER Technical Field This invention relates to a control system for a stepping motor, of the kind including a stepping motor having a first and a second pair of windings. Background Art Stepping motors are generally used to move some form of apparatus from one position to another with a high degree of accuracy. Stepping motors are designed with a number of null positions, e.g., 200 null positions for a single rotation of the motor's rotor. In a preferred mode of operation, when the rotor is to be advanced to the next null position, the advancement should take place quickly and without an undershoot and/or overshoot so as to facilitate the positioning of the driven apparatus in the quickest possible manner. Generally speaking, the rotor tends to overshoot the null position and to then reverse direction back towards the desired null position and to continue this oscillatory behavior for a period of time, which in turn decreases the accuracy of the positioning of the rotor to the null position while increasing the settling time of the rotor in its new null position. U.S. Patent No. 3,753,592 discloses--a stepping motor control system which includes two one- shot devices which controls the drivers for a pair of windings so as to effect a one step-forward function followed by a one step-backward function to decelerate the motor, with the object of achieving nonoscillatory single step operation. Disclosure of the Invention It is an object of the present invention to provide a control system for a stepping motor, of the kind specified, whereby overshoot is minimized. Therefore, according to the invention, there is provided a control system for a stepping motor, including a stepping motor having a first and a second pair of windings, characterized by control means adapted to selectively apply a first voltage level to a selected winding of said first and second pairs of windings to cause said motor to step towards a null position, and to apply a second voltage level of a higher level than said first voltage level to said selected winding prior to and during the null position in order to dampen oscillations. It will be appreciated that in a stepping motor control system according to the invention, the provision of the higher level voltage prior to and during the null position serves to minimize oscillations. A further advantage is that a high degree of accuracy in rotor positioning is achieved for single-step operation of the motor * Another advantage is that the rotor is effectively held in its null position with minimum overshoot. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Pig. 1 is a simplified schematic diagram of the preferred embodiment of the invention. Pig. 2 is a table, to be used in conjunction with the description of the embodiment of Fig. 1, for enabling an understanding of the operation of the preferred embodiment. Figs. 3A, 3B and 3C, assembled in accordance with the map of Fig. 3, illustrate in block schematic form a first portion of the preferred embodiment of the invention. Fig * 4 illustrates in block schematic form the second and remaining portion of the preferred embodiment which includes the driver circuitry for the stepping motor. Best Mode for Carrying Out the Invention Referring now to the simplified schematic diagram of Fig. 1, the stepper motor used in the preferred embodiment is comprised of two pairs of motor windings, WI and W2, each having a center tap, 10 and 12, respectively, connected to windings Al, Bl and A2, B2, respectively. The center tap 10 is con¬ nected to a +5V source by means of a diode CR6. A diode CR7 connects the center tap 12 to the +5V source. A +12V source is connectable by MOS switches T2 and Tl, when enabled, and 6.2 ohm resistors to the center taps 10 and 12, respectively. When T2 is not enabled, the +12V source is isolated from the center tap 10, and when Tl is not enabled the +12V source is isolated from the center tap 12. In the quiescent state a pair of MOS switches, T3 and T4, when enabled, connect the windings Al and Bl in circuit to ground so as to cause current to flow from the +12V source, through the associated resistor and motor winding, to ground. In a like manner, the second pair of motor windings, A2 and B2 are connected in circuit to ground by means of MOS switches T5 and T6, respectively, when they are enabled. Each of the MOS switches has an enabling gate labeled G for receiving signals for turning the respective MOS switch ON or OFF. Referring now to the table of Fig. 2, in conjunction with the simplified circuitry of Fig. 1, with the motor's rotor (not shown) at rest in the State 1 position, switches Tl, T2, T3 and T5 are ON (enabled) and switches T4 and T6 are OFF. In order