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Beyond velocity and acceleration: jerk, snap and higher derivatives (2016)

(iopscience.iop.org)
181 points EndXA | 1 comments | 19 Jun 24 10:16 UTC | HN request time: 0s | source
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londons_explore ◴[19 Jun 24 11:39 UTC] No.40727286[source]▶
I wish designers of vehicles - particularly cars, trains and busses, would work to minimize jerk, snap and crackle.

Turns out if you minimize those, you get a far more comfortable ride. It matters far more than acceleration.

Finite element models of the whole system (tyres and suspension components and flexing elements of the vehicle body and road/track) can quickly allow analysis of the jerk, snap and crackle, and allow tuning of damping and drive system control loops to make a far more comfortable ride.

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constantcrying ◴[19 Jun 24 13:19 UTC] No.40728030[source]▶
>I wish designers of vehicles - particularly cars, trains and busses, would work to minimize jerk, snap and crackle.

They do.

>Turns out if you minimize those, you get a far more comfortable ride. It matters far more than acceleration.

They know that this is the case. And put a lot of effort into making sure your car has the desired feel.

Besides your comfort these considerations are extremely important for the durability analysis for the vehicle.

>Finite element models of the whole system (tyres and suspension components and flexing elements of the vehicle body and road/track) can quickly allow analysis of the jerk, snap and crackle, and allow tuning of damping and drive system control loops to make a far more comfortable ride.

Finite element simulations are undesirable, they are extremely calculation expensive for those kind of large models and somewhat unsuitable. They are used in crash tests.

For the application you described multi body systems are used, where the car is decomposed into its functional components, which can be modeled either as stiff or flexible. With that you have a reasonably accurate model of a car which you can use to test on a virtual test track.

Basically every competent car manufacturer is doing this.

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amelius ◴[19 Jun 24 15:52 UTC] No.40729433[source]▶
I have two questions:

1) does this hold for all 3 of jerk, snap and crackle, like OP suggested?

2) In applications where no humans are involved (robot actuators etc.), would it make sense to minimize jerk, snap and crackle too?

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1. thequux ◴[19 Jun 24 21:11 UTC] No.40732424[source]▶
In response to #2, consider that every material is fundamentally "springy"[1], and many engineering materials deflect a human-noticable amount when enough force to move them is applied. Thus, you can model every connection between an actuator and an object as a spring. When the actuator starts accelerating, it applies a force through that spring, which causes the spring to extend, and the force actually applied to the object is provided only by the extension of the spring. It's only once the spring is applying the same force as the actuator is that the two objects are moving at the same speed. However, at that point, the actuator and object are moving at different speeds, so the spring is still extending. As a result, you end up with an oscillation in the velocity of the object, which is almost never desirable. For a start, if one of the parts is metal, this causes fatigue, which will cause the part to fail much sooner. Secondly, you generally want the object being moved to follow a precise path, and that oscillation will show up as ringing[2]

[1] Yes, this is a vast oversimplification, but the model I'll build using it is reasonably accurate.

[2] Most 3d printing enthusiasts are familiar with this issue; e.g. https://www.simplify3d.com/resources/print-quality-troublesh... . However, most of the advice you see amounts to "make everything stiffer", which helps, but the real solution is to be less jerky.