Force density
Force output per unit mass of actuator, typically expressed in kilonewtons per kilogram. The key metric for comparing actuation technologies in mass-constrained applications such as legged robots and aircraft. Hydraulic actuators achieve force densities that electric actuators cannot match at the multi-kilonewton scale with current motor and battery technology. As electric motor technology improves and battery energy density increases, the force-density crossover point shifts: tasks that required hydraulics at 10 kN in 2016 may be achievable with optimized electric drives in 2026, which is why design decisions should reference current datasheets rather than decade-old rules of thumb.
Defined in 2 GAGE programs, which carry 3 distinct definitions of it. The wording above is taught in Foundations of Robotics and Physical AI.
How each discipline defines it
The same term does different work depending on who is using it. These are the definitions as each program teaches them, unedited.
Force output per unit mass of actuator, typically expressed in kilonewtons per kilogram. The key metric for comparing actuation technologies in mass-constrained applications such as legged robots and aircraft. Hydraulic actuators achieve force densities that electric actuators cannot match at the multi-kilonewton scale with current motor and battery technology. As electric motor technology improves and battery energy density increases, the force-density crossover point shifts: tasks that required hydraulics at 10 kN in 2016 may be achievable with optimized electric drives in 2026, which is why design decisions should reference current datasheets rather than decade-old rules of thumb.
The amount of force an actuator can produce relative to its size or mass. Current soft pneumatic and elastomer-based actuators generally have lower force density than rigid electric or hydraulic actuators, which limits their suitability for high-force tasks regardless of the shape-uncertainty benefits they offer elsewhere.
See the definition earlier in this glossary; repeated here as a reminder that force density, not just raw force output, is the correct comparison metric when weighing a soft actuator's capability against a rigid one of comparable size or mass.
Terms it appears with
Not an alphabetical neighbourhood: these are the terms taught in the same lessons, ranked by how often they appear together.