Hybrid stepping motor unit

A hybrid stepping motor unit includes a stepper-type primary motor and a secondary motor. The rotor shafts of the stepper-type primary motor and the secondary motor are connected to a common output shaft. The stepper-type primary motor is energized in response to an electrical command signal from an external source, such as a computer. When the stepper-type primary motor is energized, the stepper-type primary motor produces torque and attempts to drive the load combination of the interconnected rotor shafts, the common output shaft, and any mechanical load on the common output shaft, such as a machine tool component. This produces a reaction torque on the stepper-type primary motor. A transducer converts the reaction torque into an electrical control signal. The secondary motor is energized in response to the electrical control signal from the transducer. When the secondary motor is energized, the secondary motor produces torque which augments the torque of the stepper-type primary motor to drive the load circulation. After the rotor shaft of the stepper-type primary motor rotates a predetermined angular distance, a reaction torque on the stepper-type primary motor is no longer produced. The electrical control signal from the transducer ceases, and the secondary motor is de-energized. Consequently, application of torque to the common output shaft is discontinued. The hybrid stepping motor unit responds to successive electrical command signals in like manner to drive the common output shaft a preselected angular distance in response to each electrical command signal.

BACKGROUND OF THE INVENTION 
This invention relates to electromechanical transducers, that is, devices 
which convert an electrical input into a mechanical output. More 
particularly, this invention relates to motors and, especially, stepping 
motors, which convert discrete electrical signals into mechanical torques 
that act on a mass, such as a shaft, or other mechanical load, to rotate 
the mass in a series of steplike angular displacements of essentially 
uniform magnitude. Specifically, this invention is directed to a hybrid 
stepping motor unit. 
Conventional stepping motors have many uses. A stepping motor can be used, 
for example, to convert a series of digital pulses from a computer into 
steplike angular displacements of a shaft which is an element of a drive 
train for positioning a machine tool component. A stepping motor can also 
be employed to position an optical code disc in an electronic typesetting 
system or to position the pen of a chart recorder. Other exemplary uses 
are well known to those who are skilled in the art, several being 
described in "Sigma Stepping Motor Handbook", published in 1972 by Sigma 
Instruments, Inc. 
Prior art stepping motors are of two basic types. One type of system is the 
conventional open loop stepper motor. This type of stepping motor is 
described, for example, in the aforementioned Sigma Instruments, Inc. 
publication. A second type of system is the conventional closed loop 
step-servo motor. An example of this type of stepping motor is the Digital 
Control C200 Series ID0376-SP-3M which is manufactured by Inland Motor 
Division of Kollmorgen Corporation. 
An open loop stepper motor characteristically has a high heat loss to power 
ratio. The open loop stepper motor is, therefore, impractical for high 
horsepower applications. 
Despite limitation to applications which require only fractional 
horsepower, however, open loop stepper motors advantageously provide a 
relatively stiff drive mechanism for a load. Stated differently, in low 
horsepower applications where open loop stepper motors are generally used, 
each discrete electrical command signal, under nominal load, is translated 
into a corresponding steplike displacement of the load. As the load on the 
open loop stepper motor increases, however, the displacement of the load 
lags the command signal by an increasing fraction of a step. At full load, 
the step lag is exactly onehalf step. Any load beyond full load will cause 
the open loop stepper motor to loose one, or more, steps. Nevertheless, 
the stiffness of the open loop stepper motor, measured in ounce-inches of 
torque per step lag, is relatively high compared to that of a closed loop 
step-servo motor. 
The closed loop step-servo motor, on the other hand, characteristically has 
a lower heat loss to power ratio than the open loop stepper motor. The 
closed loop step-servo motor is, therefore, generally used in applications 
which require high horsepower. 
In comparison with the open loop stepper motor, however, conventional 
closed loop step-servo motors provide a more compliant, or less stiff, 
drive mechanism for a load, since the closed loop step-servo motor itself 
receives no electrical drive signal until the load displacement lags the 
electrical command signal by a minimum of one full step. Unlike the open 
loop stepper motor, which carries full load with a lag of only one-half 
step, the closed loop step-servo motor carries only the first increment of 
load with a lag of one full step. Moreover, in order to provide a smooth 
progression of torque increments, the closed loop step-servo motor 
requires serveral full step lags in order to develop full torque. 
