D. S. D. Stilling

Controlling angular oscillations through mass reconfiguration: a variable length pendulum case

The control of angular oscillations or energy of a system through mass reconfiguration is examined using a variable length pendulum. Control is accomplished by sliding the end mass towards and away from the pivot as the pendulum oscillates. The resulting attenuation or amplification of the angular oscillations are explained using the Coriolis inertia force and by examining the energy variation during an oscillation cycle. Simple rules relating the sliding motion to the angular oscillations are proposed and assessed using numerical simulations. An equivalent viscous damping ratio is introduced to quantify the attenuation/amplification phenomena. Sliding motion profiles for achieving attenuation have been simulated with the results being discussed in detail.

Distributed Soil Displacement Associated with Surface Loading

Soil compaction that is an inevitable result of agricultural practices alters physical properties of soil and tends to be undesirable as it adversely affects water and nutrient penetration and subsequent root growth and development. Furthermore, additional energy is spent to till the soil. Although a tremendous amount of research has been conducted in the area of soil compaction, the focus has been primarily with respect to surface soil sinkage. Realizing that the observed soil sinkage is the cumulative effect from compacting subsurface layers, this paper discusses the deflection, compaction and transferred force through the soil from a surface load. The surface load is comparable to loads associated with equipment movement and other tillage practices. A given volume of soil of known compaction and moisture content was loaded at the surface with a slowly applied force. The distribution of the load and displacement profile from the surface to depth was measured to provide further insight into the subsurface soil structures. For instance, the nonlinear exponential decay of the soil displacement from the surface to a given depth converged to zero at the location of a hard, compact layer or a point where no soil movement occurred, regardless of the initial soil compaction.

Quantification of antagonist muscle coactivation in children with spastic diplegia

The purpose of this study was to investigate the use of the Pearson Product‐Moment correlation coefficient to quantify muscle coactivation using electromyography. The subjects were two children with spastic diplegia. Surface electrodes were used to record muscle activity from the soleus and tibialis anterior muscles during voluntary attempts at dorsiflexion and plantarflexion of the ankle against resistance. The linear envelope signals were smoothed with a Hamming filter at a cutoff frequency of 4 Hz and intervals of the resultant pairs of curves subjected to the PPM test. Both subjects demonstrated significant coactivation on their right side and independent activity on the left, indicated by high positive and negative coefficients, respectively. This method shows promise for description of side differences in diplegics and for assessing the effects of physical therapy and other interventions. Clin. Anat. 11:314–319, 1998.

Propagation of Soil Failure Front Associated with various Agricultural Tillage Tools

Current North American soil tillage practices loosen soil to depths of 75 to 150 mm (3 to 6 inches). As the soil is tilled, the failure path precedes the motion of the tillage tool. Previous research has examined the forces within the soil to predict the soil failure. This paper experimentally quantifies the rate and the path of the failure front through the soil. From high speed digital film analysis, the failure front was analysed for three tillage tools operating at 4 km/h speed. The tillage tools included sweep, knife opener and an elliptical tool. The depth of operation was either 75 mm or 100mm. Soil conditions, namely its moisture content and the level of compactness were recorded. For the sweep tillage tool, temporal profiles of the failure or crack growth were quantified from the high speed videography. If the tool operational speed can be increased to overcome the speed of the failure front, the soil disturbances will be minimal to non existent.