Tools for the implementation of enhanced PID controllers and their use in electro-hydraulic servo applications

PID as a control concept is adequate for many hydraulic servo drives. However, when payloads are high, the drive´s natural compliance and nonlinearities can become significant. In the worst cases, severe degradation in performance may result. In these situations, PID control alone does not provide an adequate solution and other strategies must be adopted. Whilst conceptually the required strategies are quite simple, the tools for implementation are not to be found in most drive control products on the market, most of which are in any case targeted at electric rather than hydraulic drive applications. For many machine builders, the tools for enhanced control strategies need to be pre-prepared, ready to use and easily configured to achieve the desired control behaviour. One approach to achieving these aims is by the use of a control function block library, included as an integral part of the drive controller. This paper describes the essential characteristics and some examples of the so-called programmable servo controller (PSC) used by the author in electrohydraulic servo control applications. The PSC function block approach to controller design provides the tools whereby the machine designer is easily able to enhance the normally used PID approach to control. In difficult control applications, the use of nonlinear and logical functions allows significantly more performance to be achieved from the same drive package compared to using PID alone. This is demonstrated in a high-payload drive and in many other applications which have been undertaken using PSC