MARTY: Electric Drivetrain and Steering Actuation
Electric Drivetrain
MARTY is equipped with a powerful and responsive electric drivetrain. The 400HP of available system power allows access to a wide range of drifting conditions: fully-autonomous tests have been conducted at speeds of up to 60km/h at 40 degrees of sideslip. The drivetrain, including the motors, gearboxes, battery packs, and electronic management systems, were built-to-order by Renovo. The Stanford team, with guidance from Renovo, designed and fabricated the mechanical solution to package this into the DeLorean.
The drivetrain was first installed with a single battery pack in the rear of the car that provided 150kW of available power at 370V. We used this initial configuration to conduct autonomous drifting experiments on small, low-speed circles. This validated the basic approach and gave us confidence in the project. The next step was to explore more complicated and challenging scenarios, at higher speeds and larger sideslip; this required more power. Thus, we installed a second battery pack in the front of the car, bringing the total system power to the current value of 400HP at 740V, with 20kWh of capacity.
Stage 1: 370V Single-pack Configuration
Bottom view of installed motor cradle, showing motors, gearboxes, and front transmission tunnel mounts.
Wide view of the installed drivetrain from the bottom, including the motor cradle, driveshafts, and battery pack.
Rear view of installed motor cradle, showing location in transmission tunnel. Mounts for battery subframe on the crossmember are also shown.
View of empty motor cradle installation position and rear crossmember mounting locations, with body off.
View of rear engine compartment with bumper off, showing rear battery pack and power electronics modules.
Close-up view of rear engine compartment with bumper off, showing rear battery pack and power electronics modules.
View of rear engine compartment with all components fully installed.
The packaging solution was designed so the motor cradle could be installed from either above, with a crane, or from below, with a lift table. In this video still, the rear of the motor cradle is being carefully lifted up into the mounts on the rear cross-member. From left: Conor Sullivan and Aaron Sellars.
The front mounts are then lifted into place, pivoting the cradle about the rear bolts. From left: Matt Brunner and Jon Goh.
The first battery pack is packaged in the rear engine compartment of the car, where the V6 Peugeot-Renault-Volvo (PRV) engine was originally located. It installs from the bottom into a welded steel subframe, which is in turn bolted to the frame of the car through custom mounts. The power electronics modules are located above the rear battery pack.
Two motors drive the left-rear and right-rear wheels independently, through a gear box with a fixed 4:1 ratio. The gears are straight-cut, and thus produce an awesome racecar/spaceship-esque whine - this can be heard in the on-board footage. In the experiments in Automated Vehicle Control Beyond The Stability Limits, the wheels are electronically controlled to operate at the same slip, similar to a physical 'locked' differential. Each motor can provide a peak torque of 700Nm, and they are mounted back-to-back between two machined aluminum plates that form the 'motor cradle'. The gearboxes are installed on the outside of this cradle.
This motor cradle installs from the bottom into custom mounts located on the front of the transmission tunnel, near the original transmission mounts; and rear frame cross-member, inboard of the original engine mounts. It turns out that the DeLorean uses Porsche 930-style CV joints, which are fairly common in the aftermarket community. We were thus readily able to source custom driveshafts, rated for the higher torque and power, from The Drive Shaft Shop.
The DC-DC converter and chargers are installed on an aluminum version of the original fuel tank cover plate. This photo shows the Stage 2 configuration, with two chargers.
This assembly is then installed into the original fuel tank mounting location.
Water pumps and hoses for the motors and power electronics modules.
A DC-DC converter steps the high voltage down to provide 1.2kW of 12V power for the autonomous computer systems. We installed the DC-DC converter and charger in the front 'V' section of the frame, where the fuel tank was originally located. The high- and low-voltage wiring harnesses for the drivetrain were made by Renovo.
The DC-DC converter, charger, motors, and power electronics modules are water-cooled. The original radiator was retained, but the rest of the cooling subsystem was replaced with rubber/silicone hoses connected by an exapseratingly vast variety of neccesary fittings.
Stage 2: Upgrade to 740V Dual-Pack Configuration
A second battery pack was installed in the front trunk, a.k.a `frunk', to bring the total system power to the current value of 400HP at 740V, with 20kWh of capacity. This posed an interesting packaging challenge, with constraints from the steering rack, lower steering column, radiator, plumbing, and front suspension - especially with our relatively large steering range (+/- 38 degrees). We also had to decide what portions of the original fiberglass frunk to cut away - i.e. saving the major structure that holds up the bumper would avoid additional reinforcement work, and minimizing cutting the thick foam-filled sections would help reduce overall messiness.
Similar to the rear, this battery pack installs from the bottom into a steel subframe. While the subframe for the rear battery pack was manually cut and mitered, in this case we got the tubes laser cut at Tube Services. This included mitering, fishmouthing, and tab-and-key slots to help hold the assembly together for welding.
This subframe, in turn, is bolted to new mounts that were welded on to the rollcage and front frame cross-member - this was an interesting process, that is described in more detail below. To accommodate the battery pack, all of the central, thin section of fiberglass was removed, as well as only about an inch of the foam-filled siderails. The original radiator was replaced by a compact custom order from Griffin Radiator, and is installed in front of the battery pack. A second charger was also added to the fuel tank area.
Top view of frunk, showing front battery pack.
Bottom view of frunk, showing subframe and pack mounting locations.
