Stepper motor damping circuit

A method and an apparatus is disclosed for electrically damping the rotation motion of a stepper motor. The voltage of the non-energized side of the motor windings is monitored and compared with the energized side. Once the voltage on the non-energized side reaches a predetermined value greater than the voltage on the energized side, the non-energized voltage is shunted to the energized voltage.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for controlling a stepping 
motor and more particularly is directed to a method and apparatus for 
electrically dampening a stepping motor. 
One of the basic problems with regard to operation of a stepping motor at a 
high rate of speed is the ability to stop the stepper motor as it arrives 
at a final position with little or no overshoot or oscillation of the 
rotor. The prior art discloses a number of approaches to the problem of 
how to effectively dampen a stepping motor in order to achieve a quick 
stop time. The two main methods taught by the prior art are by mechanical 
friction and electrical damping. 
Mechanical frictional devices reduce the mechanical overshoot and the 
audible noise by coupling a rotational inertial mass via a viscous medium. 
This is usually done by using disc or drums rotationally coupled through a 
thick silicon oil. These mechanical dampers must be tailored to a specific 
type motor, and even then they present a limit on the maximum stepping 
rate due to the load involved. 
Electrical dampening has been taught in the prior art in two ways. The 
first way taught is to distort or correct the driving electrical signal so 
as to slow the motor during the later portions of each drive pulse. This 
method is very load-sensitive since the stop or slowdown signal must be 
changed. A much larger consumption of power is required in this type 
method and the maximum stepping rate is severely reduced as the full step 
period would be apportioned between the go and the stop modes. The second 
method taught in electrical dampening is by the use of dynamic braking. 
Dynamic braking can be used to utilize the loading of the generator effect 
of the moving motor to minimize the mechanical overshoot following a stop 
position change. When the stepper motor makes a step, each coil, whether 
or not it is energized, produces a voltage by generator action. The 
overshoot produces a voltage also and this voltage can be clamped or 
shorted by diodes, resistors, or combinations of both. Unfortunately, 
stepper motors have, like most multi-phased motors, quite good intercoil 
coupling, which results in transformer action between driven and undriven 
coils. The phase of this coupling is such that a drive signal on one coil 
can be transformed to another coil in a polarity similar to a generated 
signal on the second coil. This results in the diode or resistor network 
clamping a transformer coupled drive signal which is undesired because of 
the power wasted, poor step response, and excessive heat due to the coil 
being partially shorted during energization of the first coil. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide a circuit that can improve 
means for electrically dampening a stepping motor in order to prevent any 
overshoot or oscillation about the final position to which it is being 
rotated. 
Another object of this invention is to provide a quieter operating stepping 
motor without utilizing any additional power or command signals to the 
coils. 
Still another object of the present invention is to provide a more 
simplified dampening circuit than has heretofore been possible which 
utilizes the generator signal from any or all of the coils to actuate the 
dampening function. 
The above objects are given by way of example. Thus other desirable objects 
and advantages achieved by the invention may occur to those skilled in the 
art. The scope of the invention is to be limited only by the appended 
claims. 
BRIEF SUMMARY OF THE INVENTION 
The above objects and other advantages are achieved by the present 
invention. A method and apparatus is provided for electrically dampening a 
stepping motor. The present invention shorts all non-energized motor 
windings just following every drive pulse forcing the motor to act as a 
short-circuit generator just after every moving member has reached its 
desired position and the kinetic energy which was previously dissipated as 
mechanical overshoot and audible noise is now dissipated in the shorting 
elements and the winding of the motor. The invention is such that the 
shorting is removed during the drive pulse so that excessive power 
dissipation is avoided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description of the invention follows, referring to the drawings in which 
like reference numerals denote like elements of structure in each of the 
several Figures. 
Referring now to FIG. 1, a step command circuit 10 provides digital pulses 
at a predetermined step frequency which ultimately determines the speed of 
the stepping motor. Step command circuit 10 is operatively connected to a 
decade counter 12. Decade counter 12 is of a type in which each digital 
step received from the step command circuit 10 will generate a digital 
pulse on an individual output line of the decade counter 12. For example, 
in a decade counter that has ten output lines, it will take ten digital 
pulses from the step command circuit 10 to complete the counting sequence. 
The first digital pulse from the step command 10 will activate the first 
output of the decade counter 12 only, the second digital pulse from the 
step command circuit 10 will activate the second output of the decade 
counter only, etc. The decade counter 12 is connected to a series of 
switching devices 14 which comprise NOR gates 16a through 16d, series 
resistors 18a through 18d and NPN transistors 20a through 20d. The 
collectors of the transistors are connected to the individual coil 
windings 21a through 21d within the stepping motor, the transistors acting 
as switches. The other end of the coils within the stepper motor are 
attached to the power supply. As will be appreciated by those skilled in 
the art, decade counter 12 is wired to NOR GATES 16a through 16d in such a 
pattern as to provide switching of transistors 20a to 20d in a pattern 
required to drive a typical stepper motor. When transistors 20a through 
20d are turned "on", current is drawn through the drive coil within the 
stepper motor, thus powering the stepper motor. The problem with the prior 
art drive circuit shown in FIG. 1 is that the stepper motor drive coils 
will also act as a voltage generator due to the momentum of the rotor 
within the motor itself and intercoil coupling. In a typical stepper motor 
that is powered by a five-volt continuous power supply, it is possible 
that the stepper motor driver coil can generate voltages as high as 60 
volts D.C. The present invention contemplates shorting all non-energized 
motor windings just following every drive pulse in order to have the motor 
act as the short circuit generator just after the moving member has 
reached its new desired position. The kinetic energy which was previously 
dissipated as mechanical overshoot and audible noise will be dissipated in 
the shorting elements and the windings of the motor. 
