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Comment by petra | Hacker News Reader

Is it theoretically possible to achieve the kinds of performance needed from batteries and actuators ?

Actuators, yes. At last. The power to weight ratio of motors has improved considerably in recent years. The current record is 5KW/kg [1], for a Siemens motor for aircraft. Tesla's motors are around 3.5KW/kg, although that may be peak, not continuous. Water cooling, as used by Tesla and Schaft, helps a lot. Schaft's innovation was to apply water cooling to small motors.

You can run electric motors far above their continuous rated values for short periods. Also, electric motors specifically designed for brief overloads (high-temperature insulation, temperature sensors) are quite possible. Every automobile starter motor is such a motor. With synchronous brushless motors ("brushless DC" and "variable frequency synchronous" motors are the same thing; motors above a few KW tend to be called the latter) and big power IGBTs, you can have huge torques briefly without much difficulty. If you have the electric power available.

Batteries, maybe. Running time between charges is going to be a problem for a long time to come. There's a huge battery industry trying to get energy density up, with modest success. For many applications, trailing a power cord most of the time is an option. Especially if the robot can plug itself in, which the Hopkins Beast was doing in the 1960s.

[1] http://www.gizmag.com/siemens-world-record-electric-motor-ai...

The power density of electric motors is pretty amazing. Take for instance quadcopters, a couple of brushless motors can lift their own weight, batteries, and still have plenty of thrust left over to accelerate[0]. Yes those power densities are probably for continuous power.

The problem is not power density, but torque density. Brushless motors spin really, really fast with low torque, which is the exact opposite of what we need for robots.

[0]https://www.youtube.com/watch?v=8p5uDf9i_Yc

Electric motors have maximum torque at zero speed. Maximum power is at half of no-load speed.[1] You can design motors for higher torques at lower speeds; it's a standard design parameter.

[1] http://lancet.mit.edu/motors/motors3.html#tscurve

The maximum torque an electric motor can produce is proportional to the magnetic field in the windings which is proportional to number of turns and current. Increasing the number of turns means more mass, increasing the current means more heat. More heat is particularly insidious, because as temperature goes up resistance increases, which makes more heat, which increases temperature, and so forth and so on.

SCHAFT found a solution to the more current problem with their ultracapacitor driven water cooled motors. Except one cannot drive said motors continuously and alone they still don't have that much torque, so if one wants more torque more windings must be added.

You can design motors for higher torques at lower speeds but the torque density suffers. Luckily we have compact high reduction gearing to transform high speed low torque to low speed high torque.

You've made some good points in this thread about actuation. Can you point me to more info on SCHAFT's actuation strategy? I've been trying to turn up info, but haven't found anything great.

I think the argument here is that this maximum torque is just not enough.

>> For many applications, trailing a power cord most of the time is an option.

If the charging time was close to zero, would this problem be solved, for many applications ?

Or the other alternative, phinergy's aluminum air battery, which has 2000 wh/kg, but cannot be recharged, just replaced and "recycled", but probably in a cost effective way ?

Primary batteries have higher energy densities than rechargeable batteries. The military uses high-energy-density primary battery technologies for one-shot items like torpedoes. This might be worthwhile if you were building robots to help decontaminate Fukushima. Commercial applications, no.

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