Consequently, the drive stiffness of the closed loop step-servo motor, 
measured in ounce-inches of torque per step lag, is low relative to that 
of the open loop stepper motor. This low drive stiffness of the closed 
loop step-servo motor has a highly detrimental effect on the resolutional 
integrity under load compared to the resolutional integrity under load of 
an open loop stepper motor. 
In summary, open loop stepper motors have high heat loss to power ratios 
and are, therefore, suitable only for fractional horsepower applications. 
Open loop stepper motors do, however, have relatively high drive stiffness 
which enables them to maintain high resolutional integrity under load, 
which also puts a minimal demand on the source of electrical command 
signals, such as a computer. Closed loop step-servo motors, on the other 
hand, have relatively low heat loss to power ratios and are, therefore, 
suitable for multihorsepower applications. However, closed loop step-servo 
motors have low drive stiffness which does not enable them to maintain 
good resolutional integrity under load, which puts a high demand on the 
computer or other electrical command signal source. 
OBJECTS OF THE INVENTION 
A first object of this invention is to provide a motor which has 
characteristics of operation that make it superior to conventional 
stepping motors. 
A second object of this invention is to provide a motor which provides a 
stiffer drive mechanism for a load and is effective in response to 
electrical command signals to position a load, such as a machine tool 
component, with a higher degree of resolutional integrity than 
conventional stepping motors. 
Another object of this invention is to provide an open loop motor that is 
capable of carrying full-rated torque without danger of missing steps. 
A further object of this invention is to provide an efficient motor of any 
desired horsepower rating from fractional to multihorsepower, for example, 
10 or more horsepower. 
Another object of this invention is to provide a motor which, due to 
improved drive stiffness, has a high natural drive frequency so as to 
provide superior acceleration and deceleration times. 
Another object of this invention is to provide a motor the cost of which is 
comparable to that of conventional closed loop step-servo motors of equal 
horsepower rating. 
SUMMARY OF THE INVENTION 
The present invention provides a unique hybrid stepping motor unit which 
differs from stepping motors of either the conventional open loop stepper 
type or the closed loop step-servo type. More particularly, the hybrid 
stepping motor unit of the present invention includes a low horsepower 
stepper-type primary motor. The hybrid stepping motor unit of the present 
invention further includes a secondary motor of relatively high 
horsepower. The rotor shafts of the stepper-type primary motor and the 
secondary motor are connected to a load, such as a common output shaft, 
which may in turn drive any additional load. 
The stepper-type primary motor has a drive circuit which energizes the 
stepper-type primary motor in response to an electrical command signal 
from an external source, such as a computer. When the stepper-type primary 
motor is energized, the stepper-type primary motor produces a torque and 
attempts to drive the interconnected rotor shafts and the common output 
shaft. This produces a mechanical reaction, or back torque, on the 
stepper-type primary motor. The secondary motor has a drive circuit which 
energizes the secondary motor in response to an electrical control signal 
which is produced by a suitable transducer in response to the reaction 
torque on the stepper-type primary motor. When the secondary motor is 
energized, the secondary motor produces a torque which augments the torque 
of the stepper-type primary motor to drive the interconnected rotor shafts 
and the common output shaft. 
After the rotor shaft of the stepper-type primary motor rotates a 
predetermined angular distance, a reaction torque on the stepper-type 
primary motor is no longer produced, and the electrical control signal 
from the transducer ceases so that the secondary motor is de-energized. 
Consequently, application of torque to the common output shaft is 
discontinued. The hybrid stepping motor unit responds to successive 
electrical command signals in like manner to drive the common output shaft 
a preselected angular distance in response to each electrical command 
signal. 
In a preferred embodiment of a hybrid stepping motor unit in accordance 
with the present invention, the stepper-type primary motor is mounted 
directly upon the secondary motor to form a highly compact unit. The rotor 
shaft of the stepper-type primary motor is connected through resolution 
gearing to the rotor shaft of the secondary motor. Preferably, the rotor 
shaft of the secondary motor also comprises the common output shaft. 
A bracket or the like provides a deformable connecting means between the 
stator housing of the stepper-type primary motor and the stator housing of 
the secondary motor. A transducer, which may be of the strain gauge or 
other convenient type, is mounted on the bracket. When the stepper-type 
primary motor is energized, a reaction torque on the stepper-type primary 
motor is produced. This results in deformation of the bracket. This 
deformation is sensed by the transducer which produces an electrical 
control signal proportional to the reaction torque. This electrical 
control signal is fed to the drive circuit for the secondary motor. In 
response, the drive circuit for the secondary motor energizes the 
secondary motor which produces torque to augment the torque of the 
stepper-type primary motor to drive the common output shaft. 
