Position Control of Pneumatic Piston Using Continuous Integral Sliding Mode Control

Pneumatic actuators operate with various nonlinear forces acting on them, such as the stribeck frictional effect, variable frictional coefficient across different positions, momentary compression of air, slippage due to lack of lubrication, irregularities in the air supply tubes, the dead-zone effect of valves, time-varying friction, etc. Thus, position control of pneumatic systems becomes challenging as control logic like proportional-integral-derivative (PID) controllers starts to lose quality under such a highly disturbed environment. A high amount of un-modelled dynamics results in an irregular response of controllers across different stroke positions. This paper models a pneumatic system and advocates using the continuous integral sliding mode control (ISMC)-based approach to control piston positions accurately. Sliding mode controllers use high-frequency switching control, hence having limited practical usability. ISMC controller is a combination of two controllers, one a discontinuous control which creates a perturbation-free environment for the nominal control working alongside. Continuous ISMC replaces the discontinuity in ISMC with a continuous term, thus having great potential for practical use. A small and crisp sensor unit comprising one position sensor and two small pressure sensors is used. To best utilise resources, other physical parameters are accurately estimated by the software using advanced sliding mode-based approaches like uniform robust exact differentiator (URED) [14], hence keeping a tap on the cost of the proposed solution. Continuous ISMC uses sliding mode control to eradicate all disturbances in the system, hence providing a disturbance-free controller like PID or LQR to control the system variables. The paper presents results from both simulations as well as actual hardware setups. The hardware setup includes a double-acting pneumatic piston and two 5/2 E-P proportional flow control valves. A pair of power amplifiers supply current to the E-P valves. A Control system running in a Python environment sends digital signals to a Digital to Analog Converter (DAC); an analogue output from the DAC then controls the output-current controller. The DAC works alongside the AT90USB1287 microcontroller, which provides the additional utility of acquiring pressure sensor readings using an on-chip ADC. Robust and accurate control of pneumatic piston caries is a massive advantage as pneumatic actuators are low cost, spill-free and environment friendly.