MEANS AND METHOD FOR STEPPING MOTOR STABILIZATION

Abstract: A mid-range stabilization circuit for use in a multiphase stepping motor control system having motor current controllers responsive to first and second reference signal inputs, the current controllers providing a level of current to the motor phases proportional to the reference signals. The stabilization circuit includes means for measuring the levels of the reference signals and means for limiting the levels of the reference signals such that the motor current controllers can provide current chopping when the motor is operating at a mid-range velocity.

Description: This invention relates to the control of stepping motors and, more particularly, to a novel means and method for reducing the mid-velocity instabilities of such motors when driven by a chopping drive. Stepping motors are well known devices for providing incremental motion and are widely used in applications where accurate positioning is required. Such a motor includes a rotor and a stator, with the stator having a plurality of salient poles energized by the passage of electric current through coils wound upon the poles. The coils are so arranged as to provide at least two electrical phases. The rotor includes at least one pair of N-S poles which are flux-linked with the stator poles, so that successive energizations of the phases provide incremental, or stepwise, rotary motion of the rotor. Stepping motors and their drive systems frequently exhibit instabilities in the form of torque modulation at the motor shaft, with this phenomenon generally occurring at speeds between 1000 and 4000 steps per second. The exact range depends on the motor, the motor drive system, and the load being driven. In the range of instability, the motor's torque versus speed characteristic exhibits abrupt decreases in torque and in extreme cases may even lose synchronism and stall. One approach to reducing this mid-range instability has been to mechanically damp the shaft. This is satisfactory in some applications, but is generally disadvantageous, as mechanical damping decreases the amount of useful torque available. In U.S. Patent 4,081,736, assigned to the assignee of the present invention, there is described a digital circuit for providing mid-range stability in stepping motors. That circuit measures the extent of decay current after a change of energization and either increases or decreases the time interval before the next energization, depending on whether the rotor is moving at a greater or lesser velocity, respectively, than the stator field velocity. That circuit has been found satisfactory, but requires a microprocessor for operation. In German OS 23 48 121, there is described a stepping motor control circuit in which "torque breaks" due to self resonance are controlled by means of providing a constant current to the total motor throughout its entire speed range. Variations in back EMF are compensated for by varying the bus voltage to the motor. While the control circuit described has the effect of providing for mid-range stability, the circuit will result in variations in torque ripple being present and poorly regulated current because of the method of providing constant current with an inductive member. Consequently, a motor employing such a control circuit would be unsuitable for use in a microstepping mode. It is an object of this invention to provide a simpler means and method for stabilization of a stepping motor system. According to one aspect of the invention, there is provided a method of operating a multiphase stepping motor control system having motor current control means responsive to a reference signal input for motor stabilization, characterised by:- (a) measuring the level of a plurality of reference signals; and (b) reducing the level of the reference signals while the motor is operating at a mid-range velocity such that current controllers responsive to the reference signals to provide a current level to the motor phases proportional to the reference signals can limit the level of currents to the motor phases so that the current controllers can provide chopping when the motor is operating at a mid-range velocity, thereby to provide for mid-range velocity stability of the motor. According to a second aspect of the invention a multiphase stepping motor control system having motor current control means responsive to a reference signal input for motor stabilization is characterised by a circuit comprising:- (a) current controllers responsive to reference signals and operable to provide a level of current to the motor phases proportional to the reference signals; (b) means for measuring the level of the reference signals; and (c) means for reducing the levels of the reference signals while the motor is operating at a mid-range velocity, such that the current controllers can limit the level of currents to the motor phases so that the current controllers can provide current chopping when the motor is operating at a mid-range velocity. Preferred features of the invention include a means for reducing the levels of the reference signals comprising a voltage clamping circuit which receives an input from the means for measuring the level of the reference signals and which is connected to act on the reference signals. The means for measuring the levels of the reference signals preferably comprises: (a) a signal conditioner having one of the reference signals as an input; and (b) a frequency-to-voltage converter having the output of the signal conditioner as an input and providing an input to the voltage clamping circuit. The system preferably includes an activate and disable circuit connected to receive the output of the frequency-to-voltage converter and connected to provide the input of the voltage clamping circuit, operable to cause the voltage clamping circuit to reduce the levels of the reference signals to the motor current controllers during operation of the motor in the mid-range and to not reduce the levels of the reference signals to the motor outside the mid-range. It is an advantage of the preferred embodiment that the system does not require a microprocessor. An embodiment of the invention will now be described by way of example with reference to the drawings in which: Figure 1 shows the current profile of a stepping motor operating below mid-range; Figure 2(a) shows the current profile of a stepping motor operation in its mid-range; Figure 2(b) shows the current profile of Figure 2(a) modified in accordance with the invention; Figure 3 is a graph of motor speed versus maximum current in a motor operated in accordance with the invention; and Figure 4 is a block diagram illustrating the stabilizer system interconnections. Referring to the drawings, Figure 1 shows graphically in terms of current ("I") versus time ("T") in one phase of a stepping motor a full step of the motor's rotor when the motor is operated below mid-range, or the range of instability. The full step takes place between times "T₁" and "T₂", with "I th " being the maximum current reached. The sawtoothed section at the top of the profile results from current chopping in the motor drive. Normally, a stepping motor system is relatively stable when this current profile exists. Figure 2(a) shows a single-step current profile of a stepping motor with a conventional drive in the motor's range of instability. Here, the pulses driving the motor are so rapid that chopping is unable to take effect. It has been found that it is this type of motor profile that leads to the instability problems discussed above. Figure 2(b) illustrates how the current profile is m odified using the stabilizer system. Here, the level of the maximum current "I th " has been reduced sufficiently to permit chopping and thus overcome the problems of instability. At motor velocities above the mid-range, the current profile becomes nearly sinusoidal due to the back EMF generated by the motor and the motor is stable, even though, at these higher velocities, the pulse rate is so great that chopping cannot be employed. Figure 3 shows graphically how the stabilizer system might be applied to a stepping motor having an instability range over the velocities of 2000 to 4000 steps per second. It can be seen that the stabilizer system is applied to begin to decrease the value of "I th " at 2000 steps per second and progressively decreases that value as motor speed increases until, at 4000 steps per second, "I th " is returned to its maximum value. Figure 4 is a block diagram of the stabilizer system 10 employed in a two-phase stepping motor drive system. In the conventional portion of the system, an isolator and signal conditioner 11 receives pulse and direction information as inputs and provides an input to an up/down counter 12 the output of which depends on whether the motor is to rotate clockwise or counterclockwise. The output of the up/down counter 12 is the input to step logic circuits 13 and 14 which determine the switching sequences of the motor phases. The digital outputs of the step logic circuits 13 and 14 are converted to analog reference signals "V IA " and "V IB ", respectively, in digital-to-analog converters 15 and 16, respectively, and are the inputs to motor current controllers 17 and 18, respectively. The reference signals "V IA " and "V IB " are equal in magnitude but displaced in time. The motor current controllers 17 and 18 control the current level in the motor phases "A" and "B", respectively, and provide current chopping at low motor velocities. The circuit of the stabilizer system 10 includes a signal conditioner 19 the input of which is "V IA ". The output of the signal conditioner 19 is an input to a frequency-to-voltage converter which provides a voltage output proportional to the frequency of the input signal. The voltage output of the frequency-to-voltage converter 20 is the input to an activate and disable logic circuit 21 which, when the frequency of motor as indicated by the level of the reference signal "V IA " is in the unstable range, provides an input to a voltage clamping circuit 22 which limits the levels of reference signals "V IA " and "V IB " so that the level of motor currents is correspondingly limited so that the current controllers 17 and 18 can provide chopping and thus stabilize the motor. The velocities at which activating and deactivating occur may be manually set depending on the motor being driven, or the limits may be built into the activate and disable circuit 22. As illustrated in Figure 3, the levels of reference signals "V IA " and "V IB " would be increasingly limited as the velocity of the motor is increased through the range of instability. It will be understood that what has been disclosed is a novel circuit and method for providing mid-range stabilization in a stepping motor. It will be further understood that, although the invention has been described as applied to a two-phase stepping motor, it may be applied as well to any multiphase stepping motor having other than two phases.