After the rotor shaft of the stepper-type primary motor rotates a 
predetermined angular distance, the stepper-type primary motor ceases to 
attempt to drive the common output shaft. Consequently, a reaction torque 
is no longer produced, and the electrical control signal from the 
transducer ceases. As a result, the drive circuit for the secondary motor 
discontinues energization of the secondary motor so that no further torque 
is applied to the common output shaft. The hybrid stepping motor unit 
responds to successive electrical command signals in like manner to drive 
the common output shaft a preselected angular distance in response to each 
electrical command signal. 
One important advantage of the hybrid stepping motor unit of the present 
invention is that it provides a stiffer drive mechanism than conventional 
stepping motors. This means that the hybrid stepping motor of the present 
invention has a higher resolutional integrity under load than is 
attainable by means of prior art stepping motors, such as open loop 
stepper motors and closed loop step-servo motors. 
A second advantage of the hybrid stepping motor unit of the present 
invention is that it has a higher natural drive frequency than 
conventional stepping motors and provides superior acceleration and 
deceleration times. 
A further advantage of the present hybrid stepping motor unit is that the 
initial cost of the hybrid stepping motor unit of the present invention is 
comparable to the cost of conventional closed loop step-servo motors of 
similar horsepower rating. 
These advantages of the hybrid stepping motor unit of the present invention 
will be discussed more fully hereinafter.

GENERAL DESCRIPTION 
A hybrid stepping motor unit embodying the present invention is illustrated 
in block diagram form in FIG. 1. The hybrid stepping motor unit, which is 
designated generally by the numeral 10, includes a stepper-type primary 
motor 12 and a secondary motor 14. 
The rotor shaft 16 of the stepper-type primary motor 12 and the rotor shaft 
18 of the secondary motor 14 are connected to a common output shaft 22. As 
shown in FIG. 1, preferably the rotor shaft 16 of the stepper-type primary 
motor 12 and the rotor shaft 18 of the secondary motor 14 are 
interconnected by resolution gearing 20, and the output shaft 22 comprises 
an extension of the rotor shaft 18 of the secondary motor 14. 
The stepper-type primary motor 12 is energized by a stepper-type primary 
motor drive circuit 24. The secondary motor 14 is energized by a secondary 
motor drive circuit 26. 
The hybrid stepping motor unit 10 also includes a reaction torque 
transducer 28. When the stepper-type primary motor drive circuit 24 
energizes the stepper-type primary motor 12, the reaction torque 
transducer 28 detects a reaction torque on the stepper-type primary motor 
12. The reaction torque transducer 28 produces an electrical control 
signal which is proportional to this reaction torque. The electrical 
control signal is input via a lead 45 to the secondary motor drive circuit 
26 which in turn energizes the secondary motor 14. Consequently, the 
secondary motor 14 produces a torque to augment the torque of the 
stepper-type primary motor 12. 
A tachometer 30 produces an electrical feedback signal as the rotor shaft 
18 of the secondary motor 14 rotates. This electrical feedback signal is 
fed back to the secondary motor drive circuit 26 in a conventional manner 
for damping purposes. 
With reference to FIG. 1, the hybrid stepping motor unit 10 is shown in 
conjunction with an exemplary application, that is, the drive for a load, 
such as the table of a milling machine, 32. In operation, a digital 
command source 34, which, for example, may comprise a programmed general 
purpose digital computer, supplies discrete electrical command signals to 
the stepper-type primary motor drive circuit 24. In response, the 
stepper-type primary motor drive circuit 24 energizes the stepper-type 
primary motor 12, which responds by applying torque to the rotor shaft 16. 
The mechanical resistance to angular motion of the rotor shaft 16 and the 
load of the rotor shaft 18, the common output shaft 22, and the load 32 
results in a reaction torque on the stepper-type primary motor 12. This 
reaction torque is detected by the reaction torque transducer 28 which 
converts the reaction torque to an electrical control signal, the 
magnitude of which is dependent upon the amount of reaction torque. 
