Sliding Mode Control for Industrial Controllers

This chapter presents sliding mode approach for controlling DC-DC power converters implementing proportional integral derivative (PID) controllers commonly used in industry. The core design idea implies enforcing sliding mode such that the output converter voltage contains proportional, integral and derivative components with the pre-selected coefficients. Traditionally, the method of pulse width modulation (PWM) is used to obtain a desired continuous output function with a discrete control command. In PWM, an external high frequency signal is used to modulate a low frequency desired function to be tracked. However, it seems unjustified to ignore the binary nature of the switching device (with ON/OFF as the only possible operation mode) in these power converters. Instead, sliding mode control utilizes the discrete nature of the power converters to full extent by using state feedback to set up directly the desired closed loop response in time domain. The most notable attribute in using sliding mode control is the low sensitivity to disturbances and parameter variations (Utkin, Guldner, & Shi, 2009), since uncertainty conditions are common for such control systems. An irritating problem when using sliding mode control however is the presence of finite amplitude and frequency oscillations called chattering (Utkin, Guldner, & Shi, 2009). In this chapter, the chattering suppression idea is based on utilizing harmonic cancellation in the so-called multiphase power converter structure. Moreover, the method is demonstrated in details for the design of two main types of DC-DC converter, namely the step-down buck and step-up boost converters. Control of DC-DC step-down buck converters is a conventional problem discussed in many power electronics and control textbooks (Mohan, Undeland, & Robbins, 2003; Bose, 2006). However, the difficulty of the control problem presented in this chapter stems from the fact that the parameters of the buck converter such as the inductance and capacitance are unknown and the error output voltage is the only information available to the designer. The problem is approached by first designing a switching function to implement sliding mode with a desired output function. Chattering is then reduced through the use of multiphase power converter structure discussed later in the chapter. The proposed methodology is intended for different types of buck converters with apriory unknown parameters. Therefore the method of observer design is developed for estimation of state vector components and parameters simultaneously. The design is then confirmed by means of computer simulations.