Deep Applications of Overlap

Max

Figure 1 Four-way servo valves or proportional valves usually have four throttle edges: P→B, A→T, P→A, and B→T, as shown in the figure above. When the spool moves to the left, port P connects to port B and port A connects to port T; when the spool moves to the right, port P connects to port A and port B connects to port T. For servo valves, they are mostly zero-overlap in most cases, which means that when the PB edge opens, the AT edge also opens; when the PA edge opens, the BT edge also opens, as shown in the figure below:

  Figure 1

  Figure 2

However, in the actual manufacturing process, it is difficult to achieve complete zero overlap for the four throttle edges; there must be some positive overlap and some negative overlap. When the overlap is within ±3%, we can consider that the zero-overlap requirement is met.

To facilitate the analysis of the impact of overlap on the system, we assume an extreme case, as shown in the figure below:

  Figure 3

The PB edge is zero-overlap, and the AT edge has 20% positive overlap. That is to say, when the PB throttle edge opens, the AT throttle edge is still in the dead band (i.e., closed state) until the spool continues to move and the opening degree reaches 20%, then the AT throttle edge gradually opens. What will be the system's response during this process?

When P connects to B, the pipeline is filled with oil and starts to build pressure, but the oil return is not opened at this time, which can easily lead to pressure overshoot in chamber B. At the same time, there is no oil return at port A, which will also cause the pipeline in chamber A to be blocked and the pressure to rise, which is very dangerous for the system.

To reduce the occurrence of this situation, in actual manufacturing, the overlap of the AT edge is smaller than that of the PB edge. Before the PB edge is filled with oil, the AT edge is already open to keep the system's oil return unobstructed, as shown in Figure 4:

  Figure 4  

In the actual zero-overlap situation, the gain of the AT edge is also designed to be as large as possible compared with that of the PB edge, which can keep the oil return unobstructed and reduce back pressure, as shown in Figure 5:

  Figure 5  

Finally, let's take a practical application example.

Electro-hydraulic feedback pressure servo valves usually have only three oil ports: port P, port A, and port T. Under normal circumstances, the AT edge has partial overlap relative to the PA edge, which means that near the "null position", when port A is filled with oil, port T also has micro-leakage. If the PA edge is zero-overlap, then the AT edge is negative-overlap. At this time, the flow curves of the PA edge and the AT edge are shown in Figure 6:

  Figure 6  

The above is a detailed analysis of overlap. If you understand these, you will basically understand the spool valve.