Electric-powered actuating device usable for the positioning control of valves and the like, featuring a motor, mechanical and spring means capable of driving said valve or other attached devices into a predetermined safety position in case of electrical signal or power failure; said actuating device having electrical amplifying means capable of providing pulsed current to drive said motor in one direction in response to an input signal, a constant current to hold said motor in a selected position, or no power, to allow the mechanical and spring means to reverse the motor direction; additional means are provided to limit the travel to a selected position despite further increases in input signal.

BACKGROUND OF THE INVENTION 
This invention constitutes a major improvement in electric actuators 
heretofore used for the driving of valves and other devices following the 
command signal of an automatic controller or computing device. A typical 
example for prior state of the art device is found in U.S. Pat. No. 
3,150,752 by Baumann. Here a stepping motor converts electrical impulses 
into small turns of a lead screw which in turn drives a reciprocating 
valve stem up or down. With either power or signal failure, the actuator 
will remain in the last position of the lead screw, a condition that does 
not satisfy requirements of most automatic control valve applications. 
A fuel control valve to a boiler should fail-close, in order to avoid a 
possible overheating, if electrical failure occurs. Certain coolant 
control valves, on the other hand, should fail-open. 
There is, therefore, a great need for so-called fail-safe electric valve 
actuators to replace the currently used pneumatically actuated and 
fail-safe spring-diaphragm actuators. Presently used electric actuators 
employ gear drives or threaded spindles to convert the relatively 
high-speed electric motor revolutions into higher forces but slower output 
motion. It is inherent in these high mechanically amplified devices that 
their efficiency is less than 30%, negating any possibility of reversing 
the motion by spring means to achieve the desired "fail-safe" action. 
Other attempts have been made to drive the motor into a safe position upon 
an electric line failure by means of a relay switched battery. However, 
such a solution is not only awkward and space consuming but offers only 
limited reliability unless the battery is maintained periodically. 
Operators using gear trains as mechanical amplification of forces suffer 
from wear and, more importantly, back-lash which impedes the desired 
accuracy of the actuator. 
This invention overcomes these and other disadvantages of the current state 
of the art devices. Instead of gear speed reduction, the invention uses a 
double-pitched cable drive, which not only offers no back-lash caused by 
play between meshing gear teeth, but achieves great mechanical 
amplification with up to 80% efficiency. Such high efficiency in turn 
allows the use of mechanical springs to reverse the rotation of the 
electric motor drive in order to achieve a desired safety position 
following a power failure. 
Connecting the spring-loaded actuating stem directly onto a valve shaft, 
eliminates problems of conventional mechanical override mechanisms to 
absorb thermal expansion of a valve shaft, featured, for example, in U.S. 
Pat. No. 3,150,752. The absence of gears and mechanical override devices 
leads to a dramatic simplification, great cost savings and a substantial 
increase in reliability over present similar devices. 
An amplifying circuit is further designed to drive the motor only in a 
direction opposite to the direction of the spring force. Upon reaching the 
desired valve position, the motor drive current is replaced by a locking 
DC current. For reverse action, the DC current is switched off, allowing 
the spring force to drive the motor backwards. This "One-Way" switching 
action greatly simplifies the electronic control circuit, leading to 
important cost savings and increase in reliability. 
Another important object of the invention is the provision of electronic 
means to limit the actuator travel whenever the feed-back voltage as 
function of the travel reaches an adjustable value of the input signal 
voltage, which then blocks all further stepping power to the actuator 
motor. This prevents the motor from reaching a stalled position and 
prevents it from going out of control. All this is accomplished with 
electronic means and without resorting to cumbersome mechanical switching 
means such as utilized in U.S. Pat. No. 4,097,786, for example.

DESCRIPTION OF MY INVENTION 
Referring to FIG. 1, an electric motor 3, preferably a so-called DC hybrid 
motor and capable of operating continuously with 24 V DC at approximately 
200 steps per minute, is connected to an actuator frame 4. The output 
shaft 5 of said motor has a drive pin 6 slidingly engaging a slotted 
opening 7 of a spindle 8 which is supported on each end by ball bearings 
9. Spindle 8 is divided into two threaded sections; the first one 10 has, 
in a preferred configuration, a pitch diameter of 1.000 inch, while the 
second one 11 has a pitch diameter of 0.900 inch. A steel cable 12 is 
wound around the larger pitch diameter 10, then through a pulley 13 and 
finally around the smaller pitch diameter portion 11, before each end is 
suitably anchored within spindle 8. 
Threaded portions 10 and 11 have opposite pitch angles to enable the cable 
to move towards the centrally located pulley 13 without undue strain. 
Pulley 13 is suitably fastened to a yoke 14, the lower part of which 
engages the top portion 15 of an actuator stem 16, whose lower end 17 is 
configured to engage valve stems and the like. Actuator stem 16 has a 
shoulder portion 18 which supports a spring 19 compressively engaging a 
lower plate 20 against a bolt 21 which in turn is fastened to stem 16. 
