Article Transactions of the Institute of Measurement and Control 1–15 Ó The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0142331217690224 journals.sagepub.com/home/tim Linear matrix inequality-based model matching H 2 output feedback control of a new leak tightness machine working based on hydrostatic pressure aging method Barıs x Can Yalc xın and Ahmet Koyun Abstract Fluid leaks owing to broken pipes can be a serious problem for any hydraulic system. The main reasons for pipe breakage are spontaneously changing hydrostatic and hydrodynamic pressure values inside the pipe or faults occurring during the pipe manufacturing process. Therefore, different kinds of leak tightness tests are required in many standards for approval of the pipes used in both academic researches and industrial applications. Hydrostatic pressure aging is the most common method among leak tightness test procedures. However, conventional test machines cannot reach above 700 bar owing to their mechanical specifications. In this study, the design of a new leak tightness test machine that can reach above 1200 bar and its Linear matrix inequality-based model matching H 2 output feedback control have been achieved. The efficacy of the test machine and proposed controller have been shown with both simulation and experimental results. Keywords Linear matrix inequality, model matching, H 2 output feedback, linear systems, hydrostatic pressure aging, impedance control Introduction In the industrial fields, including hydraulic and pneumatic systems, water distribution systems, chemical industry, phar- maceutical industry and nuclear engineering, many different kinds of pipes have commonly been used (Gellin, 1980; Kyriakides and Ju, 1992; Li and Zhou, 2015; Sadowski and Rotter, 2013; Yuan and Kyriakides, 2014). All these fields contain harsh working conditions, which means that the pipes can easily be affected by many different factors such as vari- able loads, temperature and pressure parameters during dif- ferent applications (Ye et al., 2016; Zheng et al., 2015). For safety, environmental and financial reasons, all kinds of manufactured pipes, especially the new generation ones, are expected to endure over 100 years under standard working conditions (Sun et al., 2014). In order to provide this perfor- mance, the pipes have to be well tested before they can be used in any application (Rafiee and Mazhari, 2016). Hydrostatic pressure aging is the most commonly used method for testing the pipes. This method is based on applying different pressure levels inside the pipe. One of the most difficult problems of hydrostatic pressure aging method is controlling hydrostatic pressure parameter inside the pipe (Yayla and Bilgin, 2007). Position control of the pistons has to be well achieved, because they directly con- trol the volume of the oil getting inside the test pipe. In most systems, the position parameter of the piston rod, instead of the pressure parameter inside the pipe, is used as output feed- back parameter. The main reason of this choice is that pres- sure sensors that are able to measure high level pressures are very expensive. Yalc xın et al. (2016) have designed a leak tightness machine that can reach 700 bar during the test. A servo motor coupled with a rack and pinion mechanism limits the maximum pres- sure value around 700 bar. Constructive capabilities of servo motor with rack and pinion mechanism are not enough to produce necessary torque for the pistons to reach above 700 bar. After reaching a specific torque level, gear teeth profiles of both rack and pinion are damaged. To deal with this prob- lem, a stronger material has to be used. However, this way brings a serious financial disadvantage to industrial compa- nies. To exceed this specific torque level in an economic way, a new mechanism structure is needed to create higher force levels on the pistons. In this study, a new design consisting of a hydraulic motor working with a servo valve, instead of a rack and pinion mechanism, has been proposed to produce Mechatronics Engineering Department, Yıldız Technical University, Bes xiktas x, Istanbul, Turkey Corresponding author: Barıs x Can Yalc xın, Mechatronics Engineering Department, Yıldız Technical University, Bes xiktas x, 34349, Istanbul, Turkey. Email: bariscanyalcin@gmail.com