A cascade control for Sander automation system

Hydraulic fracturing is one of the key technologies that are currently used for shale oil and gas production. Sander is the equipment that delivers various types of sand with desired sand rate demand and dumps the sand to the blender´s sand hopper for hydraulic fracturing. Due to the limited capacity of sand hopper as well as the high amplitude of internal disturbance, it is a great challenge to maintain the hopper´s sand level at a setting point. In this paper, a cascade control is developed for the Sander automation system to actively reject the internal disturbance. The cascade control employes an inner-loop PI control for tracking the disturbance and an outer-loop PI control for maintaining the sand level, respectively. A disturbance-amplitude-dependent gain scheduling algorithm is developed for the outer-loop control, which enables the control to adjust its time response to the amplitude of the disturbance. With the cascade control, the Sander automation system is able to actively reject the internal disturbance in the inner-loop and gradually compensate the static errors of sand rate and sand rate demand in the outer-loop. A relay feedback approach is developed to tune the PI gains for the inner-loop and outer-loop controls. Both simulation and hydraulic fracturing job testing results have verified that the cascade control is reliable and practical for the Sander automation system.