Actuating force

Polymers used for seat and plug seals and internal static seals include PTFE (polytetrafluoroeth ene) and other fluorocarbons, polyethylene, nylon, polyether-ether-ketone, and acetal. Fluorocarbons are often carbon or glass-filled to improve mechanical properties and heat resistance. Temperature and chemical compatibility with the process fluid are the key selec tion criteria. Polymer-lined bearings and guides are used to decrease fric tion, which lessens dead band and reduces actuator force requirements. See Sec. 28, Materials of Construction, for properties. 

Multiple valves have been constructed on a PDMS valve control layer (see Figure 3.27) [167]. Multilayer soft lithography has been used to generate the valve control layer (4 mm thick for strength) plus a fluid layer (40 pm thick) on PDMS. The small Young modulus ( 750 kPa) of PDMS allows a large deflection (1 mm) to be produced using a small actuation force ( lOOkPa on a 100- x 100-pm valve area). The response time is on the order of 1 ms. Round channels were... 

The conductance of electrolytes is caused by ions which are brought into motion by the effect of the electrical field. The velocity of the ions increases from the first moment the current is led into the solution but it attains a constant value as soon as the actuating force equals the resistance encountered by the ions on their respective path (viscosity of the medium and electrostatic... 

Fhc unpredictable eflecis of seal load could be eliminated if all valves, regardless of ANSI class, were tested at the actual seat loads they will experience at the full service differential. However, the ANSI standard calls instead for testing with the full actuator force—a force that is the sum of seat load, unbalance force on the plug, and friction forces. In some cases this test condition results in significantly higher seat load at test than in ser dce. 

With the hard testing machines (Bhushan etal, 1988 Tusakamoto etal, 1987 Wu et ah, 1988), the indentation depth is controlled, for example, by means of a piezoelectric actuator. Force transducers used in existing designs include a load cell with a range from a few tens of micronewtons to 2 N (Wu etal., 1988 Wu 1991) a digital electrobalance with a resolution of 0.1 o.N, and a maximum of 0.3 N and a linear spring whose extension is measured by polarization interferometry. 

The actuation force or movement generated during redox cycling is directly related to the concomitant changes in mechanical properties. Using a simple linear elastic model of the small-strain mechanical properties of PPy, it has been shown that the actuation strain (eo) at a constant applied stress (a) is accurately predicted from Equation 3.3... 

Microactuators, Fig. 1 Diagrams of (a) in-plane electrostatic comb-drive actuator and actuation force and (b) out-ofplane parallel plate electrostatic actuator... 

Generally, the chemical and physical properties of most engineering materials vary with temperature. The (Mily difference hes in the degree to which the property changes with temperature. Among all the properties, the material shape change with temperature has mostly been used for the development of thermomechanical microvalves. The detailed mechanisms of thermomechanical microvalves will be discussed below. Briefly, the deformation of the material shape causes an actuation force, which can be used to induce mechanical actions through... 

The torque of a musculotendinous force is equal to the magnitude of the actuator force F ", multiplied by the moment arm of the actuator r. Thus, the torque exerted by actuator i about joint j is... 

Both agonist and antagonist muscles contribute (unequally) to the net torque developed about a joint In fact, for any given joint in the body, there are many more muscles crossing the joint than there are dof prescribing joint movement llie knee, for example, has at most 6 dof, yet there are at least 14 muscles that actuate this joint One consequence of this arrangement is that the force developed by each muscle carmot be determined uniquely. Specifically, there are more unknown musculotendinous actuator forces than net actuator torques exerted about the knee that is, m > n in Eq. (6.9), which means that the matrix of muscle moment arms is not square and therefore not invertible. This is the so-called indeterminate problem in biomechanics, and virtually all attempts to solve it are based on the aj Ucation of optimization theory (see also the Qiap. 5 by Manal and Buchanan). 

However, Eq. (6.9) cannot be solved for the m actuator forces because m> n (i.e., the matrix of actuator moment arms is nonsquare). Static optimization theory is usually used to solve this indeterminate problem (Seireg and Arvikar, 1975 Hardt, 1978 Crowninshield and Brand, 1981). Here, a cost function is hypothesized, and an optimal set of actuator forces is found, subject to the equality constraints defined by Eq. (6.9) plus additional inequality constraints that bound the values of the actuator forces. If, for example, actuator stress is to be minimized, then the static optimization problem can be stated as follows (Seireg and Arvikar, 1975 Crowninshield and Brand, 1981) Find the set of actuator forces that minimizes the sum of the squares of actuator stresses ... 

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