機器人的腿是怎麼動的

How robot legs work

幾乎不用齒輪的馬達

A motor that barely uses gears

機器人的腿需要兩件互相衝突的事：要夠有力，也要能感覺。 準直驅就是那個折衷——它讓機器狗開始跑得起來。

Robot legs need two things that fight each other: they must be strong, and they must be able to feel. Quasi-direct drive is the compromise that let robot dogs start running.

QDD ＝ 準直驅 ・ 一顆又大又扁的馬達 ＋ 一個很小的減速機

QDD  =  quasi-direct drive  ·  a big flat motor + a very small gearbox

問題

The problem

落地會痛。

Landing hurts.

腳一踩到地面，衝擊會往回傳——沿著小腿、穿過減速機，一路進到馬達。 這個關節必須撐得住，最好還能察覺得到。

When a leg hits the ground, the shock runs backwards — up the shin, through the gearbox, into the motor. The joint has to survive that, and ideally notice it.

機身 robot body 髖關節 hip 膝關節致動器 馬達 ＋ 減速機 knee actuator motor + gearbox 衝擊沿著腿 往回傳 shock travels back up the leg 撐得住這一下嗎？ 感覺得到這一下嗎？ Can it survive the hit? Can it feel the hit?
腳一落地，力量就沿著傳動系統往回跑進馬達。 準直驅的一切設計，都是為了讓這條路保持暢通。
The foot strikes, and the force runs backwards through the drivetrain into the motor. Everything about quasi-direct drive follows from wanting that path to stay open.

取捨

The choice

這個關節有三種做法。

Three ways to build that joint.

齒輪是拿轉速換力量。用多少齒輪，決定了其他所有事—— 包括馬達要做多大，以及這條腿推不推得回去。

Gears trade speed for strength. How many gears you use decides everything else — including how big the motor has to be, and whether the leg can be pushed back.

設計 馬達 齒輪 把腿推回去 力量 DESIGN MOTOR GEARS PUSH THE LEG BACK STRENGTH 直接驅動 Direct drive 又大又重 huge & heavy 沒有 none 1:1 直接傳進來 goes straight in 準直驅 Quasi- direct 又大又扁的盤式 big, flat, pancake 6:1 還是傳得進來 still gets through 大減速機 Big gearbox 又小又輕 small & light 100:1 完全被擋住 stopped dead
同樣三個零件，三種減速比。減速機讓馬達可以做小—— 但到了 100:1，它也把回傳的路封死了：腳上被推一下，馬達完全感覺不到。
Same three parts, three ratios. The gearbox is what shrinks the motor — and at 100:1 it also slams the door on the return path, so a shove at the foot never reaches the motor.
1:1

直驅

Direct drive

連小石子都感覺得到。但沒有齒輪又要有力，馬達就得做到非常巨大——腿根本背不動。

Feels every pebble. But to be strong without gears the motor must be enormous — too heavy to carry on a leg.

靈敏 ・ 太沒力
Sensitive · too weak
≈6:1

準直驅

Quasi-direct drive

一顆又寬又扁的馬達，加上一級小減速。力量足以起跳，又保留了感覺落地的能力。

A wide pancake motor plus one small gear stage. Strong enough to jump, open enough to feel the landing.

折衷方案
The compromise
100:1

大減速機

Big gearbox

用很小的馬達就能有巨大的力量。但它是麻的，而且衝擊會啃壞齒輪。

Enormous strength from a tiny motor. But it is numb, and shock loads chew the gear teeth.

有力 ・ 麻木 ・ 易損
Strong · numb · brittle

原因

The reason

齒輪會讓馬達變得很重。

Gears make a motor feel heavy.

你推的不只是那條腿——你同時得把裡面的馬達轉起來。 而齒輪會把那個旋轉部件「感覺起來的重量」，放大成減速比的平方倍。

Push on the output and you are not just moving the leg — you are spinning up the motor inside. Gears multiply how heavy that spinning part feels by the ratio squared.

你從這裡推 you push here 腿 leg 齒輪 gears ² × 減速比平方 × ratio squared 馬達轉子 （很重的旋轉部件） motor rotor (the heavy spinning bit) 你的手感覺到的 WHAT YOUR HAND FEELS 6 × 6 36× 準直驅 — 你還推得動 quasi-direct drive — you can still move it 100 × 100 10,000× 大減速機 — 推不動，也麻木 big gearbox — immovable, and numb
關鍵就在「平方」。從 6:1 換到 100:1，齒輪只多了 17 倍， 但從外面推起來，馬達卻重了 278 倍。 這就是高減速比關節推不回去、也感覺不到任何東西的原因。
Squaring is the whole story. Going from 6:1 to 100:1 is only 17× more gearing, but it makes the motor feel 278× heavier from the outside. That is why a high-ratio joint cannot be pushed back — and why it cannot feel anything either.

好處

The payoff

於是，電線就成了觸覺。

Now the wires are the sense of touch.

在這種馬達裡，扭力和電流基本上是同一個數字。 所以只要腿推得動馬達，控制器就能從電流讀出那個推力—— 整條腿上不需要任何觸覺感測器。

In this kind of motor, twist and electric current are the same number. So if the leg can push the motor, the controller can read the push off the current — with no touch sensor anywhere in the leg.

腳踩到地面 foot hits the ground 推動 pushes 轉子被轉動 因為只有 6:1 rotor turns only because 6:1 改變 changes 線圈裡的電流 current in the windings 被讀成 is read as 力 牛頓 force in newtons 力量感測器 force sensor 不需要—— 馬達本身就是感測器 not needed — the motor is the sensor
因為回傳的路是通的，馬達同時就是這條腿的觸覺感測器。 機器狗能察覺地面、在冰上救回自己、被推的時候順勢放軟—— 全都只是靠盯著電流。
Because the return path stays open, the motor doubles as the leg's touch sensor. This is what lets a robot dog notice the ground, catch itself on ice, and go limp when you shove it — all from watching amps.

一句話總結

In one breath

準直驅馬達是一顆又寬又扁的電動馬達， 只配一級很小的減速——大約 6:1， 而不是 100:1。 放掉大部分的齒輪，等於放掉一些力量，換回齒輪會摧毀的東西： 這個關節還推得動，所以它還感覺得到。

A quasi-direct drive motor is a wide, flat electric motor with just one small gear stage — around 6:1 instead of 100:1. Giving up most of the gearing gives up some strength, and buys back something gears destroy: the joint can still be pushed, so it can still feel.

哪裡看得到

Where you'll find it

MIT Mini Cheetah — 6:1 宇樹 Unitree 機器狗 Unitree robot dogs 足式機器人的膝與髖關節 legged robot knees & hips 行走外骨骼 walking exoskeletons 力控機械手臂 force-controlled arms

兩個值得知道的簡化。在這類馬達裡，扭力與電流很接近但並非完美對應—— 齒輪摩擦與磁性效應會讓它模糊，這也是真實機器人仍然需要校正的原因。 另外，準直驅不是這個問題唯一的答案：串聯彈性致動器的做法正好相反， 它保留高減速比，在減速機和腿之間刻意放一根彈簧，量彈簧被壓縮多少。 準直驅是拿掉齒輪；串聯彈性是留著齒輪、再加上緩衝。

Two simplifications worth knowing. Torque tracks current closely in these motors but not perfectly — gear friction and magnetic effects blur it, which is why real robots still calibrate. And QDD is not the only answer to the same problem: a series-elastic actuator instead puts a deliberate spring between a high-ratio gearbox and the leg, measuring how far the spring squashes. QDD removes the gearing; series elastic keeps it and adds a cushion.