Advancing the Capabilities for Miniaturization of Hydraulic and Pneumatic Drives (Part 1)

Present-day challenges in miniaturization of executive drives

Current miniaturization processes face several developmental and technical challenges such as:

Decreasing of mass-to-dimensions rates vs. keeping high specific power;

Reduction of transmitted power vs. retaining high fast response;

Downsizing the electro-mechanical components of the control units of executive drives vs. maintaining invulnerability to harmful environmental stimuli such as heat fluxes, humidity, vibrations, electro-magnetic and radiation fields, liquid and gaseous corrosive chemicals, whose influence becomes more dangerous upon downscaled embodiments.

The commercial challenge is characterized by the demand of decreasing costs vs. the rising costs of a new generation of manufacturing equipment and instrumentation. The present-day miniaturized drives do possess the mass-to-dimension rates of yesterday's instruments and transducers. As a result, the modern manufacturing equipment and instrumentation must have a higher accuracy rating, which pushes costs upward.

The challenge of miniaturizing executive drives is followed by the corresponding downsizing of their control units, each with the appropriate feedback loops, communications, power supplying sources, and various auxiliary elements. The miniaturization of the electrical, mechanical, pneumatic and hydraulic embodiments of these components feature their own specific challenges. Those components need to enable the rated operational reliability and accuracy while maintaining high power-to-mass rates in accordance with the designs for:

Moving mechanical parts within contacting kinematic or power gears of various types; and

Downsized electric circuitries that are, in most cases, vulnerable to the previously mentioned harmful environmental or man-made stimuli.

By miniaturizing mechanical drives, there is a reduction of absolute values of transmission capacities (kW). The relative value of transmission capacity Nr (kw./lb, e.g. kw. per lb of drive's weight) depends upon the type of transmission gear, its kinematic scheme, and its strength characteristics. The miniaturization of mechanical drives is usually limited by contradictions between the specification figures and appearance of physical changes, namely:

Application of open-type contacting micro scaled pairs may entail vulnerability of those mechanical pairs to dust-laden ambient, and to damping action of surface tension along solid-liquid interfaces due to moisture condensation in conditions of alternative temperature drops;

Small surface roughness for decreasing friction of contacting pairs should predict influence of adhesive forces that hinder from relative motion of said pairs;

Fine tolerances for sliding joints and for fixed detachable joints will result in commensurability of thermal strain values with values of those fine tolerances that may provoke a casual seizure thereupon;

Minimizing dimensions of leverages must imply increasing resonance frequencies of linkages that may cause an accidental friction resistance in dry swivel joints;

High accuracy of dimensions and shape of micro scaled mechanical parts necessitates growing efforts in manufacturing of machining and measuring tools, which must be of higher accuracy grade than said mechanical end products.

Utilization of miniaturized electrical circuitries in control parts of mechatronic units, as well as electro-pneumatic or electro-hydraulic control units, may induce crucial failure modes in the complex interaction of cross-domain signals of MEMS devices^[2]. It is because those electrical circuitries undergo unpredictable changes through exposure to heat flux, high frequency vibration, influence of radiation and electro-magnetic fields, corrosive affect of liquid and gaseous chemicals. In spite of the miniaturization of hydraulic and pneumatic drives (mini H/P-D) to have a lesser rate of reduction in relative transmission capacity Nr^[3], the present-day engineering of mini H/P-D also have the following contradictions between the customized specification figures and the occurrence of obstructive physical events:

Decreasing of actuating forces in mini drives vs. increasing of relative values of opposite friction forces in hydraulic and pneumatic executive motors;

Minimizing of flow area and wall thickness of tubes and hoses with compelled keeping of relatively high pressure (especially liquid pressure) must enable appropriate rupture strength of those ducts;

Decreasing of orifice sizes in hydraulic ducts and of yawns between moving contact pairs provokes occurrence of capillary and silting events that strongly increase flow resistance there through;

Enabling lubrication for friction pairs in pneumatic drives results in silting of minimized tolerances;

Use of valve-type distributing and control units with miniaturized moving mechanical parts consequently increases the resonance frequencies of those parts and casual occurrence of self-oscillations;

Downscaling hydraulic power packs with inevitable increasing circulating factor for working liquids through reservoirs of small volume contributes to overheating of standard oils with sequent changes in viscosity and combined air content, which may result in drift of hydraulic outputs;

Utilization of the solely scaled down but traditionally designed electro-mechanical leverages in first stage of proportional or servo valves induces vulnerability of said leverages to aforesaid environmental or man-made harmful stimuli.

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