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Description  |
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BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the control of stepper motors, and, more particularly, to a control circuit for directly generating drive current command signals for a stepper motor drive.
2. Discussion of the Related Art
Stepper motors are used in a wide variety of applications due to their low cost, ruggedness, simplicity of construction, and wide acceptance, among other factors. A common type of stepper motor includes a permanent magnet-type rotor, and a
stator having a body portion with inwardly extending teeth. Coils are wound on stator poles which are connected together to form motor phases. The art is literally replete with variations of this type of stepper motor. For example, a common
configuration is a two-phase, 200-step per revolution (i.e., 1.8.degree. mechanical per step) stepper motor. In a non-microstepping mode of operation, current in any given motor phase in any given time is switched either fully on or fully off. It is
known, however, to employ a "microstepping" mode of operation, wherein the magnitudes of the currents in the various motor phases are adjusted in accordance with a pre-determined function. For example, in a stepper motor having two phases, a drive
circuit may cause electrical current to flow through the stator windings (phases) in accordance with the sine/cosine law.
One approach taken in the art to generate current command signals applied to the drive circuit is to first generate a desired velocity signal (i.e., rotational change of rotor per unit time), and then further process the desired velocity signal
to obtain the current command signals. The structure for accomplishing this under this approach may include means for generating a desired velocity signal, a pulse generator, an up down counter, ROM's containing sine and cosine data, and
digital-to-analog (D/A) converters. The pulse generator, up/down counter, ROM's, and D/A's are all external to the desired velocity generating means. The velocity signal may be a digital word representing the number of pulses to output over the next
pre-determined time period. The pulse generator receives the velocity signal and then uniformly outputs the desired number of pulses over the next pre-determined time interval. The pulse generator's output is coupled to the counter, which then either
counts up or down depending on the state of a clockwise/counterclockwise (CW/CCW) direction line (generated by a control). The counter's output defines an address applied to the ROM's. The output of the ROM's is then applied to the D/A's, whose outputs
define the current command signals for each phase.
A problem with the foregoing approach is that several hardware components must be used, which increase the cost and size of the control device (commonly referred to as an indexer). In addition, the magnitude of the current commands, the number
of steps per motor revolution, as well as other parameters are difficult to vary, being commonly adjusted by way of dip switches.
Accordingly, there is a need to provide an improved stepper motor control circuit that minimizes or eliminates one or more of the problems as set forth above.
SUMMARY OF THE INVENTION
It is an object of the present invention to reduce the number of components in a stepper motor control circuit, and thus also the cost and size of the same. In addition, it is a further object of the present invention to improve the ease with
which operating parameters, such as the number of steps per motor revolution, and current command settings, may be selected.
Preferably, the control method is implemented in software, and such parameters as current levels and pulses per revolution may be set using software commands, thereby eliminating the need for dip switches or the like. This approach thus saves
money as well as test time, relative to conventional approaches.
To achieve these and other objects, and in accordance with the present invention, a method and apparatus for controlling a stepper motor is provided. In a first preferred embodiment, the method includes several basic steps. The first step
involves generating a first velocity signal indicative of a desired change in a rotational position of the motor per unit time. Next, dividing the change in the rotational position into a first pre-determined number of increments. The third step
involves generating a first current value signal using the first pre-determined number of increments and at least one function table. Next, storing the first current value signal in a first transfer table. The next step involves generating a second
velocity signal for the next predetermined period of time. The next step involves dividing a change in rotational position of the motor associated with the second velocity signal into a second pre-determined number of increments. The next step involves
generating a second current value signal using the second pre-determined number of increments and the function table. Finally, storing the second current value signal in a second transfer table. Importantly, the step of generating the second current
value signal and storing it in the second transfer table occurs while the first current value signal is retrieved from the first transfer table and is used to generate excitation signals that are applied to the motor windings. The foregoing is
implemented, in a constructed embodiment, by programming a microprocessor having direct memory access (DMA) capabilities to thereby eliminate a number of discrete hardware components. Particularly, in the constructed embodiment, the DMA controller
transfers the first current value signal to generate the excitation signals while the microprocessor core calculates the second current value signal.
In a second preferred embodiment, the method for controlling a stepper motor again includes several basic steps. The first step involves generating a velocity signal during a first update interval. The first update interval may include a
plurality of interrupt intervals and the velocity signal may correspond to a desired change in rotational position of the motor per unit time. In a second step, the change in the rotational position of the motor--as represented by the velocity
signal--is divided by a predetermined number to obtain an incremental position change value. In a third step, the incremental position change value is used to access a function table and generate a first current value. The third step may take place
during a first interrupt interval of the plurality of interrupt intervals. A fourth step involves generating a first current in the motor responsive to the first current value. The first current may be supplied to a first phase coil of the motor. Like
the third step, the fourth step may take place during the first interrupt interval.
