AN ELECTRICALLY OPERATED HYDRAULIC COXTROL VALVE 723 



It is important that the spring be able to center the valve when the 

 coil currents are balanced. This means that the stiffness of the spring 

 must be greater in magnitude than the negative stiffness created by the 

 magnetic fields when 10 ma is flowing in each coil. On the other hand, the 

 19-ma current should be able to pull the armature against the pole piece 

 and therefore must produce more force than the spring. If the shim and 

 saturation limiting were not used it would be impossible to find a straight 

 spring line that would fulfill both these rec[uirements; i.e., its reflection 

 would be between these curves without crossing either of them. The 

 family of curves representing the net magnetic forces for the various 

 intermediate values of current unbalance fall between the extreme cases 

 shown. The intersection of one of these curves and the reflection of the 

 spring line is the position which the armature will assume for that par- 

 ticular coil current. The reason for the relatively large margin of 

 force shown for the 19-ma wide-open condition will be explained later. 



Fig. 11 is plotted to show the net forces on the armature. The curves 

 are the difference between the spring line and the two magnetic pull 

 curves of Fig. 10. It can be seen that the forces are such as to cause the 

 armature to move to the center in the balanced condition. In the case of 

 maximum signal, it will move all the way to the shim stop in the direc- 

 tion of the coil which is carrying the current. 



When there is no magnetic field present the armature resonance is 

 about 320 cps. When measured statically, or at very low frequencies, 

 with the coils energized, the negative stiffness of the field greatly reduces 

 the effective stiffness of the spring, as seen in Fig. 1 1 . However, when the 

 valve is driven experimentally to find resonance, it occurs near 320 cps. 

 This apparent increase in stift'ness with frequency results from eddy 

 currents that retard the change in flux to the extent that the negative 

 stiffness virtually disappears. Eddy currents reduce the effective in- 

 ductance of the coils from about 40 henries at very low frequency to less 

 than 10 henries at 000 cps. 



It is difficult to locate the resonance experimentally because of the 

 large amount of damping provided by the extremely thin oil film be- 

 tween the plunger and the inserts. High resonance frequency of the 

 vaWe is desired so that it is safely above any frequency encountered in 

 the servo operation, thereby eliminating one consideration in the eciuali- 

 zation. Also, a high resonance means that missile acceleration along the 

 valve axis causes little displacement of the unbalanced mass of the 

 armature. 



A 250 cps differential dither voltage is superimposed on the push-pull 

 dc signal to overcome the effects of static friction. The resulting 1 ma 

 differential current produces a magnetic force about equal to that re- 



