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There are a number of different types of active suspension including pneumatic and magnetic, but to my knowledge, in practice, these are all classified as semi-active, since there are other non-active components (e.g. spring and sway bars) that are also part of the vehicle's suspension.

It seems to me that if we could have a single actuator at each wheel and omit all of the currently used passive components (and their weight!), that this would be very attractive for some applications in which cost is not the major impediment, such as some racing series and for some very high end sports cars.

So, are there technical impediments to creating a pure active suspension? If so, what are they?

I haven't yet found any article or source which explains this.

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To answer your specific question, no, there are no technical impediments.

That said, there are reasons why passive components are still used.

Cost

As you might have guessed, cost is potentially the biggest reason active suspensions are still uncommon. You mention some applications where cost is less important (it's never unimportant), but consider cost per performance vs simple cost. A well designed passive suspension will handle a majority of driving cases, will only have a single resonant frequency (ignoring harmonics, which are hard to activate on a vehicle), and generally perform well to minimize NVH. These systems cost very little to implement because we've used them for quite some time now. As such, incremental gains to existing passive suspensions don't require extensive investment and always provide increasing levels of performance. The cost per performance, then, is minimal.

On the other side, active suspensions allow for perfect body control. You can completely isolate the cabin from any wheel movement. Of course, doing so requires a multitude of sensors and actuators that the car wouldn't normally have, and that cost significantly more than their passive counterparts. There's also the cost of developing a control system (you can't just throw a PID controller in there), and robust actuators in a small enough package for the vehicular space requirements. All of this is feasible but costly. For all that investment, you get (perhaps) an order of magnitude attenuation on top of any passive systems? 10dB isn't terribly impressive under most driving scenarios, so unless you are switching driving styles/surfaces significantly, it's not worth it.

Parallelization

For lack of a better term, it makes no sense to go full active when you can complement a passive suspension with an active suspension (ex. ClearMotion). This significantly lowers development cost while providing the same (or better, as we'll soon see) gains as a purely active system. To provide an analogy, you are effectively saying, "We can model a resistor with a suite of transistors, so we should replace every resistor with this suite of transistors." Yes, we can. No, we shouldn't.

Power Consumption

I don't consider this a technical impediment because power can always be made available, but active systems require absurd amounts of power. Imagine how you would replace a spring with active components. Electromagnetic levitation? Lead screw with strain gauges? Any method of replacing a passive suspension with an active suspension will consume enormous amounts of energy to do the same job.

I don't have the source, unfortunately, but ClearMotion (formerly Levant) systems could only sustain a 1g turn for approximately 10 seconds before the system had to deactivate to prevent overdrawing the electrical capacity of the vehicle. Sure, that's quite a high load for quite a while, but imagine a particularly turny road. My Focus EV owner's manual actually says in it that the power steering rack (electronic, of course) will decrease steering assist if you turn too much because of the power requirements (and overheating). That's a power steering system, which doesn't require nearly as much power as a semi-active suspension. You can always get more power to the system, but at some point, you need to ask whether or not it's worth it.

Weight

You mention reducing weight. The best way to do that would be to use newer composite springs with lightweight dampers. That's pretty much all there is to the system, so that's all the weight it adds. Conversely, an active system needs large conductors to power it, possibly air compressors for air springs, electromagnets for magnetorheological damper fluids, travel sensors, etc. Some add to the unsprung mass. Some don't. Either way, you can end up with more weight, especially if you factor in power generation in the form of a larger battery, supercapacitor, or a bigger alternator.

So while we can solve the technical challenges, they aren't challenges that are really worth solving.

  • 1
    Much of the technology - in the hydraulics world - exists, The extending legs for some of the oil rigs dealt with undulation sea bed and were controlled to within millimeters while dealing with hundreds of tons of load. Is it cost effective etc see above - well done @Hari Ganti – Solar Mike Mar 24 '17 at 21:11
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To piggyback off what Hari Ganti says, it's not so much that the tech doesn't exist, but rather that it's not efficient enough (currently) to facilitate such investment.

