Therapeutic traction apparatus for applying traction to a patient by means of a drawing cable

A traction apparatus for medical purposes, wherein an adjustable traction generated by an electromotor is transmitted to the body of a patient by means of a drawing cable. The traction appearing in the drawing cable is continuously measured and compared with the selected traction magnitude, and the drawing cable is wound or unwound by the electromotor in dependence on the difference established by the comparison until this difference has at least approximately been reduced to zero. Preferably, an intermittent traction is exerted, in such manner that holding periods in which the full traction is applied are interrupted by rest periods, in which an adjustable rest force different from zero is exerted. The traction appearing in the drawing cable may be measured by passing the cable over a shaft on which a flexional load is exerted by the traction and by arranging strain gauges on said shaft.

BACKGROUND OF THE INVENTION 
U.S. Pat. Nos. 3,168,094 (Siltamaki) and 3,835,847 (Smith) disclose 
traction apparatus in which the traction is controlled by means of an 
adjustable spring-loaded lever responding to the tension in the drawing 
cable. This lever operates a switch when the desired traction has been 
reached, whereby the motor is switched off, and a timer is actuated to 
reverse the motor after a predetermined delay. An arrangement of this kind 
has the disadvantage that no correction is possible for an excessive 
traction magnitude, due for instance to a shift of the position of the 
patient. 
U.S. Pat. No. 3,710,787 discloses a similar arrangement wherein the lever 
is constituted by the motor, which is pivotably suspended. In order to 
prevent an excessive traction magnitude, the disclosure relies on a 
limitation of the current that may be supplied to the motor. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide an improved apparatus of the 
above-mentioned kind, wherein the traction exerted on the patient is 
accurately kept at a desired value. 
Further objects of the invention will appear from the following 
description. 
According to the invention, the generated traction is continuously and 
electrically measured and compared with the selected traction intensity, 
and the drawing cable is wound or unwound in dependence on the difference 
established by the comparison until this difference has at least 
approximately been reduced to zero. 
The apparatus according to the invention may be constructed as a separate 
device adapted to be attached to a wall, a bed or a chair, but it may also 
be incorporated in a traction table or a traction chair. 
Since it must be possible to increase or to decrease the traction 
intensity, according to circumstances, the electromotor used in the 
apparatus must be readily reversible, and must also be able to generate a 
large torque in the stationary condition. These exigencies may be 
satisfied in an appropriate manner by the use of a D.C. motor. 
In many cases, the treatment is performed in such manner that the exertion 
of the full traction is interrupted by rest periods, and it is generally 
desirable that the traction is not completely suppressed in these rest 
periods. The apparatus according to the invention may be easily 
constructed in such manner that an adjustable rest force different from 
zero is exerted in the rest periods. The use of a D.C. motor is also very 
useful in this respect, because there is a linear relation between the 
supplied current and the exerted torque in such a motor, so that the 
traction intensity may be readily controlled by a variation of the 
supplied current. 
In general, it is considered desirable that the full traction is gradually 
switched on and off. For this purpose, the apparatus according to the 
invention may be carried out in such manner that the course of the 
traction as a function of time during the transitions from zero or from 
the rest force to the full traction, and from the full traction to zero or 
to the rest force is continuously adjustable. 
It is preferred to insert an electromagnetic coupling between the motor and 
the drawing cable. This has the advantage that the drawing cable may be 
rapidly drawn out in the decoupled condition in order to connect it with 
the patient, and that the patient may interrupt the traction at any time 
by disabling the electromagnetic coupling by means of an emergency switch. 
In addition, the patient is automatically decoupled when the voltage 
supply is interrupted. The use of an electromagnetic coupling also 
provides for an additional protection, since the traction intensity can 
never become larger than the value corresponding with the slipping torque 
of the coupling. In a preferred embodiment of the invention, the slipping 
torque of the coupling is automatically adapted to the selected traction 
intensity, in such manner that the slipping coupling is always slightly 
larger than the value corresponding with the selected traction intensity. 
