Feed Forward and Integral Control of an EGR Valve

Automotive embedded control systems need to implement real-time closed-loop control strategies for controlling valves, motors, etc. The implementation needs to focus on use of low cost hardware and efficient software with minimal foot-print so as to adequately meet the application requirement. This paper highlights the low cost hardware and software design concepts by way of a case study related to control of progressive EGR valve. The control strategy is based on "map-driven set-points" where percentage opening of the valve is stored in the form of 16x16 matrices. The set-points are accessed based on instantaneous throttle and engine rpm values which form the row and column indices of the map.

The closed loop control algorithm eliminates the need for multiplication by implementing "feed-forward with integral control algorithm." A feed-forward map specifies the most likely PWM duty cycle to be applied to the valve for a given set-point. This allows the control strategy to function in an open-loop manner for initial portion of the control cycle. When the valve response to the feed-forward settles down a finite error still exists between the actual output and the desired set-point. However this error is small enough to be "integrated out" by a simple integral algorithm based on summation. This approach has been successfully tried out in real life application and proven to be extremely useful in delivering required closed-loop accuracy at lowest cost. Although the case-study is specifically based on progressive EGR valve, the strategy can be gainfully employed for different control applications with similar control requirements.