The electrical control signal from the transducer 28 is input via the lead 
45 to the secondary motor drive circuit 26. The secondary motor drive 
circuit 26 in response to the electrical control signal from the reaction 
torque transducer 28 energizes the secondary motor 14. The secondary motor 
14 responds by applying torque to the rotor shaft 18. The rotor shaft 18 
is connected to the load 32 through the common output shaft 22 which may 
comprise an extension of the rotor shaft 18. In order to minimize energy 
dissipation during acceleration, however, the common output shaft 22 may 
be distinct from the rotor shaft 18, and gears (not shown) may be 
interposed between the rotor shaft 18 and the common output shaft 22. 
When the rotor shaft 16 has rotated a predetermined angular distance, the 
stepper-type primary motor 12 no longer produces torque. The electrical 
control signal from the reaction torque transducer 28 ceases, and the 
secondary motor drive circuit 26 discontinues energization of the 
secondary motor 14. Consequently, the secondary motor 14 also no longer 
produces torque, and the hybrid stepping motor 10 regeneratively assumes 
its initial state to await another electrical command signal from the 
digital command source 34. 
The hybrid stepping motor unit 10 responds to successive electrical command 
signals in like manner to drive the output shaft 22 a preselected angular 
distance in response to each such signal to drive the load 32. 
DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 2 and 3 show a preferred embodiment for the hybrid stepping motor 
unit 10 of the present invention. The hybrid stepping motor unit 10 
includes a stepper-type primary motor 12 and a secondary motor 14. The 
stepper-type primary motor 12 is mounted on a bracket 35 in any 
appropriate manner. The bracket 35, for example, may be attached to the 
stator housing 13 of the stepper-type primary motor 12 by means of bolts 
36. The bracket 35 is in turn mounted on the secondary motor 14. Bolts 37, 
for example, may secure the bracket 35 to the stator housing 15 of the 
secondary motor 14. 
A pinion 17 is mounted on the rotor shaft 16 of the stepper-type primary 
motor 12. A pinion 19 is likewise mounted on the rotor shaft 18 of the 
secondary motor 14. The pinions 17 and 19 may be secured to the rotor 
shafts 16 and 18, respectively, in any suitable manner, as by 
press-fitting the pinions 17 and 19 on the respective rotor shafts 16 and 
18. 
As shown in FIGS. 2 and 3, the pinions 17 and 19 are meshed so that the 
rotor shafts 16 and 18 are in driving relation to one another. The bracket 
35 by which the stepper-type primary motor 12 is mounted on the secondary 
motor 14 may have an arcuate slot 38. Consequently, when the bracket 35 is 
mounted on the stator housing 15 of the secondary motor 14 by means of 
bolts 37, the position of the pinion 17 with respect to the pinion 19 can 
be adjusted so that pinions 17 and 19 are in proper meshed relation to 
minimize gear backlash. In the preferred embodiment, the common output 
shaft 22 comprises an extension of the rotor shaft 18 of the secondary 
motor 14. 
The torque of the stepper-type primary motor 12 is applied through the 
rotor shaft 16, the pinion 17, the pinion 19 and the rotor shaft 18 to 
drive the output shaft 22. Also, the secondary motor 14 applies torque 
through the rotor shaft 18 to drive the output shaft 22. 
In accordance with the present invention, the torque which is applied to 
the output shaft 22 by the stepper-type pirmary motor 12 and the secondary 
motor 14 is additive. Stated differently, the torque of the secondary 
motor 14 augments the torque of the stepper-type primary motor 12. 
Consequently, rotation of the rotor shaft 16 of the stepper-type primary 
motor 12 and rotation of the rotor shaft 18 of the secondary motor 14 are 
in opposite directions for the configuration which is shown in FIGS. 2 and 
3. That is, the stepper-type primary motor 12 rotates the pinion 17 in the 
clockwise direction as shown by the arrow 39 in FIG. 3, and the secondary 
motor 14 rotates the pinion 19 in the counterclockwise direction as shown 
by the arrow 40 in FIG. 3 or vice versa. 
With reference to FIGS. 2-4, when the stepper-type primary motor 12 is 
energized and applies a torque to the rotor shaft 16, a reaction torque 
between the rotor and the stator of the stepper-type primary motor 12 is 
produced. This reaction torque is transmitted to the bracket 35. 