Whenever stem 17 is prevented from further down-travel such as the seating 
of an attached valve plug, then yoke 14 can continue to move plate 20 down 
by overcoming the preselected load of spring 19 and allowing portions 15 
to slide through an opening 20a of plate 20. 
Guide pins 22 prevent yoke 14 from rotating, while stem thread 23 as part 
of pully assembly 24 is engaged to provide tightening of cable 12 and 
pre-compression of a second spring 25 during the initial assembly phase. 
This second spring 25 allows to pull up stem 16 whenever the motor is 
de-energized or, in case of power failure. 
Any left-hand rotation of motor shaft 5, caused by a suitable current 
switching by the electronic circuit shown in FIG. 2, will turn spindle 8 
via pin 6. This will wind further cable 12 onto the larger pitch diameter 
portion 10 (1.00.times.3.14 inches/turn) while, at the same time, 
unwinding from the smaller diameter portion 11 at the rate of 
0.90.times.3.14 inches/turn. The end result is that pulley 13 advances at 
one-half the difference between the up and down motions of cable 12, i.e. 
0.5 (1.00.times.3.14-0.90.times.3.14)=0.157 inches/turn of motor shaft 5. 
Expressed differently, the pulley motion=1.57 times the difference in 
pitch diameter per motor turn. The theoretical mechanical advantage thus 
derived in the preferred embodiment can be calculated by comparing the 
work done by the motor to the work produced in pulling a force attached to 
the pulley. In one example, a motor with 15.8 inch-lbs. torque will 
produce a work output of 15.8.times.2.times.3.14=99.27 inch-lbs. per turn. 
Since the pulley 13 motion is 0.157 inch, the theoretical force that the 
pulley may move against is now 99.27/0.157=632 lbs.; a mechanical 
advantage of 40:1, (632/15.8). 
This high-force amplification is possible without need for a large number 
of successive gear trains with their associated backlash and wear 
problems. The differential cable system is accomplishing this task at 
efficiencies approaching 80%, a feat not possible with 40:1 gear train 
ratios (the resultant gear efficiency is generally below 50%, which would 
make the whole system unworkable). 
Since the efficiency of the cable system is well above 50%, the force of 
spring 25 is able to back-drive the motor shaft 5 in the reverse direction 
should the electronic logic system of FIG. 2 call for a reverse in 
direction of else, should either the controller signal or the motor 
current fail. In the latter case, actuator stem 16 will move up, propelled 
by the force of spring 25, to open an attached valve (not shown). 
In order for the logic system (FIG. 2) to sense the motor stem position, 
motor 3 has a second, opposed shaft 26 engaging by means of gears 27 and 
28 the shaft 30 of a rotary potentiometer 57 calibrated to typically 
produce a 1 to 15 volt DC output signal in proportion to the motion of 
stem 16. 
A separate circuit board 29 contains electronic logic and switching 
elements, is more closely described below and shown in FIG. 2. 
While the shown configuration has actuator stem 16 retract on power 
failure, a suitable reversal of the mechanical element in the invention 
could provide means to pull the stem down on power failure, in order to 
provide a "Fail-Close" valve position for example. 
The electronic circuit shown in FIG. 2 may be described as follows: 
Refering to FIG. 2, a 4-20 mA controller signal enters the circuit from an 
externally generated current source, such as a process control computer of 
conventional art and as a function of a sensed controlled variable, at 
terminals 50 and 51. The positive portion of the signal being applied to 
terminal 50 and the negative portion being applied to terminal 51. The 
controller signal is then converted at terminal 50 to a controller signal 
voltage by a factory preset potentiometer 52, shunted to ground. 
A feedback voltage is generated by potentiometer 53. The feedback voltage 
span and zero adjustment are provided by potentiometer 54 in conjunction 
with potentiometer 55. 
The controller signal voltage and the feedback voltage are then double 
compared. The first comparison occurs with the controller signal voltage 
applied to the inverting input of voltage level comparator 56, and the 
feedback voltage applied, through potentiometer 57, to the non-inverting 
input of comparator 56. The second comparison occurs with the feedback 
voltage applied to the non-inverting input of voltage comparator 58, and 
the controller signal voltage applied to the inverting input of comparator 
58. The resulting signals from these comparisons are used to indicate the 
relative relationship between the actual actuator stem position, as 
indicated by the feedback voltage, and the desired actuator stem position, 
as indicated by the controller signal voltage. Potentiometer 57 provides 
the circuit with an adjustable sensitivity. 
