Position indicator

A position indicator of potentiometer type includes two parallel wires, each having first and second ends, and a movable, rotatable contact cylinder around which the wires pass round at least one turn. The potential contact is made via the first end of a first wire and the second end of a second wire, whereas the potentiometer current is arranged, when in use, via a second end of said first wire and a first end of said second wire, the two being coupled through the contact cylinder.

The invention relates to an electrical position indicator of the 
potentiometer type. 
Although it is not often constructed in this manner in practice, the 
pedagogical picture of the Wheatstone bridge is a wire, against which lies 
a movable contact, which is coupled to the zero indicator. Those skilled 
in the art are aware that precision bridges are not presently made by 
means of such resistance wires, but rather with sets of resistors which 
can be regulated in fixed steps. The same is true of precision 
potentiometers. 
However, the original resistance wire in older versions of the Wheatstone 
bridge has come into use as a position indicator. It is of course not 
coupled as a Wheatstone bridge but as a potentiometer. A voltage is 
applied between the ends of the wire, so that an evenly distributed drop 
in voltage is obtained over the same. If the drop in voltage is then 
measured from one end to a movable contact (without drawing any current 
through the contact), a value is obtained which varies linearly with the 
displacement of the movable contact, provided that the wire is of uniform 
gauge. 
A disadvantage of such a level indicator is that it is often difficult to 
achieve satisfactory contact between the wire and the movable contact. As 
troublesome as such difficulties can be under laboratory conditions, they 
are even more so under operating conditions, e.g., in a factory or shop, 
where it can be difficult to protect the wire adequately against various 
contaminants. 
It is known to improve the contact between the moving contact and the wire 
in such devices by winding the wire once or twice around a pin or cylinder 
serving as a contact member, which is usually rotatable so that it rotates 
and rolls forward on the wire when its support or "carriage" is moved 
along the same. 
It is a purpose of the invention to achieve an improved position indicator 
of potentiometer type of this generally described type.

As will be evident from FIG. 1, the level indicator comprises two wires 1 
and 2. One end of each is fixed and the other end is attached to a tension 
spring 3 and 4, respectively. The parallel wires are furthermore wound 
once around a contact drum 5, in grooves cut in the outside surface of the 
drum. If the wires were wound on a cylindrical surface, they would travel 
in one direction or the other as the drum is turned to displace it along 
the wires. If there were no obstruction, the wires would be displaced by 
the pitch distance for each turn of the drum. By retaining the wires in 
grooves, the corresponding slippage in the grooves serves to achieve 
metallic contact. 
As seen from FIG. 1, coupling wires are coupled to the ends of wires 1 and 
2. The spring end of wire 2 and the fixed end of the wire 1 are 
individually clamped as at 6 and 8, respectively. The remaining connecting 
points on the wires, namely, the fixed end of wire 2 and the spring end of 
wire 1, are coupled to the same clamping screw 7. 
In the potentiometer circuit or the like for which the device according to 
FIG. 1 is intended, a current is set up between clamping screws 6 and 8, 
producing a current which passes via the spring end of wire 2, via contact 
drum 5 over to wire 1 and from there on to its fixed end and on to 
clamping screw 8. It can also be seen that the contact point, at contact 
drum 5, is sensed as to its potential via clamping screw 7. 
This circuit provides a number of advantages. Firstly, there is no need for 
a loose cable or particular switch-in device to the contact point. 
Secondly, there is a safe metallic contact in the contact point, which is 
a great advantage since tottering contact due to vibrations, for example, 
is eliminated as long as there is any current at all in the 
"potentiometer." The "low-ohm" portion of the system thus guarantees 
contact for the "high-ohm" portion, thereby eliminating many causes of 
error. 
It will be apparent that the high-ohm portion of the system is freed of 
problems such as contact resistance, charging effects and continuity 
problems. A number of causes of thermoelectromotive forces are also 
eliminated. 
The embodiment is shown in more detail in FIGS. 2 and 3. The wires 1 and 2 
are mounted in a type of box made of aluminum profiles with plastic ends, 
and the contact cylinder 5 is rotatable about a shaft 11, which is fixed 
to a plastic slide 12. Slide 12 is slidably arranged on a pair of rods of 
drill rod 13 and 14 which extend parallel to the wires 1 and 2. The slide 
will thus be movable along a path with precision bearings. A fork-shaped 
driver 15 is fixed to slide 12. This driver is accessible through a slot 
in the box, so that a bar, for example, can be set in the fork. The bar 
can be a portion of a machine part or the like whose linear displacement 
is to be measured with the aid of the position indicator. 
While FIGS. 2 to 3 do not show in detail how an electrical junction is made 
to clamping screws corresponding to clamping screws 6, 7 and 8 of FIG. 1, 
no particular skill is needed to arrange this detail after inspection of 
FIG. 1. It is thus evident that the wording "clamping screw" should be 
broadly construed, being a convenient expression for any means by which 
the wires may be coupled to well-known measuring apparatus or the like. 
In the embodiment shown, the wires are 0.25 mm music wire. The contact 
cylinder has an external diameter of 33 mm, and is of brass and is slide 
journalled on a 10 mm shaft. Manufactured versions were made with 
measuring intervals of lengths varying between 200 and 1500 mm, which 
proved to function well.