Direct Driven Pump Control of Hydraulic Cylinder for Rapid Vertical Position Control of Heavy Loads: Energy Efficiency Including Effects of Damping and Load Compensation

Abstract

The ever-tightening government-enforced regulations for more energy efficient and less polluting machines and the simultaneous fast development of electric drives have caused hydraulic systems to lose ground to electric drives. One promising solution to improve the status of hydraulics in this competition are the Direct Driven Hydraulic (DDH) systems, aka electro-hydraulic actuators (EHAs), which are characterized by a closed circuit type and a servo motor driven speed-controlled pump controlling the actuator. Due to this topology, they offer a possibility of reaching higher energy efficiencies compared to traditional open circuit type valve-controlled systems and simultaneously they offer the high accuracy and dynamics of these. Typical applications where DDHs have been used are, in the area of mobile equipment, modern commercial and military aircrafts and some lift trucks, and in the area of stationary applications, mostly presses. In all of these, the actuators produce relatively slow motions. In this experimental study, a DDH system is applied to a stationary industrial vertical position control application where a very rapid movement of a heavy load is required. This brings out some unwanted fluctuation phenomena not encountered with slower motion velocities. Here we are striving for avoiding these phenomena by adding damping to the system. In addition, it is studied whether the good energy efficiency of DDH systems could be enhanced with load-compensation. The presented measurement results include the system behavior regarding the smoothness of positioning, the fluctuations of pressures, forces, and power, and finally the energy consumption with three different system configurations: basic DDH, load-compensated DDH, and load-compensated and damped DDH. The measured energy consumptions are compared against results gained in simulating a conventional valve-controlled system driving the same application. The measurement results manifest that energy consumption wise significant benefits are achievable with DDH, especially in combination with hydraulic load compensation. However, without added damping the motion involved marked vibrations in the end of the upward and downward strokes. Added damping eliminated these vibrations, but at the cost of reduced energy efficiency. Due to this, the solution for the fluctuation and vibration problem should be sought by developing a control strategy that produces a smoother but as fast motion.

Experimental Study on Fast and Energy-Efficient Direct Driven Hydraulic Actuator Unit

“…With the P-controller, it has previously been possible to achieve a rise time of approximately one second [26,27]. In these tests, as well as in the tests of the current study, the P-controller's task was facilitated by using a rate limiter, which replaced the step-wise change in the position reference with a steep ramp.…”

“…In addition, the motion was plagued with a series of pressure spikes causing unwanted fluctuations in the linear velocity of the actuator piston during the stroke. In tests where load compensation was not used, the pressure spikes in the lifting chamber of the actuator cylinder could exceed 200 bar [26,27]. The problems with motion vibrations and high chamber pressures became even more pronounced when the rise time was shortened below one second.…”

“…The reason for testing the suitability of the DDH system for this specific application is that the currently used valve-controlled system, although satisfying the dynamic requirements, is considered to be too energy consuming. Prior studies [25][26][27] conducted with this test rig proved that the DDH system enables achieving a 53-87% reduction in energy consumption compared to a valve-controlled system. The magnitude of reduction depended on applying or not applying load compensation.…”

“…In the measurements of the DDH system, two control methods were tested: position control with a P-controller (applying a correction proportional to the position error), and S-curve trajectory control with an additional PI (Proportional-Integral; applying an integral term to the position error, in addition to the proportional term) control for accurate final positioning. The results of previous tests conducted with P-control are described in [25][26][27]. Although the achieved energy savings with DDH in these tests with this controller were more than satisfactory compared to traditional valve-controlled implementation-even in the cases where load compensation wasn't applied-the dynamic performance of DDH with the P-controller was unsatisfactory.…”

“…The system is applied in a full-scale test rig that emulates a real industrial application, a material-handling machine, which consecutively vertically lifts and lowers a fixed mass of 1325 kg over a transition range of 260 mm at high velocity. Previous stages of this study [24][25][26][27] manifested the high energy efficiency of the system, but the dynamic performance was considered unsatisfactory when using a traditional P-controller (in which the position of the cylinder is only controlled by applying a correction proportional to the difference between the desired position and the measured position when producing the speed command for the servomotor). The shortcomings appeared in the form of strong pressure fluctuations during movements and marked vibrations at the end of the actuator strokes.…”