for the motor's rotor to advance to State 2, in a clockwise direction, it is necessary to cause switch T3 to go OFF and for switch T4 to come ON. The switches T5 and T6 remain in the same state as existed for State 1 without change. Also switch Tl remains unchanged in its ON condition. At the time of transition of switch T3 from the ON to the OFF state and T4 from the OFF to the ON state, switch T2 is turned OFF and remains OFF for a predetermined interval of time, Δt, which interval is sufficient to permit the rotor to approach the null position corresponding to State 2. As the current in Bl rises (in an approximately linear manner) and the current in Al falls, the effective magnetic null position of the rotor slews smoothly between the position corresponding to State 1 and State 2. The rotor attempting to follow the effective magnetic null position, first lags the slewing null, for the first half of the interval then leads and decelerates to approach the final null position (State 2) just at the end of the time, interval Δt. During this interval windings Al and Bl are supplied from the +5V source by the diode CR6. Upon approaching the desired null position, switch T2 is turned ON to effectively apply the current source from the +12V through the winding Bl serviced by switch T4 and the 6.2 ohm resistor. This current feed, applied as the rotor moves towards the null position, causes the rotor to strongly lock to the null position without oscillatory overshooting. In-order to increment the stepper motor from State 2 to State 3 in a clockwise direction, from the table we see that the switches T3 and T4 remain in the state determined by State 2, switch T2 remains ON and switch T5 is to be turned OFF with switch T6 being turned ON. Upon that occurrence, switch Tl is turned OFF for a period, as previously described, of predetermined length. If the motor's rotor is to be rotated in a counterclockwise direction, the sequence of states shown for the various switches is approached from the initial (present) state, and the next state to be achieved is the one above the initial state rather than the one below. The motor of the preferred embodiment is a 200 step motor of the type manufactured by Superior Electric Co., Bristol, CN, having their part number M061-FC02. In the table of Fig. 2, only the steps corresponding to Steps 1-7 are shown for simplifi¬ cation purposes, it being understood that the remain¬ ing Steps 8-200 repeat in a like pattern. Referring now to Figs. 3A, 3B and 3C, assembled in accordance with the map of Fig. 3, the controller circuitry is provided with a right and a left limit detecting apparatus so that the mechanism in which the stepper motor functions is limited in its movement between two acceptable ranges. Other types of circuitry and/or mechanical limiting may be used as desired, the main purpose of disclosing such circuitry in the present invention is to provide the environment for which the present invention has particular appli¬ cability. The left and the right limit circuitry are identical in construction and are comprised of an LED and a photosensitive transistor pair, generally indi¬ cated as items 11 and 13, coupled to inverters 14 and 16, respectively. The light beam from the LED causes the phototransistor to conduct and to effectively ground the input to the respective inverter. When the right or left limit is reached, the light beam from the particular LED experiencing the limit is inter¬ rupted by a physical masking, such as an arm project¬ ing from a carriage, thereby causing the respective phototransistor to turn OFF, which in turn causes the voltage on the input to the respective inverter to rise from ground. The output of the inverters 14 and 16 are directed to other circuitry (not shown) and to an input to AND gates 18 and 20, respectively, for indicating that one or the other of the limits has been reached. In the preferred embodiment of the invention, the left limit and right limit signals act to cut-off the enablement of AND gates 18 and 20, respectively. The AND gate 18 is adapted to also receive stepping pulses from a computer (not shown) via an inverter 15, when the stepping motor's rotor is to be stepped in the left direction. In a like man¬ ner, AND gate 20 receives step right pulses from a computer, via an inverter 17,- when the stepping mo¬ tor's rotor is to be stepped in the right direction. It is to be understood that clockwise and counter¬ clockwise rotation of the motor's rotor is translated into left and right movement of the apparatus driven by the motor. Each pulse received, either on the step left or the step right input, corresponds to one step of the stepping motor in the indicated direction. The