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Designer's case file: counterbalance mechanism positions a light with surgical precision.

An integral part of a successful surgical procedure is the surgical light, which is positioned above the operating table to provide maximum illumination to the medical staff. The light can be suspended at various levels above the tabletop, depending on the position of the patient. The light remains fixed until the surgeon or a member of the operating team needs to move it.

Because a surgeon examines different areas of the operative site during a procedure, the light is repositioned many times to provide the best illumination possible. The quality of motion during repositioning is therefore important. It must occur easily and with a nearly uniform input force. Moreover, doctors and other medical personnel located at various positions around the surgical table must have access to the light.

If the light becomes difficult to maneuver, a surgeon may quickly grow tired of the constant strain to reposition it. Additionally, the light may become hard to position precisely where it is needed. Finally, the surgeon may be forced to pull or push the light with effort and suddenly find a lessened resistance to motion, resulting in an overshoot of the desired location.

The quality of motion during repositioning is based on the counterbalance achieved. Thus, designing an improved light arm means designing a counterbalance mechanism that minimizes the imbalance.

All items of support and counterbalance must lie in a plane above the height of the user's head. This prevents inadvertent accidents to the user and allows unimpeded access to the patient during surgery. Offsetting counterweights, used in the past to counterbalance the weight of the surgical light, are therefore unacceptable because of the obstruction they pose. Any mechanism employed should be enclosed and pose minimal obstruction to allow ease of cleaning.

Human Factors

A human factors layout of the surgical area (Figure 1) identifies the correct location of the counterbalance, the articulation range needed to maintain focus for various surgical procedures, and the amount of reach required to change the light's position.

From studying the human factors, several design requirements were formulated:

* The vertical support interface to the rotating suspension should lie above 2 meters (82 1/2 inches) to provide head clearance.

* The suspension should allow approximately a 0.914-meter (36-inch) offset from the vertical support to extend the light over the surgical table and to provide coverage at any point along the table surface.

* The suspension should allow approximately 0.508 meter (20 inches) of vertical travel to compensate for patient posturing and to maintain proper focus.

* The light should be balance in all locations and not drift from a set position during the procedure.

* Due to the reach required, positioning forces should be kept below 44.48 Newtons (10 pounds).

* Positioning forces should be uniform at all orientations of the suspension. The positioning force variation was targeted at [+ or -] 8.89 Newtons (2 pounds).

Minimizing the Imbalance

To allow vertical positioning of the light, the suspension must rotate about a fixed pivot. The combined weight f the suspension components and light produces an applied moment about the fixed pivot thta varies as a function of the angle of rotation (see Figure 2). this applied moment must be counterbalanced by a restraining moment if the light is to remain in a fixed position. Ideally, the restraining moment should equal nearly the applied moment at all orientations of the light. This way, external forces necessary to reposition the light are minimal and only need to overcome friction existing within the system.

The design problem is therefore to minimize the difference between the applied and restraining moments. Any differences, referred to as imbalance, must be compensated for by friction within the system. The induced friction will also resist any external positioning load and is to be minimized if low positioning forces are to be achieved.

Offsetting Counterweights

Make the Light Too Heavy

Previous designs relied on offsetting counterweights or diagonally placed linear spring counterbalance methods such as those shown schematically in Figure 3. The counterweight method adds considerable weight to the ceiling structure and violates head clearance in various orientations. It is thus undesirable from the user's perspective. While the linear spring allowed the basic mechanism to be located above head clearances, it did not counterbalance applied moments unless friction was induced. In fact, friction brakes were incorporated in all such designs to prevent drift. Wear at the friction pads necessitated frequent periodic adjustment and maintenance to prevent drift.

The reason for the large friction requirement on the diagonal linear spring concept is clear when the applied moment is plotted against the restraining moment produced from the spring (Figure 4). The applied moment varies as the cosine of positioning angle, whereas the restraining spring moment varies nearly linearly with spring deflection. The resulting difference in moment is imbalance and must be compensated for by friction within the system.

Proposed Design

An improved design will enable the restraining moment induced by the counterbalance to closely track the applied moment produced by the weight--in other words, a counterbalance mechanism that minimizes the imbalance.

Though the linear spring designs used earlier did not allow an optimum balance, their use is desirable because of their availability, low cost, and ease of assembly. Therefore, the proposed design alters the linear attributes of the spring by allowing the force to act through a simple linkage such that the countermoment generated by the spring force more closely approaches the cosine function posed by the suspended weight.


