Energy efficiency of water hydraulic FST, PMT, and servo motor system

Low energy efficiency and high installed cost are the two main problems that prevent water hydraulics to be popular in application. This research introduces two novel systems: water hydraulic fluid switching transmission (FST) and pump motor transmission (PMT) that only use cheap ON/OFF valves for lessening the initial cost and reducing the energy consumption; beside, this paper also introduces a conventional servo motor system (SMS) for comparison. Moreover, both FST and PMT systems can recover the kinetic energy of a flywheel in a deceleration process and store it into an accumulator for using in the next working cycle. The experimental results show that the FST system only need from 33.2 to 47.3% of total energy consumption of the SMS to complete a full cycle and even much reducing in PMT system with the reduction from 76.0 to 86.0%. A method to estimate the saved energy stored in the accumulator will be introduced in this study as well and found out that from 8.2 to 11.6% and from 8.7 to 13.7% of the total energy consumption of the FST and PMT systems were recovered. The SMS shows advantage in transient response with the shortest rise time and smallest overshoot while the steady state error is only slightly smaller than the PMT system. The steady state error of FST system is quite large, but it keeps almost same value for all reference velocity; thus, such system is appropriate for applying in high velocity systems. With acceptable velocity response and extremely improving in energy efficiency, the PMT system is promising to replace conventional water hydraulic system in many applications.