Machine Design

A Closer Look At Proportional Control

A new proportional valve positions the solenoid's moving parts outside the core of the coil. This eliminates the need for tightly toleranced parts and prevents the armature from rubbing against the housing.

The critical specifications engineers evaluate when selecting high-performance proportional flow-control valves (PFCVs) include linearity, frequency response, and hysteresis. Other important characteristics are repeatability, power consumption, leakage, life expectancy, and cost.

The ideal valve should fall near the high-performance end of all of the above specifications while being simple enough to keep costs reasonable. Designers recently developed a valve that delivers linearity, high frequency response, and low hysteresis using a unique solenoid construction.

In the past, valves generated proportional flow by dithering on/off solenoids or by using linear solenoids. However, dithering increases the complexity of the electronic circuitry and yet does not yield the linearity necessary in most closed-loop applications. Thus, linear solenoids are often preferred because they produce truly proportional flow.

The two main types of linear solenoids differ in how they eliminate stiction. One uses a pulse-width-modulated electrical input signal, which reduces the frequency response of the valve. The second type suspends the moving part (the armature) to eliminate metal-to-metal contact. The latter design, though typically preferred, is complex and expensive to manufacture.

In conventional proportional solenoids, reshaping the magnetic flux to produce linearity requires complex, tightly toleranced parts and difficult welding or brazing processes. For example, a valve might have an axisymmetric tubular construction consisting of three parts made of different materials. Manufacturing would typically require a complicated fixture to assemble the parts and facilitate welding or brazing. Within this tubular construction, movements must be precisely controlled to ensure the armature never touches the sides of the housing.

One way to avoid these complex operations is by relocating the moving parts out-side the core and directing the magnetic flux paths — that linearize force displacement — outside the coil. This new design, developed by Teknocraft, Melbourne, Fla., keeps moving parts outside the core of the coil. This eliminates the need for tightly toleranced parts and prevents the armature from rubbing against the housing. It also eliminates welding or brazing.

Two other goals of PFCV designs are high frequency response and low hysteresis. These are generally achieved by eliminating metal-to-metal contact between the armature and its surroundings and minimizing armature weight. In high-performance valves, designers typically suspend the armature between a pair of thin, flat springs to eliminate metal-to-metal contact.

In the new design, however, no part of the magnetic circuit is captured between the two flat suspension springs. This simplifies the design, reduces the number of parts within the assembly, and eliminates the need for tight tolerances.

Minimizing armature size and using inexpensive nonmagnetic materials for the other components in the suspending mechanism makes for a lightweight armature assembly. The valves yield a frequency response above 250 Hz and hysteresis between 3 and 5%.

The valves accommodate a wide range of flows with orifice sizes ranging from 0.032 to 0.625 in. The valve flow turn-down ratio, which indicates resolution, is at least 20,000:1. This means the valves handle flows from 200 lpm to 10 cc/min without losing proportionality or linearity.

The valves have a two-way poppet made of either metal or elastomer. Operating pressures range from 1 to 250 psi. A diaphragm isolates magnetic and electric components from working media, making the valves suitable for liquid and gaseous applications.

PFCVs can replace on/off valves in a variety of applications. One example is a medical ventilator in which a pair of valves precisely meter, mix, and deliver air and oxygen. The valves handle a maximum flow rate of 200 lpm with a resolution of 0.01 lpm. Another example is in a portable turbine engine for an electrical generator where the valves meter fuel, which can be diesel, gasoline, propane, or natural gas. Other applications include water purification systems, mass-flow meters, oxyacetylene welding, and fuel cells.

This information supplied by Teknocraft Inc., Melbourne, Fla.

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