output signals from AND gates 18 and 20 are directed as an input to a dual 2-to-4 line decoder, 26 and 28, respectively, of the type manufactured by National Semiconductor and bearing their part No. 74LS139. The input signals from the AND gates 18 and 20 constitute enabling signals and are received on the enabling inputs labeled 1G/. When an enabling signal is pre¬ sent on the enabling input of either decoder 26 or decoder 28, the state of the signals received on the respective inputs, labeled A and B, will cause coded output bits to appear on the output terminals labeled YQ through Y3. In the absence of an enabling signal, all of the outputs, YQ through Y3, will be at a high logic level. The outputs, Y]_ and Y2, from the decod¬ ers 26 and 28 are cross-coupled to a pair of four input AND gates 30 and 32, respectively. The remain¬ ing two inputs to the AND gates 30 and 32 are received from the associated output terminals Y Q and Y3 from each of the decoders 26 and 28, respectively. The output of AND gate 30 is designated a PULSE 1 signal with the output of AND gate 32 being designated a PULSE 2 signal. A PULSE 1 signal will be present on the output of the AND gate 30 when all of the inputs, labeled 1 through 5, are at the high logic level. In a like manner, a PULSE 2 signal will be present at the output of the AND gate 32 if all of its inputs, la¬ beled 1 through 5, are at a high logic level. The output line from AND gate 30 is directed to the clock¬ ing input, CK, of a D-type flip-flop 22. The output line from the AND gate 32 is directed to the clocking input, CK, of a D-type flip-flop 24. A D-type flip- flop places the data present on its D input to the complementary outputs Q and Q/ during the positive- going transition of the signal present on its CK input. The S/ and C/ inputs correspond to set and clear inputs of the flip-flops, which inputs are con¬ nected to a high logic level by an inverter 39 which has its input connected to ground. The Q/ output of flip-flop 22 is denoted as QI/ and is fed-back to the D input of flip-flop 22 and to an output line labeled QI/. The Q output is denoted as QI and is directed to the inputs labeled B of the line decoders 26 and 28 and to a like labeled output line. The output Q2, of the D-type flip-flop 24, is directed to the inputs labeled A of the line decoders 26 and 28 and to a like labeled output line. The Q2/ output of the D-type flip-flop 24 is connected to its D input and to an output line labeled Q2/. In operation, upon a receipt ' of a left and/or a right negative pulse on the inputs of the inverters 15 and/or 17 respectively, the circuit thus far de¬ scribed will generate the signals QI, QI/ and Q2, Q2/ in the order and logic state which will cause the OFF and ON conditions of the switches T3, T4 and T5, T6 in accordance with the table of Fig. 2. The PULSE 1 signal is generated each time the signals QI and QI/ toggle or change their complementary states. In a like manner, the PULSE 2 signal is generated each time the signals Q2 and Q2/ change state. The PULSE 1 signal is directed to a B labeled input of a timing circuit 36, which in the preferred embodiment of the invention is a one-shot multivibrator of the type manufactured by National Semiconductor under their part No. 74LS123. Upon receipt of the PULSE 1 signal and the lapse of a predetermined timed interval, a signal Rl appearing on the like labeled output line connected to the Q labeled output of timer 36 will change state. The time duration of the timer 36 is selected by the size of a capacitor connected between the terminals numbered 14 and 15 and the size of the resistor connected to the +5V source. In the pre¬ ferred embodiment of the invention the capacitor was sized at 0.1 microfarads and the resistor was sized at 68K ohm, so as to provide a timing interval equal to 2.5 milliseconds. In a like manner, the timing cir¬ cuit 38 receives the pulse signal PULSE 2 and provides a change in state of the output signal, R2, 2.5 milli¬ seconds after receipt of the PULSE 2 signal. Referring now to Fig. 4, a hex buffer 50 receives on its inputs the signals R2, Rl, Q2/, Q2, QI/ and QI from the output lines appearing in Figs. 3B and 3C. The outputs from each of the buffer amplifi¬ ers in the hex buffer are connected by 56K pull-up resistors to a +12V source. Each of the outputs is also directed to a gate input, labeled G, of a MOS switch T2, Tl, T4, T3, T6, and T5, respectively. A stepper motor 100, having two pairs of center tapped windings, WI and W2, is provided with a first voltage level equal to +5V through the diodes CR6 and CR7 that are connected to the center taps 10 and 12, respectively. The center taps lines are further connected