FIG. 2 shows the modification to the prior art circuit shown in FIG. 1 in 
which the step command circuit 10 is connected to a sequence driver 12 
such as the decade counter shown in FIG. 1. The sequence driver 12 is 
connected to a first switching circuit 14 which was exemplified in FIG. 1 
with the combination of NOR gates, series resistors, and transistors. The 
switching circuit 14 is then connected to the individual drive coils 
within the stepper motor. A series of diodes 22a through 22d are 
operatively connected at their anodes to the stepper motor drive coils 21a 
through 21d on the same side as the first switching circuit 14 is 
connected. On the cathode side of diodes 22a through 22d, a voltage 
sensitive switching circuit 24 is provided. The output of switching 
circuit 24 is connected to the power supply side of the drive coils within 
the stepper motor. The operation of the present invention can be 
appreciated by following a single switching step of a stepping motor. 
Assume that it is desirable to power coil 21a within the stepper motor. 
First switching circuit 14 will switch the unpowered side of the drive coil 
21a to essentially ground. Unpowered coils 21b through 21d are essentially 
in what is known in the art as a floating mode, i.e. the unpowered side of 
the coils has a voltage equal to the powered side of the coils. Because 
the stepper motor acts as a voltage generator due to intercoil coupling, 
the non-energized coils 21b through 21d will build up a voltage in excess 
of the power supply voltage. The kinetic energy will thus be dissipated as 
mechanical overshoot and audible noise. In the present invention the 
generated voltage in the non-energized coils will pass because of forward 
bias through the respective diodes 22b through 22d into the voltage 
sensitive switch 24. Once the generated voltage exceeds a predetermined 
level, the voltage sensitive switch 24 will then short the unpowered side 
of the non-energized coils to the powered side, thus dissipating the 
kinetic energy of the non-energized coils. The present invention 
contemplates energizing the voltage sensitive switch 24 when the 
non-powered side of the non-energized coils reaches approximately twice 
the voltage of the powered side. As mentioned previously the non-powered 
side of the non-energized coils in a five-volt stepper motor may reach 
voltages as high as 60 volts. The present invention contemplates clamping 
or shorting this voltage at approximately 12 volts. 
In a specific embodiment of the present invention, FIG. 3 shows the basic 
stepper motor drive circuit of FIG. 1 with the diodes 22a through 22d 
including voltage sensitive switch 24. Voltage sensitive switch 24 
comprises an SCR 26 which has its anode 28 operatively connected to the 
cathode side of diodes 22a through 22d and the cathode side 30 connected 
to the powered side of the coils of the stepper motor. Two resistors 32 
and 34 are provided in series across the anode 28 of SCR 26 and cathode 30 
of SER 26 with the gate 36 of SCR 26 attached at the junction point of the 
two resistors 32 and 34. The resistive values of resistors 32 and 34 in 
conjunction with the type of SER 26 chosen will determine the voltage 
which as known in the art will fire the SCR 26. As mentioned above, the 
present invention contemplates the SCR firing at a voltage which is equal 
to approximately twice the power supply voltage of the stepper motor. In a 
five-volt stepper motor such as mo 61 D manufactured by Superior Electric 
it has been found that a resistive value of 22KOHMS for resistor 32 and a 
resistive value of 1 KOHMS for resistor 34 in conjunction with a 2N5061 
SCR works quite satisfactory at clamping the generated voltage at 12 volts 
D.C. It will be appreciated by those skilled in the art that the voltage 
sensitive switch network 24 can be repeated for each individual coil of a 
stepper motor, thus eliminating the need for diodes 22a through 22d. It is 
believed, however, that using one voltage sensitive switch in conjunction 
with diodes 22a through 22d acting as isolators would be the less 
expensive and best mode of the invention. 
Turning now to FIG. 4, it may be desirable to inactivate the voltage 
sensitive switch during high speed, continuous operation of the stepper 
motor since one would not be concerned with stopping the motor at any 
given location. A switch 38 is provided which would be set in one position 
when the motor is to operate at high speed and a second position when the 
motor is to operate at low speed and permitting auto-dampening to occur. A 
photo coupler 40 is provided with the collector tied to the gate of SCR 26 
and the emitter connected to the power supply of the coils within the 
stepper motor. When switch 38 is set in the high speed position, SCR 26 is 
essentially disabled, thus preventing the voltage sensitive switch network 
24 from clamping the non-powered side of the individual coils within the 
stepper motor. When switch 38 is set in the low speed position, the 
voltage sensitive switch 24 is enabled, thus allowing automatic clamping 
or shorting of the non-powered side of the non-energized coils within the 
stepper motor.