784 THE BELL SYSTEM TECHNICAL JOTTRNAL, MAY 1957 



«hu\vs the oil flowing from left to right and simulates the intake port of 

 the valve. The extra land on the plunger directs the oil stream toward 

 the escape passage to reduce turbulence and cavitation and increase oil 

 flow. The right illustration illustrates the reverse flow of oil, right to 

 left, and represents the exhaust port of the valve. 



Fig. 18 is a plot of the measured Bernoulli force on the J-7 valve. It 

 will be noted that there is little relation between the curves for various 

 pressure drops. No simple ecjuation has been formulated to account 

 for the forces observed. Although Fig. 18 shows the Bernoulli force to be 

 large, Fig. 11 shows that full signal produces enough net force on the 

 armature to overcome this force at any valve position. 



A large part of the development effort on the J-7 model was expended 

 on the problem of reducing the forces caused by oil flow. For the same 

 flow conditions, the J-7 has about one-fifth the Bernoulli force of earlier 

 designs with simple rectangular grooves in the plunger. 



Subsequent to the initial manufacture of the J-7 valve, the design 

 of the annular grooves and body inserts has been improved continually. 

 Consequently, the Bernoulli force has been further reduced to permit 

 higher operating pressure, and hence more gain, without creating a hy- 

 draulic oscillation problem. 



THE J-7 VALVE AS A SERVO ELEMENT 



For any given set of operating conditions, the transfer function (ex- 

 pressed as cubic inches of oil flow per milliampere of control current 

 unbalance per lb per square inch of pressure drop) can be extracted 

 from the information presented above. However, the resulting family 

 of curves for various load torques would be of little use to the servo 

 designer. The pressure drop available for use by the valve is different 

 for each curve, and they are all ciuite nonlinear, as is apparent from the 

 data. 



The overlap of the valve ports results in another type of nonlinearity 

 that complicates the loop equalization problem. Examination of Fig. 12 

 will show that the effect of the overlap is a small dead area in the region 

 of zero output of the valve. Small signal levels will cause the valve arma- 

 ture to operate in and around the vicinity of the dead zone, resulting in 

 very little oil flow. Thus, the gain of the valve for very small signals is 

 lower than for signals of greater magnitude. 



As mentioned earlier, the effect of the nonlinearities of the valve is 

 greatly reduced by use of the relatively fast-acting local loop which 

 encompasses the valve, actuating cylinder, and amplifier. This inner, or 

 secondary, loop contains sufficient gain to insure that, in spite of the 



