Undesired Forces Research Articles

Magnetic attitude actuation for passive satellites

The orientation of a passive communications satellite with respect to the earth can be adjusted by using an electromagnetic actuator which is rigidly mounted on the structure of the satellite. The actuator consists of three mutually-orthogonal air-cored coils on the skin of the satellite. If any one of the coils is excited by current, the magnetic field generated by it tends to rotate the satellite until the generated field and the terrestrial magnetic field are aligned. The activation of the coils also produces various undesirable forces which must be supported by the satellite structure. If the actuator is mounted on an ultra-thin structure, such as the Echo II Satellite balloon, these forces can have damaging effects. In this paper equations are derived for the adjusting torque and for the undesirable forces. The conclusion reached is that the compressive force upon the coil loops caused by the terrestrial field poses the most serious danger to the satellite structure.

Are Class II elastics necessary?

Class II elastics create an undesirable force on the mandibular arch which generally has detrimental effects. As yet there is no evidence to support the claim that Class II elastics stimulate mandibular growth. There is considerable evidence to support the claim that extraoral forces, when applied on a growing child, are capable of withholding the forward movement of the maxillary denture and the forward growth of the maxillary alveolar bone. I consider the position of the mandibular incisors anteroposteriorly in the face to be very important in case analysis and treatment planning. An excellent method of determining the optimum position for the mandibular incisors, taking the chin point into consideration, was introduced by Holdaway and has been explained in detail. Important factors that must be taken into consideration by the orthodontist utilizing the Holdaway ratio are the patient's age, sex, growth potential, skeletal pattern, and soft-tissue architecture. Class III elastics, worn in conjunction with a cervical gear against the maxillary molars, are often required for retraction of the mandibular teeth to their optimum position anteroposteriorly and/or for gaining arch length in the mandibular arch. Although the procedures advocated in this article may lengthen the treatment time, this could often be considered desirable if the treatment is timed to correspond with the period of optimum growth potential. Since successful treatment with extraoral appliances is entirely dependent upon the patient's cooperation, it is of the utmost importance that the orthodontist explain this basic fact to his patients and inspire them to accept their responsibilities throughout treatment.

Dynamic comparison of current ballistocardiographic methods. II. Effect of a platform in ballistocardiographic dynamics.

Part I dealt with dynamic arrangements (Dock, Smith, Walter, Nickerson, Burger, v. Wittern) which could be treated approximately as single masses in the lower frequency range. It was shown that in addition to the cardiovascular forces sought, other undesired forces from the support were mixed in, seriously altering the record: especially for components in the important 4 to 8 cycles per second region. Second, emphasis was laid on the several meanings that could be assigned the displacement, velocity and acceleration (DVA) records, according to what dynamic aspect (motion, momentum or force) of the cardiovascular action they were referred. It was explained that all direct-body and stiff-bed ballistocardiographic records were "compound", as they could not be referred to any one of these dynamical quantities. In part II, we will discuss the platform methods in detail, showing how the platform mass comes into the record. The arguments for and limitations of the various platform methods are considered, including the "shin-platform" and the very light pendular platforms now being tried.

Doubly chorded fractional-slot windings

The pattern mostly used for laying out balanced fractional-slot windings is that for maximum distribution factor of the main wave. From this pattern, a large number of other balanced patterns can be obtained by a cyclic shifting of coils from one phase to the other inside the same repeatable group. The distribution factor of the main wave of these latter patterns is always smaller than that of the former pattern. The distribution factors of the harmonics are partially smaller, partially larger than those of the former pattern, so that the values of the harmonic (differential) leakage reactance of the different patterns do not differ much from each other. The advantage of the pattern with cyclic shift of coils inside the same repeatable group is that it makes possible to reduce the distribution factors of one or several undesirable electromotive force or magnetomotive force harmonics. The aim of this article is to show that the same effect of reduction of certain harmonics can be achieved by shifting of repeatable groups with respect to each other. This method of reduction of harmonics has the advantage that it is applicable to lap as well as to wave windings while the cyclic shift of coils is limited to lap windings. Further, the shift between the repeatable groups, necessary to achieve desirable results, is seen immediately, while with the cyclic shift of coils inside the same repeatable group the trial-and-error method is to be applied. On the other hand, the shifting of repeatable groups reduces the maximum possible number of parallel circuits.