A method in accordance with the second preferred embodiment may include several additional steps. For example, the method may include the step of repeating the first current value generating step and first current generating step during each
interrupt interval of the update interval. The method may also include the steps of generating a second current value and generating a second current during the first interrupt interval for use in controlling a second motor phase of the motor.
The second preferred embodiment is advantageous relative to the first preferred embodiment because it may be implemented with fewer components. In particular, the first preferred embodiment uses a pair of transfer tables and a DMA controller to
access the transfer tables. Both the transfer tables and DMA controller may be eliminated in the second preferred embodiment, which relies on current command generation directly from a function table during each interrupt interval of the update
interval.
Other objects, features and advantages of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings illustrating features of this invention by way of example, but not by
way of limitation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified, schematic and block diagram view of an improved stepper motor system according to a first embodiment of the present invention.
FIG. 2 is a simplified, schematic and block diagram view showing, in greater detail, the control means shown in block diagram form in FIG. 1.
FIGS. 3 shows exemplary waveforms generated by the control circuit according to the present invention.
FIG. 4 is a simplified flow chart diagram showing the overall methodology of a first embodiment of the present invention.
FIG. 5 shows, in greater detail, the general step of determining current value signals for an inactive transfer table, shown in block diagram form in FIG. 4.
FIG. 6 shows the current value determining means 30 of FIG. 2 in greater detail.
FIG. 7 is a simplified, schematic and block diagram view of an improved stepper motor system according to a second embodiment of the present invention.
FIG. 8 is a simplified, schematic and block diagram view showing, in greater detail, the control means shown in block diagram form in FIG. 7.
FIG. 9 shows the current value determining means 90 of FIG. 8 in greater detail.
FIG. 10 is a simplified flow chart diagram showing the overall methodology of a second embodiment of the present invention.
FIG. 11 shows, in greater detail, the general step of determining current value signals, shown in block diagram form in FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, FIG. 1 shows a motor system 10 in accordance with a first preferred embodiment of the present invention. System 10
includes a digital motor, such as two-phase stepper motor 12, and means or circuit 14 for energizing the motor phases of motor 12 in accordance with pre-determined criteria. Although the first preferred embodiment of the present invention will be
described in connection with a preferred, two-phase stepper motor embodiment, it should be appreciated that the present invention is not so limited, and may be profitably employed to electric motors having three or more phases.
Motor 12 may comprise one of a plurality of conventional stepper motor configurations known to one of ordinary skill in the art. In a constructed embodiment, a two-phase, permanent magnet rotor, 200-step per revolution configuration was used.
However, it should be appreciated that the number of motor phases, the number of full steps per revolution, etc. are exemplary only, and not limiting in nature. Motor 12 is diagrammatically illustrated in FIG. 1, and includes a rotor portion 22, a
stator portion 24, and windings 26.sub.A and 26.sub.B defining motor phase A and motor phase B, respectively. Energizing means 14 is configured to energize windings 26.sub.A, and 26.sub.B so that phase currents I.sub..phi.A, and I.sub..phi.B,
respectively flow therethrough. It should be appreciated, due to the conventional construction of motor 12, that rotor 22 may include a plurality of radially, outwardly extending poles, while stator 24 may include a plurality of radially inwardly
extending poles (which may have teeth formed on the ends thereof) upon which windings 26.sub.A, and 26.sub.B are wound. The tips of the stator poles define a central bore sized to accommodate rotor 22. In all respects, motor 12 is well-known and
conventional.
Energizing means 14 includes control means 16, a plurality of digital-to-analog (D/A) converting devices 18, and a drive circuit 20. Control means 16 is configured to directly generate a pair of digital words (the current value signals 27 )
corresponding to the desired current magnitude to flow through the motor phases A and B. In a preferred embodiment, control means 16 outputs a 16-bit word comprising two 8-bit words, one 8-bit word for each phase A and B. The 16-bit digital word is
updated by control means 16 at a predetermined frequency. For purposes of illustration only, in a constructed embodiment, control means 16 updates the 16-bit digital word 64 times every two milliseconds. In an alternate embodiment, control means 16
updates the 16-bit digital word 48 times every two milliseconds. It should be appreciated that the number of microsteps per revolution is selectable and may be varied and still remain within the scope of the present invention. In addition, it should be
further appreciated that the selection of the number of microsteps/revolution may be influenced by the type of drive circuit 20 used in system 10, the particular configuration of motor 12, as well as the instantaneous values of various operating
parameters of motor 12 (e.g., speed, acceleration, etc.). In a preferred embodiment, control means 16 comprises a microprocessor pre-programed in accordance with the present invention. In a constructed embodiment, a commercially available component
Hitachi model H8/3003 having on-chip RAM, ROM, a processing core, as well as an on-chip direct memory access (DMA) controller (DMAC) has been found to operate satisfactorily in carrying out the invention. Of course, other devices known to those in the
art, and equivalent thereto, may be readily substituted therefor and remain within the scope of the
present invention.
