Accelerometer based position control system for hydraulic applications
A.Drumea, P. Svasta, B.Lupu1)
Department of Electronic Technology, “Politehnica” University of Bucharest, Romania
1) Research Institute for Hydraulics and Pneumatics, INOE2000-IHP, Bucharest, Romania
Topic: E
Presentation: P
Summary: This paper presents design, implementation and testing of an accelerometer-based module for displacement measurement in electrohydraulic systems. Position control of hydraulic cylinders is widely used in hydraulic applications like presses, robotics or seismic test benches for civil engineering. The idea was to replace standard displacement transducer attached on the cylinder with an accelerometer, thus eliminating mechanical coupling between cylinder rod and cylinder body. A microcontroller based module that reads accelerometer data, calculates displacement and sends this information over wireless link to the main control unit was designed and is under testing on a standard hydraulic test bench.
Keywords: integrated accelerometer, position control, wireless.
Motivation
Control systems for industrial processes became complex mechatronic systems with multiple sensors, data processing units and actuators. Many of these systems are electro-hydraulic (using hydraulic cylinders as actuators) and are considered mechatronic systems because they combine fine mechanics, electronics and information technology to achieve better performances with a lower cost. Typical example is a linear actuator like intelligent hydraulic axis used in seismic test benches for civil engineering. Its structure contains electronic servo amplifier, servo-valve, hydraulic cylinder, load and position sensor fixed on cylinder (figure2) or load. It combines the advantages of hydraulics—high power density within a given volume, lack of complicated mechanical power transmissions, ruggedness and long service life—with the flexibility of decentralized control architectures. Integrated electronics automatically compensate the non-linearity of the hydraulic system. Hydraulic axis can work independently of higher-level controllers or as actuators. Usually, the position sensor is linear variable inductor type (LVDT). Linear Variable Differential Transformer LVDT sensors are widely used in hydraulic mechatronic systems for measuring physical quantities like displacement, force or pressure. This sensor combines good accuracy (0,1% error) with low cost, but it requires special mechanical assembly for mounting on load attached to cylinder rod.
Another solution for displacement measurement is based on integrated accelerometers. It has the advantage of elimination of complex mechanical assembly because sensor and its electronics can be placed on the moving load without any other mechanism connected to chassis. Displacement information is obtained from acceleration by double integration. But integration amplifies over time any error from acceleration sensor and, to compensate it, a second sensor is added for a reference position. Because optical sensors are impractical in industrial conditions (dust, oil, dirt), this is a simple proximity sensor and its placement is not critical, the processing unit must know it as a parameter. The structure of proposed system (fig.1) includes also a radio part for wireless communication between sensor board and main control system.
Fig.1: Structure of an electrohydraulic system with accelerometer based position sensor.Results
The paper focuses on mechanical and electrical design, implementation and testing of the electronic module based on integrated accelerometer. Modelling and simulation of this method for position control are also performed. Experimental results with both sensor types are also presented. Tests are performed using standard electrohydraulic test bench and equipment (fig.2) and a microcontroller based module (fig.3) is used for main system control ([3]).
Fig.2: Electrohydraulic test bench and equipment. / Fig.3: Control module for hydraulic applications.Final paper will show important hardware and software aspects of new module and some experimental results, static and dynamic, measured on test bench. Differences to standard solution for displacement measurement (using inductive devices like LVDT) will be presented.
References
[1] J. Gansle, “The firmware handbook. The definitive guide to embedded firmware design and applications”, Newness, Burlington, 2004.
[2] C. Nagy, “Embedded system design using the TI MSP430 series (Embedded Technology series)”, Newnes, 2003.
[3] A. Drumea, P. Svasta, "Universal electronic module for industrial control based on system on chip device", 30th International Spring Seminar on Electronics Technology, ISSE 2007, Cluj-Napoca, Romania, 2007.