A Hybrid Digital Hydraulic Servo

In many applications, control system equations are being processed in a digital computer. Digital-to-analog (D/A) converters are then used to make command signals compatible with analog final control elements. It is more desirable to design a highly integrated system which uses a digital servo and does not require D/A conversion. However, digital hydraulic servo valves have not yet been highly developed, and some types are quite complex.

It was the object of this program to design a hydraulic digital servo which uses the standard, proven two-stage electrohydraulic servo valve. This was done by having the servo valve itself perform the function of a D/A converter. It accepts digital signals in computer format and quantizes actuator velocity. Initial design studies were performed, followed by analog and digital computer studies. A breadboard unit was built and evaluated. Two main conclusions have thus far been reached:

1. 1.
   Performance comparable to analog servo systems can be obtained with the hybrid.
2. 2.
   The basic concept provides inherent redundancy, thus high reliability; the hybrid servo operates at reduced performance with most single failures.

Experimental as well as analytical data are presented in the paper.