Claims: A method of operating a multiphase stepping motor control system having motor current control means (17, 18) responsive to a reference signal input for motor stabilization, characterised by:- (a) measuring the level of a plurality of reference signals; and (b) reducing the level of the reference signals while the motor is operating at a mid-range velocity such that current controllers (17, 18) responsive to the reference signals to provide a current level to the motor phases proportional to the reference signals can limit the level of currents to the motor phases so that the current controllers can provide current chopping when the motor is operating at a mid-range velocity, thereby to provide for mid-range velocity stability of the motor. A multiphase stepping motor control system having motor current control means (17, 18) responsive to a reference signal input for motor stabilization, characterised by a circuit (10) comprising:- (a) current controllers (17, 18) responsive to reference signals and operable to provide a level of current to the motor phases proportional to the reference signals; (b) means (19, 20) for measuring the level of the reference signals; and (c) means (22) for reducing the levels of the reference signals while the motor is operating at a mid-range velocity, such that the current controllers (17, 18) can limit the level of currents to the motor phases so that the current controllers (17, 18) can provide current chopping when the motor is operating at a mid-range velocity. A system according to claim 2, characterised in that the means for reducing the levels of the reference signals comprises a voltage clamping circuit (22) which receives an input from the means (19, 20) for measuring the level of the reference signals and which is connected to act on the reference signals. A system according to claim 2 or claim 3, characterised in that the means for measuring the levels of the reference signals comprises: (a) a signal conditioner (19) having one of the reference signals as an input; and (b) a frequency-to-voltage converter (20) having the output of the signal conditioner (19) as an input and providing an input to the voltage clamping circuit (22). A system according to claim 3 and claim 4, characterised in that the said circuit (10) includes an activate and disable circuit (21) connected to receive an output of the frequency-to-voltage converter (20) and connected to provide the input of the voltage clamping circuit (22), operable when activated to cause the voltage clamping circuit (22) to reduce the levels of the reference signals to the current controllers (17, 18) during operation of the motor in mid-range and to not reduce the levels of the reference signals to the current controllers (17, 18) during operation of the motor outside the mid-range. A system according to claim 2, characterised in that the said circuit (10) includes an activate and disable circuit (21) coupled to an output of the measuring means (19, 20) and to an input of the reducing means (22) and operable such that the reducing means (22) reduces the levels of the reference signals to the current controllers (17, 18) during operation of the motor in the mid-range and does not reduce the levels of the reference signals to the current controllers (17, 18) during operation of the motor outside the mid-range.

Inventors: GIANNI, THOMAS R., GIGUERE, IRVING J.
Applicants: DANA CORPORATION
Application year: 1986
Main classification: H02P8
IPC codes: H02P8