Mechanical actuators are integral components of many engineered systems. Many of the presently available actuator systems lack the desired stroke, power, controllability and reliability. The hierarchical actuator is a natural extension of the trend toward improving the performance of actuators through increments in geometric complexity and control. The hierarchical concept is to build integrated actuators out of a combination of smaller actuators. The smaller actuators are arranged geometrically and controlled so as to extend the performance of the total actuator into ranges that are not possible with actuators that are based on a few active elements and levels of control. Precision, speed increase, force output, load sharing, efficiency under smooth load/displacement control, smooth motion, stroke amplification/reduction and redundancy are all possible. Mechanics and mechanisms of hierarchical actuators are examined, along with a few experiments to demonstrate the operating principles.
A method is presented of measuring the specific heat capacity, Cp, of pure metals and alloys in an undergraduate thermofluids laboratory using simple concepts of unsteady heat conduction in solids and natural convection in fluids taken from textbooks on heat transfer. A simple experiment was developed in which a small-diameter metallic sphere at ambient temperature (∼20° C) is suddenly immersed in a tank containing quiescent ice water at 0° C. The metallic sphere was suspended by the lead wire of a thermocouple and the temperature-time evolution was recorded. A first-order differential equation descriptive of a lumped model was used. Although the equation is nonlinear, a variable transformation converted it into the Bernoulli form, which allows an analytic solution. Introduction of two measured temperatures at two distinct times into the analytic solution allows the specific heat capacity of a metallic material to be determined by means of an algebraic procedure.
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