New trends for Shift Actuation Design
Dr Mike Soumelidis, 19/06/2020
The advent of electrification in the automotive sector (passenger, commercial and off-highway) has stimulated the development of transmission and E-Machine architectures, technology and powertrain design optimisation. The demand for increased efficiency never fades, for which multi-speed transmissions clearly remain paramount in the future of automotive solutions, allowing the E-Machine to remain within its optimum operating envelope.
Electric actuation presents several advantages over more traditional shifting mechanisms, including but not limited to: production cost reduction, fewer parts, reduced size and mass, increased reliability and reduced maintenance requirements. However, this solution also presents unique technical challenges which if not properly addressed would result in dissatisfactory shift quality. Electric shift systems can be engineered to achieve high shift quality with the appropriate control algorithms, although, this is typically more challenging than with electro-hydraulic systems.
Model Based Design
The first step toward successful implementation is to understand and validate the shift system requirements, through model-based design. Following which, the shift system solution can be designed using dynamic multi-physics modelling, representing the physical hardware of the sensors, motors, mechanical actuators, coupled to the transmission controller. Effectively, this becomes the ‘digital twin’ of the shift system which is imported into the wider powertrain and vehicle model. These models are developed iteratively to both derive lower level requirements on the electronic hardware, selecting the technology which delivers the requirements, and then implementing the proposed solution back into the model to verify that the solution meets its requirements. These steps are paramount in delivering the right system first time and enable high fidelity optimisation of the solution without committing to production or testing until necessary to do so.
Most transmission shift systems require a method of converting rotary motion into linear motion, and as such the mechanical actuator is typically driven by Motor (brushed or brushless) through, for example, a ball screw or rack and pinion. Motor characteristics are clearly important and must be considered alongside the system requirements and mechanical properties of the transmission and shifting mechanism which applies, e.g. shaft inertia or shift fork inertia for an Automated Manual Transmission (AMT), deriving the power and torque required as well as the accuracy of angular position control.
Beyond motor design and selection, the actuation system must consider peak current and voltages required, including gear-shift duty cycles, to ensure the correct switching devices and gate drivers are selected and can be reliably employed, managing heat dissipation during gear shifting. This is supported by component layout, arrangement and track routing for the PCB design. Sensor signal and data processing are primarily managed via the selected microcontroller, which includes consideration for wider system aspects for transmission monitoring, shift control (speed and torque) and protection, executing the software to manage the gear shifting functionality.
The executable code is developed using model-based-design techniques, commensurate with Functional Safety expectations in accordance with ISO 26262 where necessary and can be rapidly deployed on the target platform enabling testing using stimulated inputs prior to the integrated powertrain and system-level testing.
Romax Design Services
Romax have proven the efficiency and robustness of this methodology having delivered a working full-electric, Brushless Direct Current (BLDC) based motor transmission shift system for a Full Electric Vehicle (EV) C-Segment multispeed-speed transmission with park-lock.
With a vast array of experience in all shift system technologies and transmission types, Romax are able to swiftly assess the number of different topologies and technologies which could be employed, in order to promptly down select to the optimised solution based on the requirements and any constraints which may apply.
At Romax, we develop and deliver bespoke solutions to the electrified powertrain sector, enabling multi-speed shifting with the reliability, speed, accuracy and comfort expected from a modern transmission. Transmission shift system solutions predominantly include electro-hydraulic, and increasingly, full-electric actuation using motor drives, depending on the sector which applies and the needs of the vehicle in terms of shift quality.