The hundred-fold increase in torque density is drawn from a publication in Science Robotics [1], in which a 40-micron droplet pitch microhydraulic motor, with a torque density of 78 Nm/kg, is compared to a sub-centimeter electromagnetic stepper motor with a torque density of 0.68 Nm/kg².

But what happens when we compare ourselves with DC motors currently used for finger joints? And when we encase our actuator core in a housing that is many times heavier than the motor itself?

We chose a Maxon motor for comparison. We narrowed our search to Ø16 mm motors, to match the diameter of our motor and meet the low-profile requirements of finger joints. We zeroed in on the ECX PRIME 16M brushless DC motor, which has the highest torque density in its class. Here are some of its specifications, at a nominal voltage of 24V:

• Nominal torque (max. continuous torque): 14.6 mNm
• Weight: 43 g
• Nominal current (max. continuous current): 1.04 A
• No load speed: 15,600 rpm
• No load current: 51.8 mA
• Nominal speed: 12,900 rpm
• Stall torque: 83 mNm
• Stall current: 6.27 A
• Rotor inertia: 1.03 gcm²

With a power supply of 24V, the Sangtera MECA-1 will have the following specifications:

• Blocked torque: 100 mNm
• Weight: 6.9 g
• Number of layers: 51
• Blocked current: < 1 μA in the actuator core
• Dimensions: Ø16.5 mm, height 13.5 mm
• No‑load speed: 180 rpm
• Rotor inertia: 0.1 gcm²

The torque density of the Maxon ECX PRIME 16M is 14.6/43 = 0.34 Nm/kg, while the torque density of the MECA-1 will be 100/6.9 = 14.5 Nm/kg.

MECA-2, our second generation actuator, is in the blueprint stage. It will have 400 layers and tighter manufacturing tolerances. The MECA-2 will produce 1000 mNm while weighing less than 20 grams, a torque density of over 50 Nm/kg.

There are other notable differences. The MECA-1 will have 10x lower rotor inertia than the ECX PRIME, making it much more responsive. And the no load current is neglegible—practically no heat is generated holding position. This zero-power hold is only possible with an electrostatic motor.

The five-fold drop in absolute weight is also valuable. This makes for lighter and safer hands.