Basically, I would say, that the only technical aspect that prevents these systems from existing is advancing the existing tech.

Hari mentions:

I don't consider this a technical impediment because power can always be made available, but active systems require absurd amounts of power.

Until we have mini on-board fusion generators or there is a massive breakthrough in the laws of thermodynamics, we don't have a method to generate or store the huge amounts of power that would be used by a fully active system such as EM systems. To me, EM systems are ideal since you can instantly (almost) adjust output values and the magnetic field also provides a constant buffer. But the strain that you're talking about from a system like this is huge. We may in the coming years see this start deploying in the upper race classes, but just imagine powering 16 or so electromagnets to control 3,000 LBS at 4+ Gs... I'm mean we're talking about 12,000 lbs of sudden force and stably controlling it to change direction within a few milliseconds with an accuracy of less than an inch... that's going to be a lot of power.

That said, in the consumer market passive systems also fill the role of being a fail-safe. If you have one of those high-end cars with active magnet assists, what happens when something goes wrong mid-turn. Well, it drops back to the passive only system and an get you through the turn since it's only assisting with the job. With fully active on the other hand, your in a bad place. Even turning into a lane that boarders oncoming traffic. If you suspension fully gives suddenly you find yourself turning and moving way more or less than anticipated. If you're lucky you just nudge a guard rail or vehicle traveling in the same direction - if you're unlucky you're going off a cliff or pancaked to the front of a semi. These are the manufacturers nightmares that come into play. Just like Toyota having those issues with the fully electronic throttle control - hope for the best, plan for the worst.

As a final note - I do not believe it would be cost effective. When you're talking about massive hydraulic systems you talking huge weights and forces, but they all lack speed. As with most cases in modern times, the larger something is, the slower is maximum potential of operation becomes. Take trains for example. A single locomotive is allowed to travel (made up number) 75 mph, but add 2 million tons of cargo and it's not allowed to exceed 15 mph. So really, the unbelievably cosmic cost comes from R&D. And we already have passive and hybrid systems that work very, very, well... so is Benz invests $40M into a fully active system, will the market give it back? Doubtful. I personally wouldn't pay $500,000 for a C Class.

  • I wonder if you might be overestimating the energy requirement. Consider how a simple spring works -- energy is applied to compress the spring and most of the energy (minus a small bit) is returned when the spring expands. Regenerative electrics work in a similar way. As a capacitor charges or an inductor builds a magnetic field, the device stores energy, most of which (minus a small bit) can be returned. – Edward Mar 24 '17 at 22:09
  • @Edward Regen is a tricky subject. Ever notice how you can't get 100% regen on an EV? Recovering power will always incur a loss, meaning you still draw power. Also, your example of capacitors and inductors match up perfectly to springs and inertia. Capacitors and inductors are passive components, not active components. – Hari Ganti Mar 24 '17 at 23:23
  • This description of regenerative shock absorbers suggests 20 to 70% recovery of energy. – Edward Mar 24 '17 at 23:33
  • As for the numbers on power demand, you have to remember that power = work / time. So sure, it's not crazy to move with 3000 lbs, (shoot, it's not that hard to push a car) but moving and adjusting under 3000 lbs of load in 50 ms... now that takes a lot of power. So we do get back to - how do we store or generate sufficient power. Even if we can reclaim 50% (mean of your range) we still have to hold it. – kyle_engineer Mar 25 '17 at 1:10
  • @Edward The linked article is referring to the recovery of energy dissipated by a damper in a traditional suspension. This doesn't account for fully active components in a suspension, which require steady state draws as well (which traditional suspensions don't). Maybe you could hit those numbers on an exceptionally bumpy road, but under normal circumstances, you'd still waste a lot of power. – Hari Ganti Mar 25 '17 at 18:22

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