By offsetting the line of action of the spring from the pivot of the light arm structure, a countermoment can be generated. The line of action of the spring force is along a spring rod, R, and is constrained to pass through a pivot point, C, displaced from the suspension pivot, O. The moment arm to the spring force, h, is found to vary with the positioning angle.

The spring force also varies. It is a function of the arm position due to a change in length, as the spring is extended or compressed within the suspension arm structure. A spherical bearing at point B compensates for relative motion between the structure and spring rod. The resulting spring force produces a restraining moment, which is a function of arm position. This is shown schematically in Figure 5.

Examining the linkage relationships for the scheme as shown in Figure 6, it is possible to generate equations for applied moment, restraining moment, and positioning force as a function of arm geometry and weight distribution.

Applied moment is the influence of weight acting through the system's center of gravity taken about the suspension pivot. Taking reference 0=O degree as the horizontal suspension arm position, the equation describing applied moment is:

[M.sub.a] = WL cos 0 (1)

Restraining moment is the result of spring force acting against the suspension structure. The equation describing this counterbalance influence is:

[M.sub.r] = ([F.sub.s])(h) (2)

The spring rod does not rotate at the same angular displacement with the suspension and undergoes displacement [phi] with arm rotation. This interrelationship is determined through solution of the geometric relationships shown in Figure 6. A Regula falsi method is used to solve the complex relation involving the radial length [sub.x]. Because of the angular displacement differences, a spherical bearing is provided at B to allow relative rotation to occur.

Imbalance is the difference between the applied and restraining moments given by equations 1 and 2:

[M.sub.i] = [M.sub.a] - [M.sub.r] (3)

Any imbalance must be compensated for by friction within the pivots and within the spring support tube so that the system will not drift. The frictional moment is given by:

[M.sub.f]=[[mu].sub.O.N.sub.O.r.sub.O]+[[mu].sub.C.N.sub.C.r.sub.C]+ [[mu].sub.B.N.sub.B.r.sub.B]+[[mu].sub.S.N.sub.S.r.sub.S] (4)

[M.sub.f] must exceed [M.sub.i] in all arm rotation angles to prevent drift.

The positioning force must overcome the difference between [M.sub.i] and [M.sub.f] for motion to occur. Therefore, the positioning force at any angle is given by:

[F.sub.p] = ([M.sub.f] [+ or -] [M.sub.i])/[X.sub.p] (5)

The sign used in equation 5 is positive if imbalance works against positioning effort. Therefore, the upward positioning force differs from downward positioning force in magnitude for various [theta] position angles.

Simulation Model

To analyze system performance, a spreadsheet is used to input component weights, spring preload, spring rate, pivot geometry, and bearing friction. The spreadsheet is used to calculate applied and restraining moments, friction moment, and positioning force as a function of arm rotation angle.


Creation of the simulation model allows a degree of optimization of positioning force by studying the effects of varying spring preload, spring rate, pivot position, and bearing design parameters. Equations governing spring stress and fatigue are also incorporated into the spreadsheet to ensure that good design practice is maintained for all selected preload conditions. In this manner, the quality of performance of the counterbalance can be improved in an attempt to achieve minimum uniform positioning force while preventing drift.

Performance Assessment

The offset in linkage allows a close correlation between the applied and restraining moments and thus reduces the requirement for friction (see Figure 7). Analysis indicates that sleeve bearings are the best choice for the pivots to provide the optimum friction to compensate for remaining imbalance.

Preload variations are considered along with spring rate to determine a spring geometry that minimizes arm cross section. The spring design equations predict stresses and fatigue characteristics to be acceptable for the optimum case selected.

Since pivot bearing friction was determined to be sufficient to overcome imbalance, the influence of spring tube friction was minimized by employing a low friction liner to reduce friction.

The final positioning force achieved by the design is shown in Figure 8, which shows a uniform force throughout the range of articulation. The original design requirements are shown in Table 1 and are compared with the actual performance achieved by the design concept. It is clearly evident that counterbalance performance goals were achieved.

Final Design

The final design is shown in Figures 9 and 10. The spring and linkage are enclosed within the arm to facilitate cleaning within the sterile operating room environment. A removable side cover allows ease of assembly, adjustment of spring preload, and service if it is required.

The positioning performance exceeded original requirements and the system is easily moved into a variety of positions. The system does not drift in any arm position and was found, through life-cycle testing, to retain this capability for 10 years' design life without needing adjustment.
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Title Annotation:surgical lamp
Author:Fisher, Kenneth J.
Publication:Mechanical Engineering-CIME
Date:May 1, 1992
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