Here’s What Manufacturing Experts Found When They Tore Into The Tesla Cybertruck’s Wacky Steer-By-Wire System

Manufacturing experts at Munro & Associates are tearing into a Tesla Cybertruck right now and discovering some fascinating stuff. What’s especially cool is their recent teardown of the truck’s steering system, which is totally different than a conventional steering system in that the steering wheel is not mechanically connected to the front wheels. That may sound dangerous to you, but it turns out, Tesla has lots of redundancies built in. Here’s a look at some of them.

OK, first things first: We should look at how a typical automobile steering system works. When you turn the steering wheel, a torque sensor either near the steering wheel or built into the rack reads your steering input input and processes it through a controller, ultimately sending current to an electric motor in the steering rack. This electric motor then moves the rack left and right, which moves the tie rods in and out, pushing or pulling the steering knuckle, turning the car’s wheels. Thus, it’s the electric motor doing all the work; your hands are only giving an input telling that motor which direction it should spin and how much effort it should exert.

Still, there is a physical connection between the steering wheel and those wheels, so if the power steering stops working, you can still turn. Here’s what a typical electric power steering system looks like.

A Typical Steering Rack

Your steering wheel connects to a steering intermediate shaft, which is shown below. It has two universal joints in it to allow it to rotate even when you tilt the steering wheel up and down, and even when the car flexes a bit during driving. It’s also designed to collapse in the case of a crash:

That intermediate shaft connects to a steering rack, which looks like this:

As you can see, even if you for some reason lose power and the electric motor stops assisting you, you can still turn the wheel, which will send torque down the intermediate shaft, into the rack, and across those tie rods that are connected to the front steering knuckles that the wheels bolt to (via a wheel hub).

It’s worth noting that sometimes, in cases where packaging can be tricky, the electric motor is connected to the column/intermediate shaft itself, spinning it instead of directly acting on the rack. Nonetheless, this doesn’t change my main point, which is: On a typical steering system, there is a physical connection between the steering wheel and the front wheels.

Tesla’s Steer-By-Wire

The Tesla Cybertruck is different. It uses a so-called “steer by wire” system (Infiniti had a similar system, though there was a clutched mechanical backup; the Tesla has no such thing), which means that you replace that intermediate shaft shown above with… a wire. That’s it. There is no physical connection between the steering wheel and the wheels. If the system fails completely, turning your steering wheel will not result in the front wheels turning.

 

View this post on Instagram

 

A post shared by The Autopian (@theautopian)

As you can imagine, a complete system failure simply can not happen, which is why Tesla and ZF (the steering supplier) have built in multiple failsafes, which Munro & Associates explores in this new video.

As shown and mentioned before: There is no intermediate shaft. At all. The steering wheel connects to a contraption that includes a tilt/telescoping steering wheel position-adjuster, an electric motor, and various sensors. Below is the entire steering “column.”

Steering ‘Feel’

The column features a force-feedback motor that takes input from the steering wheel sensor and steering rack and actually “”artificially” sends forces into the steering wheel so that the driver experiences an intuitive “feel” based on the driving situation:

Munro went for a ride with Chris Mantzios, ZF’s Vehicle Dynamics & Steering Feel Specialist. He described to Sandy and the team how that motor in the steering column works with the whole system to give the vehicle a natural steering feel.

“We wanted to show how natural the steering effort feels in the vehicle…We do have a steering input that we read from our sensor,” he says in the video above. “We send that signal right to the front wheel actuator, which is in position-control…it will ask the front wheels to turn so much. Because it’s in position-control, the motor has to overcome the load to basically get to that position…we look at the motor information and we convert all that intelligently into a rack force. Then that rack-force…we do put it through what we call an inverse boost curve that will give us a torque request to the hand wheel. And our torque controller on the hand-wheel generates that torque for you.”

Here is the sensor in the column that reads the driver’s input:

Tesla’s Ethernet Cables And 48-Volt System

Those steering inputs from that sensor get sent into these ethernet cables:

Notice how these cables are sending signals and motor power through small wires/connectors. They’re able to do this because of relatively low I^2R losses since the Cybertruck incorporates a 48-volt system instead of a 12-volt system. We know that Power is equal to current times voltage; so if you need a certain amount of power for an electric motor to turn front wheels, then if you quadruple the voltage, you can divide the current by four. Less current means fewer I^2 losses, which are generally what dictate wire thickness.

I have to say: The flat ethernet cables are fascinating; typical harnesses in cars are round, and feature relatively fat insulated wires. To see this flat thing with small connectors at each end — very interesting.

Redundancy Is Key To Steer-By-Wire’s Safety

Notice that there are two cables — that’s in order to provide redundancy to the steering wheel input signal, which gets sent to a module. That module isn’t shown in Munro’s video since it’s been analyzed by a client (probably an automaker or supplier looking to gain insight into Tesla’s work), but its job is to interpret the steering signal that the driver gives, and to then send an output signal to the motors on the steering rack:

Look at that rack. It looks totally different than a typical electric power steering rack, as not only is there no input shaft, but there are two motors driving the sliding rack. Those are there for redundancy in case one fails. Here’s a look from behind:

You can see that each motor has its own sensor to tell the module where the wheels are facing:

In addition, there’s a third sensor that acts as a so-called “tie breaker” in case there’s a discrepancy between the two motors’ sensors. “This is a triple-redundant system,” Munro says:

Update: It’s worth noting that, in the “how steer-by-wire” works video from Munro, a ZF rep response to a question about redundancy, saying: “There’s always a focus on dueling redundancy. There’s multiple aspects: You have to have abilities to have… dual power capability, there’s dual communication, there’s dual sensing technologies.” Based on that, it sounds like there have to be two isolated power sources, though I’ll want to dig into that further.

The Cybertruck’s Rear Steering Is Simpler

Out back, things are a little simpler, with the setup quite similar to typical rear-steer arrangements you might find in Porsches and Audis (though I’m fairly sure their systems don’t have ball joints, but rather just moves the rear “knuckles” in the bushings). There’s not much in the way of redundancy, but the good news is that if the rear-steer fails, it’s only steering a maximum of 10 degrees, and you’ve still got the fronts to keep the vehicle from going out of control.

Here’s how the Audi A8’s rear-steer works:

And just for fun, here’s GM’s Quadrasteer rear-steer system from 20 years ago:

How Do You Feel About Steer-By-Wire?

Are you cool with your steering wheel not being mechanically connected to the front wheels? Do you trust ZF’s/Tesla’s redundant system (great video above by ZF showing how it all works), or is there a bit of hesitancy on your side that it could all fail, and you’d be stuck unable to turn the vehicle?

To be, it seems like a cool system, and I see the advantages — specifically that you can change the steering ratio as a function of a number of inputs, such as vehicle speed (the Cybertruck turns farther for a given angular displacement of the steering wheel at slow speeds than it does at high speeds). And it does offer packaging advantages, though those redundant motors take up space, and add cost and weight. It’s interesting, but is it better overall? It’s hard to know.