Empty frunk subframe mounted in car.
To make space for the new battery pack, we used a new compact radiator and moved it forward of the original location.
Steering Actuation
Computer control of the vehicle's steering angle is implemented through a modified electric power assist system. The upper steering column with power assist gearing was sourced as part of a bolt-in aftermarket kit from Ed Uding (DeLorean Europe). We replaced the kit's 12V motor with a 48V brushless DC motor (Anaheim Automation BLY344D), mounted through a custom adapter. The back of the motor is rigidly attached to the rollcage, in order to react the considerable torque output during automated testing. Measurements of the motor position are obtained using a digital multi-turn absolute encoder (Kuebler F3688), which is mechanically connected to the back of the motor shaft. Both the motor controller (Advanced Motion Control DPCANTE 060B080) and the encoder communicate using the CANOpen standard. Power to the motor controller is supplied through a 12V to 48V DC-DC converter (SEC America 6948).
View of electric power steering system, showing encoder, motor, steering column, and mechanical mounting solution.
Video still from the first test of the prototype system under computer control, in summer 2015.
Design, Development, and Fabrication Insights
Stage 1 of the MARTY electricification was a significant undertaking, and a massive learning experience for everyone on the team. We first measured as much of the frame and body as we could, and used that to roughly block out placements of the various components, while Renovo sized out and configured the system. Once both teams were convinced this was possible, things got serious -- we pulled the gas motor, and all the various hoses, bolts, and wiring that connected the body to the frame. And then we separated them, giving us ready access to the bare frame to refine our measurements and design.
Body separated from frame on the lift.
The bare X-frame of the DeLorean, looking very similar to that of a Lotus Espirit...
A side view of the bare chassis on the lift.
We were new to the drivetrain, the car, and the process of puting the former into the latter; so it was critical to develop processes that helped us establish confidence in what we were doing as we moved forward. Perhaps the most important of these was the use of measurement jigs and prototypes to quickly iterate on our designs. This also allowed us to physically confirm clearances and measurements before fabrication. An example from the motor cradle packaging is discussed in the following slides.
Here, a laser-cut Duron prototype of the motor cradle is installed into simple mock-up fixtures for both the front and rear mount designs. We match-drilled the rear fixtures where the prototype lay, and used the resulting hole measurement of finalize the design.
Additionally, this setup allowed us to verify our critical clearances with the frame...
...and where the body would be.
This was also very useful to iterate on parts that are generally a pain to CAD, like cooling lines.
To contrast with the temporary fixtures, here are the installed final machined aluminum mounts. Because we decided to do the machining ourselves, their design emphasized ease of manufacture and robustness.
Aluminum motor cradle mounts, machined in-house by the Stanford team.
These practical lessons regarding the value of simple measurement jigs and prototyping were invalulable to pulling off Stage 2. In contrast with Stage 1, because most of the car was already in place, this was not a 'greenfield' project; it was a far more constrained packaging problem. In particular, it was critical to account for the large range of motion of the front suspension, which we were simultaneously redesigning at the time.
This is perhaps best exemplified by the development of the bolted connection between the rollcage and the battery subframe, as shown in the following slides.
The top of the subframe bolts to the rollcage through this mount, which is made from welded steel plates.
To finalize the design of the upper mount, we first temporarily fixture the subframe to known reference locations on the front crossmember, namely the steering rack mounts.
We then used a custom measurement jig, assembled from 3d-printed parts and laser-cut Duron, to refine our measurements and update our CAD based on where the final, physical subframe lay.
Before cutting metal, we validated the new design with printed paper templates and simple cardboard prototypes.
The production parts are made from waterjet cut steel plates, die-grinded by hand to final fit. They are held in place by the same fixture. Thumbs-up indicates good to go!
Tushar welding the plates together, after they have been tacked together and the fixture has been removed.
The bottom of the subframe bolts to these mounts welded onto the front cross member. These required far less work because we were confident in our reference CAD: the steering doubler plate is our own design, and fabricated with computer-driven processes.
Perhaps the most unusual and DeLorean-specific task during Stage 2 was modifying the frunk fiberglass. As there was significant cutting, we decided that bunny suits and a full respirator were required, resulting in some fantastic candid photos.
Phill Giliver and Joe Sunde cutting fiberglass.
Jon Goh just about to start cutting more fiberglass.
Contributors
On the Stanford side, the team that developed Stage 1 in 2014-2015 consisted of Jon Goh, Shannon McClintock, Arni Lehto, and Phill Giliver, with contributions by Wyles Vance. Stage 2 was developed in Summer 2016, by the team of Jon Goh, Tushar Goel, Mike Carter, Joe Sunde, and Phill Giliver.
The electric drivetrain installation was a cooperation between Stanford and Renovo. Since then, Renovo has continued to support the MARTY project by lending their time and invaluable experience. Aaron Sellars, Matt Brunner, Jason Stinson, Chris Heiser, Nick Hori, Mike Vogel, Conor Sullivan, Elmar Grom, Ivan Pandev, Savion Ragster, and Owen Davis were an indispensable part of the team.
Stage 1, in particular, was a significant undertaking for a relatively inexperienced Stanford team. Throughout this process, we were fortunate to work with and learn from the experienced team at Renovo. They were as excited about the project as we were, and always ready to help out and share the wisdom they had earned by having done similar work on the Renovo Coupe.