A suitable reaction torque transducer 28, such as strain gauges 41, 42, 43 
and 44, is attached to the bracket 35 in any appropriate manner, as by 
means of a suitable adhesive. The strain gauges 41, 42, 43 and 44 are 
connected as shown in FIG. 5 to detect strain, or deformation, of the 
bracket 35 caused by the reaction torque which acts on the stepper-type 
primary motor 12 when the stepper-type primary motor 12 is energized by 
the stepper-type primary motor drive circuit 24 (FIG. 1). The output from 
the reaction torque transducer 28, which comprises a conventional bridge 
circuit that includes the strain gauges 41, 42, 43 and 44, is input via 
the lead 45 as an electrical control signal to the secondary motor drive 
circuit 26 (FIG. 1). The secondary motor drive circuit 26 energizes the 
secondary motor 14 whenever reaction torque is present so that the 
secondary motor 14 produces a torque which augments the torque that the 
stepper-type primary motor 12 produces to drive the common output shaft 
22. 
The stepper-type primary motor 12 and the stepper-type primary motor drive 
circuit 24 can be a small DC stepper motor plus drive circuit of 
well-known construction. One suitable motor is the Kearfott Division of 
the Singer Company Variable Reluctance Stepper Motor CR4 0192 015. One 
suitable drive circuit is the Kearfott Division Stepper Motor Driver C70 
3531 007. The secondary motor 14 can be a conventional multihorsepower DC 
motor with a built-in tachometer. One suitable motor is the Inland Motor 
Division TTR-2952-120-A-00 rare earth magnet motor with built-in 
tachometer, 5:1 resolution gearing, and a resolver mount. The 
aforementioned resolver mount may comprise the bracket 35 to mount the 
stepper-type primary motor 12 on the secondary motor 14. The secondary 
motor drive circuit 26 can be a conventional SCR pulse-width-modulated 
drive circuit. One such drive circuit is the Inland Motor Division 
SPAE-1115 Amplifier. The secondary motor 14 and secondary motor drive 
circuit 26, however, may be a conventional DC motor and DC servo 
amplifier, especially in applications which require high stepping rates. 
The reaction torque transducer 28 may comprise four Bean Bae-XX-062AA-120 
Strain Gauges which are attached to the resolver mount as shown in FIGS. 
2-4 and connected in a bridge circuit as shown in FIG. 5. 
Preferably, in the hybrid stepping motor unit of the present invention, a 
relatively high horsepower secondary motor augments the torque of a low 
horsepower stepper-type primary motor. When the hybrid stepping motor unit 
is configured so that the secondary motor produces substantially the 
entire portion of the torque necessary to drive the common output shaft, a 
small stepper-type primary motor can be used so that the high heat loss to 
power ratio of the steppe-type primary motor is insignificant. Since the 
secondary motor may be any horsepower rating, the hybrid stepping motor 
unit has application at any required horsepower. Moreover, the initial 
cost of the hybrid stepping motor unit is comparable to the cost of a 
conventional closed loop step-servo motor for any given application. 
The advantages of the hybrid stepping motor unit over either the 
conventional open loop stepper motor or the conventional closed loop 
step-servo motor will now be described. 
First, if the size of the desired step size is the same, the hybrid 
stepping motor unit provides a stiffer drive mechanism than either the 
conventional open loop stepper motor or the conventional closed loop 
step-servo motor. Consequently, the hybrid stepping motor unit has a 
higher resolutional integrity under load than conventional stepping 
motors. 
In a conventional open loop stepper motor, the output shaft rest position 
will be such as to balance the torque on the output shaft and the reaction 
torque, that is, the output shaft will not be at its unloaded position but 
at a holding position. This means that there is a step lag between an 
electrical command signal and mechanical displacement of the output shaft 
to a predetermined desired angular position. This predetermined desired 
angular position has reference to the position wherein the output shaft 
would be in an unloaded position. 
In the present hybrid stepping motor unit, the reaction torque transducer 
detects reaction torque on the stepper-type primary motor. If reaction 
torque is present, the secondary motor is energized so that the output 
shaft is displaced to an angular position wherein the stepper-type primary 
motor develops no reaction torque. Consequently, the reaction torque which 
would produce a holding position, if the stepper-type primary motor were 
operated as a conventional open loop stepper motor, in the present hybrid 
stepping motor unit tends to cause energization of the secondary motor to 
displace the rotor shaft of the stepper-type primary motor to an unloaded 
position. This means that there is less step lag between an electrical 
command signal and mechanical displacement of the output shaft to a 
preselected desired angular position, and therefore, the present hybrid 
stepping motor unit is a stiffer drive mechanism than a conventional open 
loop stepper motor. 
