Arm with gravity-balancing function

Details Arm with gravity-balancing function Arm with gravity-balancing function

1980-10-21

1983-05-17

Herrmann, Allan D.

Machine element or mechanism

Control lever and linkage systems

16 1C, 2481231, 2482921, 414 1, G05G 104, B25J 1700

Patent

active

043834558

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to an arm assembly with gravity-balancing function especially for use in the teaching and playback type of robots.


BRIEF DESCRIPTION OF THE PRIOR ART

It is conventional in the art of robots for welding or the like to previously store the trajectories of a movements within a memory unit by manual teaching operation. It is of course desirable that manual actuation of multiarticulation arms of robots require a minimum of force regardless of the direction of movements.
The arms of robots are generally of considerable weights so that they become lighter and heavier when moving downwardly and upwardly, respectively. Accordingly, there are problems that the arms exhibit uneven movements during manual teaching operation, thus deteriorating accuracy of reproduction in a servo control system. An exemplary arm assembly for use in industrial robots is disclosed in Japanese Patent unexamined publication No. 27158/1977 but unsatisfactory both theoretically and structurally with respect to its counterbalancing performance.


SUMMARY OF THE INVENTION

According to the present invention, there is provided a rotary sheave extending over the fulcrum of an arm, the rotary sheave being constantly held somewhere in a vertical plane regardless of the vertically oscillating movement of the arm. A wire extending between a working point on the center line of gravity of the arm is connected to a spring mechanism having a maximum expansion equal to the distance between the working point and the rotary sheave, thereby counterbalancing moment produced by the weight of the arm and thus ensuring wellbalanced moving behavior of the arm.
The spring mechanism in the thus improved arm embodying the present invention includes a plurality of fixed rotary sheaves and a spring member .


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explanation of the operating principles of an arm assembly according to the present invention;
FIG. 2 is a skeleton diagram of a first stage arm assembly according to the present invention;
FIG. 3 is a skeleton diagram of a second stage arm assembly according to the present invention;
FIG. 4 is a skeleton diagram of a principal part of a sheave system on which a sire is suspended in the arm assembly according to the present invention;
FIG. 5 is a front view of an example of an articulation scheme according to the present invention with a part thereof omitted; and
FIG. 6 is a plan view of the example of the articulation scheme of FIG. 5 with a part thereof omitted.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1 there is shown a basic arrangement for counterbalancing the weight of an arm. The moment produced on the arm about a fulcrum B is represented by the following equation:


______________________________________ downward W .times. h upward P .times. K ______________________________________ extending between another joint C spaced at a small distance in a vertical direction from the fulcrum B and point A on the center of gravity of the arm AB, W is the weight of the arm AB at an action point A on the gravity center, P is the tension of the spring S, K is the length of a perpendicular standing from the fulcrum B with respect to the line AC and h is the length of a perpendicular standing from the point A with respect to the line BC.
It is evident that balance condition is as follows:
Combining equation (1) and (2) to make the ratio therebetween, ##EQU1## Thus,
Assuming that the spring tension (P) is proportional to the spring length (l), the link spring mechanism can keep its balance wherever it stands in a circumference. A spring whose spring constant meets the above requirements must have a free length of zero and does not exist as a matter of fact. By this reason for simple arrangement as discussed above it is not possible to achieve its intended purpose as a practical device.
In this connection nor does the arm disclosed in Japanese Patent unexamined publication No. 27158/1977 exhibit satisfactory

REFERENCES:
patent: 1948065 (1934-02-01), Clark
patent: 2416753 (1947-03-01), Hicks
patent: 2436006 (1948-02-01), Kaiser
patent: 4208028 (1980-06-01), Brown et al.

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