The purpose of this work was to develop a simplified electromechanical model of a solenoid valve used in the Space Shuttle’s fuel system. The primary goal of this math model is to characterize and predict the valve’s current signature. A current signature is simply a plot of the current flowing through the valve’s electromagnet versus time.

It has been proposed that a detailed analysis of current signature can provide critical information about valve health (see “Solenoid Valve Status Indicator,” KSC Research and Technology [R&T] 1999 Annual Report). Health monitoring in this context implies that failures can be avoided by observing subtle changes in the current signature, which correspond to deterioration of the valve system. From this point of view, early detection of current signature anomalies could be used to avoid a catastrophic valve failure.

There are several prominent features of the current signature belonging to the solenoid valve, as discussed in the 1999 R&T Report. When the valve is energized, a sizeable dip in current occurs in the general vicinity of the midpoint of the exponential rise of the current curve. Similarly, a large dip occurs in the midpoint vicinity of the current decay. The precise shape and position of these energized and deenergized current dips are thought to yield information about the electromechanical state of the valve, which is linked to the question of valve health.

It is presently believed that the basic shape of these current dips (or spikes) is primarily caused by a back-EMF generated by the time derivative of inductance L. In conventional circuits involving inductive elements, the total time derivative of L is zero, since L is a constant. However, in the Shuttle solenoid valve, inductance is a function of the poppet-plunger assembly displacement x. The displacement is also a function of time, so that total time derivative of L is nonzero.

The dependence of valve inductance on other system parameters can lead to detection of valve performance degradation by analyzing the current signature. One of the most common failure mechanisms is contamination of the valve’s O-rings with foreign debris. Other less common failures might be related to cracks in the casing or the spring and spring-mounting assembly. Debris blocking the plunger-solenoid gap could also lead to valve failure. It is hoped that a good understanding of working characteristics using a simplified electromechanical valve model will enhance our ability to utilize current signature analysis. Valve current signature monitoring and analysis should, in the end, be able to provide critical information about valve health and performance and hopefully provide a means of detecting the onset of common failure modes well before catastrophic failure occurs.

Key accomplishments:

• Developed a lumped-parameter electromechanical model that helps explain the current signature of a Space Shuttle solenoid valve.
• The deenergized portion of the model seems to fit the measured data with reasonable accuracy.
• The energized portion of the present model fits well but only if all parameters are unconstrained (more work is needed in this area).