A third comparison takes place with the feedback voltage applied to the 
non-inverting input of voltge level comparator 59, and a travel limit 
voltage, adjustable 0 to 15 vDC by potentiometer 60, applied to the 
inverting input of comparator 59. The resulting error signal from This 
comparison is used to indicate that full travel has been achieved and that 
any further travel signal should be ignored. This logic circuit acts as a 
solid state travel limit stop and prevents the motor from stalling out 
when a mechanical stop is reached. Such stalling out is highly undesired 
since the motor will slip phase and loose power, following which the 
return spring will drive the motor back in the opposite, undesirable 
direction. 
Resistors 61, 62, and 63 provide the necessary pull-up resistance to 
generate output signals from comparators 56, 58, and 59. 
The output signals from comparators 56, 58, and 59 are then passed through 
filters, each consisting of a series resistor 64, 65, 66 and a capacitor 
67, 68, 69, shunted to ground, designed to provide a time delay to prevent 
instability in the circuit and to reduce the comparator output signals to 
more TTL compatible levels. Each signal is then passed through a Schmitt 
Trigger Inverter 70, 71, 72, which completes the filtering process by 
logically inverting the signals and defining them as either TTL logic 1's 
or 0's. 
The filtered signals from comparators 56 and 58 are applied to the two 
inputs of X-NOR gate 73. The output from this X-NOR gate is applied, in 
parallel, to resistor 74, which provides the necessary pull-up resistance, 
and to the input of AND gate 75. The filtered signal from comparator 59 is 
applied to the remaining input of AND gate 75, and the resulting output 
from this AND gate is then applied to the input of AND gate 76. A clock 
pulse signal, originating from a square wave source, such as a 555 Timer 
circuit, is applied to the remaining input of AND gate 76. The function of 
this logic network is to determine, based on the results of the three 
voltage comparisons, whether or not to pass said square wave signal onto 
the rest of the circuitry. 
The output from AND gate 76, here after refered to as "CLOCK", is applied 
to the clock input for J-K Flip/Flop 77, which is configured in toggle 
mode. The Q output from said Flip/Flop is applied as an input to X-NOR 
gate 78 and also as the clock input to J-K Flip/Flop 79, which is 
configured in toggle mode. 
The Q output from J-K Flip/Flop 79 is used as step control signal 3, and 
also as the second input to X-NOR gate 78. The output of X-NOR gate 78 is 
used as step control signal 1 and, after passing through Inverter 80, as 
step control signal 2. Resistor 81 provides the necessary pull-up 
resistance. Step control signal 4 is derived from the Q-not output of J-K 
Flip/Flop 79. 
Step control signals 1 through 4 are applied as inputs to four individual 
AND gates, 85, 82, 83, 84, with each step control signal being applied to 
the input of one and only one AND gate. The remaining input for the four 
AND gates, 85, 82, 83, 84, is provided by the filtered output from 
comparator 56. This signal, here after, is refered to as "ENABLE". 
"ENABLE" allows step control signals 1 through 4 to pass through AND gates 
85, 82, 83, and 84 to their perspective current drivers, 89, 86, 87, 88. 
Drivers 89, 86, 87, and 88, when activated, provide the necessary current 
to energize the required windings of a DC stepping motor to produce the 
desired effect as determined by the controller signal voltage vs. the 
feedback voltage relationship. 
The basic purpose and desired function of the control circuit as 
exemplified in FIG. 2 is to assure that the ultimate position of the stem 
16 is in direct proportion to the 4 to 20 mA signal that is issued by said 
external control device. When the circuit is subjected to a 4 mA signal, 
the stem 16 should, typically, be in the highest (retracted) position 
corresponding to an open valve, if such valve would be connected to the 
actuating device, while a 20 mA signal customarily calls for a fully 
closed valve i.e. a fully extended stem 16. 
The circuit converts the 4 to 20 mA signal to a corresponding voltage 
signal which is then compared to a feedback voltage that is generated by a 
potentiometer 57 (See FIG. 1). Both voltages have to be balanced to 
satisfy the steady state circuit requirements, that is, the rotary 
position of the potentiometer 57 and thereby the position of the stem 16 
has to correspond to the input signal level. If there is a positive 
imbalance between the two voltages, then suitably pulsed electric currents 
are provided to the motor 3 to move the stem 16. For a negative imbalance 
between the input signal voltage and the feedback voltage, all power to 
the motor 3 is disconnected and spring 25 is allowed to pull the stem 16 
up. After the stem 16 has moved up sufficiently to cause the potentiometer 
57 to produce a lower voltage corresponding to the input signal voltage, 
then the circuit will provide suitably constant currents to the motor 3 as 
required to hold the actuating device in this desired position. 
It should be recognized that numerous changes can be made by a person of 
ordinary skill in the art without departing from the scope of the 
following claims. For example, the amplifying circuit could be modified to 
initiate pulse signals to the motor not only in the direction opposed to 
the return spring force but also in the same direction to allow for a more 
precise descending motion. Such a more costly and complicated version is 
well anticipated by the invention even though it is not preferred. 
Another modification would consist of placing the amplifying circuit 
external of the actuator, inside the main frame of a computer, for example 
.