Session: 01-02: Pumps and Motors 1: Piston and Membrane Machines

Paper Number: 140212

140212 - Torque Ripple Minimization in a Variable Displacement Linkage Motor (VDLM) Through the Customization of Piston Trajectory 

Abstract: 

Hydraulic motors play a pivotal role in fluid power systems, converting hydraulic energy into mechanical work with high power density. A critical challenge in these machines is torque ripple, defined as periodic fluctuations in the output torque due to the reciprocating motion of the pistons. This phenomenon negatively impacts motor performance and controllability, introducing jerking motion that compromises the smooth operation of machinery. While prior research work has made advances in addressing this issue by modifying structural parameters of motors, such as the number of pistons or their trajectories, these efforts have predominantly focused on reducing the kinematic torque ripple. However, opportunities remain to enhance motor performance by addressing the dynamic torque ripple, where the compressibility effects of the working fluid are considered, especially across a wide spectrum of operating conditions. This study proposes an innovative approach to minimize torque ripple by optimizing the piston travel trajectory in a Variable Displacement Linkage Motor (VDLM), aiming to simultaneously enhance performance at different operational states, including fractional displacements. The methodology centers on the use of control points to define the profile of a periodic B-spline that characterizes a custom piston trajectory, which leads to torque ripple reductions when combined with appropriate valve timing. The VDLM under study uses five linkage modules to convert the reciprocating motion of the pistons into output rotary motion of a seven lobe cam. The drive and valve cams of this motor are synchronized as part of its external rotating case, and their profiles are shaped based on the desired piston trajectory and valve timing. Kinematic and cylinder models are used to simulate and predict the torque ripple in the VDLM  at operating conditions of interest. The kinematic model simulates the motion of the cam-linkage mechanism employed in this motor through vector loop equations and the complex number method, resulting in the prediction of piston position and velocity for any given motor displacement. On the other hand, the cylinder model assesses the pressure dynamics in a piston chamber by combining the orifice equation with a compressible fluid volume to predict the cylinder pressure and inlet/outlet flow rates. The piston velocity profiles and cylinder pressures are then combined to predict the output torque of the VDLM, allowing for the calculation of a ripple metric. With these models, an optimization process is set up to find optimum piston trajectories. This setup uses a known VDLM linkage geometry, along with the found piston trajectory, to obtain the drive cam profile needed to achieve this prescribed trajectory at 100% displacement. From this defined drive cam profile, the resultant piston trajectories at lower fractional displacements are used to evaluate the torque ripple of the motor. This is done with a weighted-average metric that quantifies torque ripple at six primary operating conditions derived from a realistic drive cycle of the motor. The optimized piston trajectory is then benchmarked against a baseline sinusoidal trajectory to assess the improvements at the same operating conditions of interest. The results demonstrate that the optimized trajectory achieves torque ripple below 5% for targeted corner conditions and remains under 12% for other varied conditions. Furthermore, the optimization yields a 10% improvement in torque ripple reduction when compared to the baseline sinusoidal trajectory at corner conditions. These findings underscore the potential of trajectory optimization to significantly enhance the torque ripple of hydraulic motors, particularly in applications requiring precise control and stability.

Presenting Author: Martin Herrera Perez University of Minnesota

Presenting Author Biography: Martin Herrera-Perez completed his B.S in mechanical engineering at the University of Miami in 2020. He is a current Ph.D. candidate who works as a research assistant at the Mechanical Energy and Power Systems laboratory at the University of Minnesota. Martin’s research interests are in model driven design of fluid power systems and efficient energy conversion with a focus on flow and torque ripple reduction in positive displacement hydraulic pumps and motors.

Authors:

Martin Herrera Perez University of Minnesota
Grey Boyce-Erickson University of Minnesota
Jonatan Pozo Palacios University of Minnesota
John Voth University of Minnesota
Paul Michael Milwaukee School of Engineering
James Van De Ven University of Minnesota