by means of 6.2 ohm, 25 watt resistors and the transistor switches T2 and Tl (when enabled) to a +12V source. Each of the windings, at their outermost end away from the center tap, is connected to ground by means of their associated switch transistors T4, T3, T6 and T5. Each of the transistor switches has a diode CRl, CR2, CR4 and CR3, respectively and a 27 ohm resistor connecting its drain terminal to a +5V source. This latter combination functions to elimi¬ nate spiking voltages from being applied to the drain inputs of the respective switch transistors. In operation, when switch transistor T4 is ON, a current will flow through the stepping motor via the diode CR6 through the coil winding Bl to ground. When T4 is ON, T3 is OFF and in a like manner when T6 is ON T5 is OFF and in the stepping operation of the motor, these switch transistors toggle in pairs, in a complementary fashion, according to the state table of Fig. 2 to drive the stepping motor's rotor towards a null position. As previously indicated, a fixed time after the transition of the turn-on and turn-off of a switch'transistor pair, the switch transistor,- either T2 or Tl, associated with that pair is turned OFF to remove the +12V from the pair that has just switched and the +12V is not reapplied until the rotor position approaches its designated null at which time the +12V is applied to firmly drive the rotor to the null position. When at rest, the +12V remains applied to center taps 10 and 12.

Claims: CLAIMS . 1. A control system for a stepping motor, including a stepping motor having a first and a second pair of windings (WI, W2) , characterized by control means adapted to selectively apply a first voltage level to a selected winding (e.g. Bl) of said first and second pairs of windings (WI, W2) to cause said motor to step towards a null position, and to apply a second voltage level of a higher lever than said first voltage level to said selected winding (e.g. Bl) prior to and during the null position in order to dampen oscillations. 2. A control system according to claim 1, characterized in that said control means includes: first and second complementary .switching means (T3, T4; T5, T6) coupled respectively to said first and second pairs of windings (WI, W2) and adapted to alternately energize the windings (e.g. Al, Bl) of the associated pair of windings (e.g. WI) ; and third and fourth switching means (T2, Tl) adapted to control the application of said second voltage level to said first and second pairs of windings (WI, W2) respectively. 3. A control system according to claim 2, characterized in that said control means includes first and second bistable toggling means (24, 22) . adapted to apply first and second pairs of complementary control signals (Q2, Q2/; Q3, Q3/) to said first and second complementary switching means (T3, T4; T5, T6) , and first and second timing means (38, 36), responsive to the toggling of a respective one of said first and second bistable toggling means (24 r 22) to apply a disabling signal to that one of said third and fourth switching means (T2, Tl) associated with the pair of windings (e.g. WI) which has been toggled, said disabling signal having a predetermined time period at the end of which said second voltage level is applied to said selected winding (e.g. Bl) . 4. A control system according to claim 3, characterized in that said control means includes: first and second voltage sources adapted to supply said first and second voltage levels, respectively; and first and second diodes (CR6, CR7) coupling said first voltage source to said first and second pair of windings (WI, W2) respectively, said second voltage source being coupled to said third and fourth switching means (T2, Tl). 5. A control system according to claim 4, characterized in that said control means includes first and second decoder means (28, 26) each adapted to receive, as inputs, outputs of said first and second bistable toggling means (24, 22) and a stepping control signal for said stepping motor, and first and second AND gates (32, 30) each coupled to " outputs of both of said first and second bistable toggling means (24, 22) and said first and second timing means (38, 36). 6. A control system according -to~claim—S,.. — characterized in that said first and second timing means include respective one-shot multivibrators (38, 36) adapted to provide said disabling signal having a predetermined time period. 7. A control system according to claim 6, characterized in that said predetermined time period is 2.5 milliseconds.

Inventors: RANGER MICHAEL HARRY, RANGER, MICHAEL, HARRY
Applicants: NCR CO, NCR CORPORATION
Application year: 1988
Main classification: H02P8
IPC codes: H02P8