SERVO VALVES

泛舟

SERVO VALVES-Fluid Power Circuits and Controls

PEN

1.  取决于负载的状况，哪怕在油缸伸出时图3中压杆侧的压力也可能比盖侧的压力要高出得多。

2.  在图4中，当用了伺服阀来替换方向控制阀DCV时，则流量控制阀是不必要的。

3.  文件没有表明如何去计算整个活塞的压降 。

4.  有关压降的系统评估概念是错误的，因为它没有把力考虑进去。以恒定速度控制时，作用在活塞上的力的总和必须为0。

5.  第7页的负载压力的概念也是错误的，因为它没有把活塞两端的面积不同考虑到进去。假如方程正确的话，应该能够使用它的公式来计算出Pa和Pb(可参考比较我最近有关VCCM公式的帖子)。

1.In diagram 3 the pressure on the rod side may be much higher than the pressure on the cap side even when extending.  It depends on the load.
2.  in diagram 4 A flow control valve is not needed when the directional control valve DCV is replaced with a servo valve.
3. The document does not say how to compute the pressure drop across the piston
4. The concept of evaluating the system in terms of pressure drops is wrong because it doesn’t take into account forces.   To control at a constant velocity the sum of forces acting on the piston must be 0.
5. On page 7  the concept of load pressure is wrong because it doesn’t take into account the different areas of the piston.   If the equations were correct then one should be able to use these formulas to calculate Pa and Pb.  See my recent thread about the VCCM equations.

More comments later.

PEN

6. 没有人能够真正使用第7页中计算流量的公式，除非他们只需要设计一个简单的阀或喷孔。作为用户我们并不知道Cd或A0的值，我们使用制造商提供的额定流量除以35 bar的平方根。阀的流量系数Kv = 流量 / 35 bar的平方根，这是实践知识。

7.我同意第12页所说的“阀应安装在液压缸上”的观点。但对于“最精确的系统是伺服泵/马达的组合”的说法，我不能同意。最精确的系统应该是“阀/液压缸”的组合，而且作者应该在这个“阀/液压缸”的组合中，在阀旁边再包括一台蓄能器。这样由于蓄能器存储了大量的能量，马达就不必作出迅速反应。由于阀的响应速度要比马达快得多，因此很重要的一点是在阀的附近安装蓄能器以获得最佳的系统性能。至于固有频率，它是取决于阀和活塞之间的液压油体积，而不是在马达和活塞之间的液压油体积。“阀/液压缸” 组合系统和“伺服泵/马达” 组合系统的固有频率应该相同。

6. On page 7.  No one uses that formula for flow unless they are designing a valve or orifice.   We, the users, don’t know the value for Cd or A0.   We use the manufacturers rated flow divided by the square root of 35 bar.  Valve flow coefficient, Kv =flow/sqrt(35bar) is practical knowledge.

7 On page 12.  I agree that the valve should be mounted on the cylinder.  I don’t agree that the most accurate system is the servo pump motor combination.  The most accurate is the valve cylinder combination.   The author should have included an accumulator by the valve in the valve cylinder combination.  This way the motor doesn’t need to respond as quickly because there is a lot of energy stored in the accumulator.  The valve can respond much faster than the motor so it is important to have an accumulator near the valve for best performance.
The natural frequency depends on the volume of oil between the valve and piston, not between the motor and piston.   The natural frequency of the valve cylinder and servo pump motor should be the same.