The present hybrid stepping motor unit, which is stiffer than a 
conventional open loop stepper motor, also provides a stiffer drive 
mechanism than a conventional closed loop step-servo motor. With the 
preferred embodiment of the hybrid stepping motor of the present invention 
as described above, for example, the relatively high torque of the 
secondary motor is produced from the same output shaft displacement as 
required to produce the torque of the stepper-type primary motor. As 
indicated above, the step lag is less than that which would occur if the 
stepper-type primary motor were operated as a conventional open loop 
stepper motor. In consequence of the high combined torque and small step 
lag, the stiffness of the hybrid stepping motor unit of the present 
invention is much higher than a closed loop step-servo motor of comparable 
horsepower rating. 
Since the hybrid stepping motor unit of the present invention has a smaller 
step lag than either a conventional open loop stepper motor or a 
conventional closed loop step-servo motor, a higher resolutional integrity 
under load is attainable over that heretofore known. That is, the hybrid 
stepping motor unit is responsive to an electrical command signal to 
rotate the output shaft through a steplike angular displacement such that 
the load is positioned with greater integrity with reference to a 
preselected desired angular position than is attainable with conventional 
stepping motors. This is a result of the fact that for a comparable step 
size the present hybrid stepping motor unit displaces the rotor shaft of 
the stepper-type primary motor to substantially the unloaded position 
whereas the conventional open loop stepper motor displaces the rotor shaft 
only to the holding position. With respect to a conventional closed loop 
step-servo motor, the resolutional integrity under load of the present 
hybrid stepping motor unit is approximated only if the size of the step 
for the conventional closed loop step-servo motor is a fraction of the 
step size for the present hybrid stepping motor unit. If a programmed 
general purpose digital computer were the source of electrical command 
signals, however, this would mean a much greater demand on the computer 
for the conventional closed loop step-servo motor than for the present 
hybrid stepping motor unit in order to obtain the same resolutional 
integrity under load. Consequently, the hybrid stepping motor unit of the 
present invention provides a stiff drive mechanism which is especially 
well-suited for use in high horsepower applications that require high 
resolutional integrity under load. 
Since the natural drive frequency is related to the stiffness, given any 
value for the inertia of the mechanical system, the natural drive 
frequency of the present hybrid stepping motor unit is higher than that of 
a conventional open loop stepper motor or a conventional closed loop 
step-servo motor. This means that better acceleration and deceleration 
times are attainable with the hybrid stepping motor unit of the present 
invention. 
It has been pointed out above that the hybrid stepping motor unit may be 
configured so that the secondary motor produces substantially the entire 
portion of the torque that is necessary to drive the output shaft. As a 
result, the efficiency of the hybrid stepping motor unit depends almost 
entirely on the efficiency of operation of the secondary motor, and the 
high heat loss to power ratio of the stepper-type primary motor is 
inconsequential. Since the operation of the stepper-type primary motor 
does not figure significantly in the determination of efficiency, there is 
no need to force the step lag of the stepper-type primary motor near the 
maximum step lag limit where maximum torque is produced and which 
represents the most efficient point of operation for the stepper-type 
primary motor. The stepper-type primary motor can operate at a small step 
lag to guard against loss of step. Any consequent loss of torque which 
results from such operation of the stepper-type primary motor may be 
compensated by an increase in the torque that is produced by the secondary 
motor by means of an increase in the gain of the secondary motor drive 
circuit amplifiers. 
The secondary motor of the hybrid stepping motor unit may be selected so 
that full horsepower is required to drive the load. Consequently, since 
the secondary motor is fully loaded, the secondary motor will operate at 
optimum efficiency. The drive circuit for the secondary motor may be 
adjusted so that any reaction torque on the stepper-type primary motor 
produces energization of the secondary motor so as to produce full torque. 
In this case, energization of the secondary motor by the secondary motor 
drive circuit is such that the overload protection for the secondary motor 
would have to operate before steps could be missed. 
Other advantages of the hybrid stepping motor unit over either a 
conventional open loop stepper motor or a conventional closed loop 
step-servo motor will be apparent to those who are skilled in the art. As 
one example, the hybrid stepping motor unit of the present invention does 
not require an up/down counter or position encoder such as a resolver, as 
are present in a conventional closed loop, digital-analog-digital 
step-servo motor. This particular advantage leads to simplification in 
installation procedure, based on elimination of calibration of a position 
encoder, as well as reduction in the number of components. 
The hybrid stepping motor unit structure shown in the drawing and described 
above is subject to modification without departure from the spirit and 
scope of the appended claims.