The winding drum used for winding and unwinding the cable is preferably 
connected with the motor through a unidirectional coupling. This is 
important for the case that the drawing cable is completely unwound from 
the winding drum. In this condition, a command to unwind the cable may 
cause the same to be rewound in the opposite direction, whereby the 
traction intensity is increased instead of decreased. This danger may be 
removed by the use of a unidirectional coupling. 
It is essential to the invention that the traction intensity in the drawing 
cable is continuously measured. This may be realized in an effective 
manner by passing the drawing cable over a shaft on which a flexional load 
is exerted by the traction, and by arranging strain gauges on said shaft 
in order to measure the generated traction. 
In order to control the exerted traction in an effective manner it is 
preferred that a voltage corresponding with the selected traction 
intensity, or the selected rest force, respectively, is supplied to one of 
the inputs of an operational amplifier, and that the amplified voltage is 
fed back to another input of said amplifier through an integrator having 
an adjustable time constant, so that the voltage of said other input 
follows the voltage of said first-mentioned input with an adjustable 
delay. In this case, it is useful that the output voltage of the 
integrator is compared with a voltage corresponding with the actual 
traction, and the difference established by this comparison is used to 
control the current supplied to the electromotor. 
If the apparatus comprises means for measuring the duration of the traction 
periods and the rest periods, it is preferable that the said measuring 
means are activated at the moments at which the full traction intensity, 
or the rest force, respectively, has at least approximately reached the 
selected value.

DETAILED DESCRIPTION 
The apparatus shown in FIGS. 1-3 comprises a D.C. motor 1 generating the 
traction, of which the shaft is connected with a reducing transmission 2. 
The output shaft of the transmission 2 may be coupled by means of an 
electromagnetic coupling 3 with a pinion 4 engaging a gear wheel 5. The 
gear wheel 5 is freely rotatable about a shaft 6 and is provided with a 
hub coupled through a unidirectional coupling 7 with a winding drum 8 
keyed to the shaft 6. 
One of the ends of a spring 10 is attached to the shaft 6, while the other 
end is attached to a frame 14. When the end of the cable 9 is to be 
connected with the patient, the electromagnetic coupling 3 is left 
inactive, so that the cable 9 may be freely drawn out by hand. During this 
operation the spring 10 is wound up, so that it may rewind the cable 9 on 
the winding drum 8 after the treatment. When the electromagnetic coupling 
has been actuated and the treatment has been started, but the required 
traction intensity has not yet been reached, the motor is actuated in the 
direction in which a greater part of the cable is wound on the drum, so 
that the traction intensity increases, until an equilibrium between the 
required and the actual traction intensities has been reached. 
The command "increase traction" corresponds with a fixed direction of 
rotation of the motor, However, if the drawing cable 9 is arranged on the 
drum in the opposite direction, which is possible if the cable has been 
completely unwound, the above-mentioned correlation between the variation 
of the traction intensity and the direction of rotation of the motor is no 
longer present. This might lead to a dangerous situation, which is 
prevented, however, by the unidirectional coupling 7. 
The drawing cable 9 is led over a pulley 11, which is freely rotatable 
around a measuring shaft 12, so that the measuring shaft 12 is subjected 
to a flexional load by the traction in the drawing cable. Strain gauges 
for measuring the traction have been stuck to the shaft 12. The drawing 
cable is led out of the apparatus over a pulley 13, and connected with the 
patient to be treated. The pulley 13 is freely rotatable around a shaft 
13a. 
According to the drawings, the traction operative in the cable works in a 
horizontal direction. However, any desired direction within a semisphere 
is possible. 
The rotating parts of the apparatus are borne by the frame 14. 