Digital-to-analog conversion (D/A) devices 18 are configured to receive, respectively, the 8-bit digital words (the current value signals 27 ) corresponding to the desired phase current levels. D/A's 18 also each receive a reference signal
REFERENCE, which sets the full scale of the current command signals I.sub.CMD.phi.A and I.sub.CMD.phi.B. D/A's 18 then generate, in response thereto, a pair of "analog" current command signals I.sub.CMD.phi.A, and I.sub.CMD.phi.B, respectively. The
current command signals, of course, as should be appreciated by those of ordinary skill in the art, are comprised of a series of small steps.
Drive circuit 20 is conventional, and well-known in the art, and is configured to generate excitation signals for phase A and phase B of motor 12 using the current command signals I.sub.CMD.phi.A and I.sub.CMD.phi.B, to thereby cause phase
currents I.sub..phi.A, and I.sub..phi.B to flow through coils 26.sub.A and 26.sub.B, respectively. The art is literally replete with various drive configurations and topologies suitable for use with the present invention. Accordingly, this block will
not be discussed in any further detail.
Before proceeding with the detailed description of the invention referenced to the drawings, a general overview of the control established by the present invention will be set forth. To control stepper motor 12's rotational position, a VELOCITY
signal is calculated by control 16 every Update Interval (2 milliseconds in a constructed embodiment). The VELOCITY signal represents the desired change in rotational position over the next Update Interval. Specifically, VELOCITY corresponds to the
number of "microsteps" to take during the next Update Interval. This change in position is divided by a pre-determined number corresponding to the number of table entries in a transfer table, and is preferably 64 in a constructed embodiment. The result
of the division is a Velocity Increment (VEL INC). The VEL INC is used to generate position data every 31.25 microseconds (in a constructed embodiment--0.002/64) over the next Update Interval. In the constructed embodiment, each new VELOCITY signal is
used to populate one of two 64 entry transfer tables by reading data from a function table (e.g., a sine wave or cosine wave) using the velocity increment to generate a table address to extract a data sample from the function table. After one of the
transfer tables is populated, and at 31.25 microsecond intervals, a data word from the transfer table is written to the output port. The data at the output port pins are the current value signals. The transfer table address is incremented following
each transfer in order to output the next table entry on the next transfer (31.25 microseconds later). After 64 transfers (2 milliseconds), the entire transfer table has been output, and at this point a software interrupt is generated. The interrupt
code will cause a new VELOCITY signal value to be calculated, and with it, 64 new table entries are determined. There are two transfer tables. A first one of the tables is outputting data every 31.25 microseconds under control of a DMA controller,
while the other is being loaded with 64 new words of data by the main control software. The roles of the two transfer tables are reversed every Update Interval by way of an interrupt. The transfer table most recently loaded will commence transferring
its data to the output port following the next interrupt, while the transfer table presently accessed by the main control will be reloaded with new current values.
Referring now to FIG. 2, control means 16 is shown in greater detail. Control means 16 includes means 28 for generating a velocity signal, means 30 for determining current value signals, a function table 32, a first transfer table 34, a second
transfer table 36, an output port 38, a velocity increment memory 40, a transfer table position memory 42, a table address memory 44 and a direct memory access (DMA) controller (DMAC) 45.
Generating means 28 is preferably implemented in software by executing a sequence of pre-programmed steps stored in program memory of control means 16. The output signal from generating means 28, namely velocity signal VELOCITY, may be, and is
in a constructed embodiment, a digital word indicative of a desired change in a rotational position of rotor 22 per unit time. In one embodiment, the VELOCITY signal count corresponds to the desired number of microsteps for rotor 22 to take during the
next Update Interval. For example, assume that a full-step for motor 12 corresponds to the well-known 1.8.degree. mechanical displacement of rotor 22. Further assume, in one configuration, that 64 microsteps per full-step is selected. Also assume
that the Update Interval is 2 milliseconds. Based on the foregoing, a VELOCITY count of 640 corresponds to | | |
