MOTOR DESIGN

Motor Design



Motor Design

Introduction

The intention with the work presented in this report is to evaluate the concept of electric power assisted steering in heavy vehicles with respect to size and energy requirements. The report describes the design and optimization of an electric power steering actuator on a relatively high abstraction level in the mechanical, electromechanical, and electric and control domains. Power electronics as well as control electronics are left out of the analysis.

The work is based on a number of assumptions in order to reduce the number of design variables, most of these assumptions have been discussed with experts from Volvo AB and Scania AB. The most important ones are:

1. The existing steering system geometry and principles are unchanged (Figure 1).

2. The steering gear ratio is the same as in today's steering system (~20).

3. The actuator is to be placed on the steering wheel side of the steering gear (Figure 1).

4. The worst case load for the assist actuator is during brakes applied steering when the vehicle is at standstill.

5. The driver is only required to supply a small part of the total steering torque.

Furthermore, factors as steering feel, safety, the harsh environment, the implications on the vehicle's electrical system and to some extent price are left out of this analysis. The intention with this work is to estimate the size of the constituent components and the electric currents at different supply voltages. Different component types are not compared, the electric machine is for example assumed to be a brushless DC, which probably is very competitive for this type of application but another machine type could very well be shown to both better and less expensive.

The design and optimization methods used have been developed during an ongoing research project in mechatronic design methods, see [1] for a detailed description.

Required kingpin torque at stand still, brakes applied maneuvering

The required kingpin torque to turn the front wheels during stand still steering consists of mainly two components, the torque necessary to overcome the friction in the tire road contact and a torque generated by the gravity load on the front wheels. In addition to these torques there is of course also friction in the steering mechanism itself, this friction is however neglected in this analysis. The flexibility in the tire makes the rim turn a finite angle before the tire/road contact patch will start to slide. In this region, before any sliding occurs, the kingpin torque is modeled as a linear spring.

Friction Torque

The modeling of the friction part of the kingpin torque that is presented in this section is inspired by the analysis made by Peter Schmitt. The analysis is based on the assumption of a rectangular shaped tire/road contact patch (Figure 3). As far as possible the notation and coordinate system orientations are based on the definitions.

Where Nw is the resulting normal force from the ground, a and b are the contact patch length (x-direction) and width (y-direction) respectively (Figure ...