FIG. 4 shows a timing program that may be used in a treatment with the 
apparatus according to the invention. The apparatus is actuated at the 
time t.sub.o, after which the traction is gradually increased up to the 
selected value K.sub.1, which is reached at the time t.sub.1 and 
maintained up to the time t.sub.2. The interval (t.sub.2 -t.sub.1) is 
indicated as the drawing time. After the time t.sub.2 the traction is 
gradually decreased to the rest value K.sub.2, which is reached at the 
time t.sub.3, and maintained up to the time t.sub.4. The interval (t.sub.4 
-t.sub.3) is indicated as the rest time. After the time t.sub.4, the 
traction is again gradually increased up to the value K.sub.1 which is 
reached at the time t.sub.5, and maintained up to the time t.sub.6, and so 
on. The intervals (t.sub.1 -t.sub.o), (t.sub.3 -t.sub.2) and (t.sub.5 
-t.sub.4) may be indicated as adaptation times. At the time t.sub.n, the 
treatment is terminated, so that the interval (t.sub.n -t.sub.o) may be 
indicated as the treating time. 
In an apparatus according to the invention, the traction intensity may be 
variable from 0 to 100 kg, while the drawing and rest times may each be 
variable between 0 and 60 seconds, the adaptation time between 1 and 30 
seconds, and the treating time between 0 and 100 minutes. 
FIG. 5 shows a block diagram of an apparatus according to the invention. 
The measuring shaft 12 is provided with a transducer 15 converting the 
traction intensity into an electric signal which is transmitted through an 
amplifier 16 to a comparator 17. The apparatus is further provided with 
adjusting members 18, 19 and 20 for the slope of the traction intensity 
during the adaptation times, the traction intensity and the rest force, 
respectively. Adjusting members 21, 22 and 23 are provided for the drawing 
time, the rest time and the treating time, respectively. The adjusting 
members 21, 22 and 23 are connected with a logical circuit 24, controlling 
a switching member 25. The values selected by means of the adjusting 
members 18, 19 and 20 are transferred to the comparator 17 by means of the 
switching member 25. 
The comparator 17 controls the logical circuit 24 and provides an input 
voltage for an amplifier 26 controlling the electromotor 1. This motor is 
connected through the electromagnetic coupling 3 with the winding drum 8. 
The electromagnetic coupling 3 may be disabled by the patient by means of 
an emergency button 27. Furthermore, the apparatus contains a stop button 
28, a start button 29 and an indicator 30 for reading out the times. 
In FIG. 6, P.sub.1 and P.sub.2 are potentiometers for adjusting the 
traction intensity and the rest force, respectively. The said 
potentiometers are actuated at the right times by means of a flip-flop 31, 
consisting of two nand-gates. The wipers of the potentiometers P.sub.1 and 
P.sub.2 are connected, through series connected resistors A and B, and 
through series connected resistors C and D, respectively, with the 
inverting input of an operational amplifier 32. 
As long as the apparatus is inoperative, the junctions between resistors A 
and B, and between resistors C and D each have a voltage 0, so that the 
potentiometers P.sub.1 and P.sub.2 are inactive. By means of the flip-flop 
31, the 0-voltage may be removed from any one of the junctions. In this 
case, a predetermined fraction of the voltage adjusted on the 
potentiometer P.sub.1 or P.sub.2 is supplied to the inverting input of the 
amplifier 32. The output voltage of the amplifier 32 is supplied, through 
a full-wave limiter consisting of diodes D.sub.1 and D.sub.2, to an 
integrator consisting of an input resistor P.sub.3 and an operational 
amplifier 33 with feed-back through a condenser 34. The output voltage of 
the amplifier 33 is fed back to the non-inverting input of the amplifier 
32, so that the voltage at this input follows the voltage at the inverting 
input with a delay determined by the adjustment of the potentiometer 
P.sub.3. In this manner, the required gradual change-over to the full 
traction intensity is obtained. The measuring shaft 12 carries strain 
gauges 35 and 36 inserted in a bridge of which the output signal is 
supplied to an operational amplifier 37. The output voltages of the 
amplifiers 33 and 37 are compared by means of resistors F and G, and the 
established voltage difference controls the motor current by means of an 
operational amplifier 38. 
The measuring means for the duration of the traction periods and the rest 
periods are preferably activated at the moments at which the full traction 
intensity or the rest force, respectively, has at least approximately 
reached the selected value. In view of the inevitable inaccuracy of the 
force control the activation of the measuring means must occur with a 
force tolerance of a few percents.