Disturbance Observer

Using a nominal model to work backward to unknown disturbances like friction and payload, then canceling them out in the control input.

The disturbance observer is one of the most widely used tools for robust motion control, proposed by Kouhei Ohnishi and colleagues around 1983. The idea is to lump friction, load changes, model-parameter error, and external forces into one ‘lumped disturbance’ and work it out from a simple nominal model. For a motor, nominally J·θ̈ = K_t·i (J the moment of inertia, θ̈ angular acceleration, K_t the torque constant, i current); when the measured acceleration and current don't match, the mismatch is the disturbance torque, which is then passed through a low-pass filter to get an estimate (the cutoff frequency sets the estimation bandwidth — too high amplifies noise) and added back into the current command to cancel it out. This makes the plant behave much closer to its nominal model, so an outer PD loop only needs to handle tracking performance while the DOB handles robustness — the two can be tuned independently. The momentum observer used for robot collision detection and the extended state observer in active disturbance rejection control share a similar idea.

ExampleAdding a DOB to a joint motor, wrapped by an outer PD position loop: when the arm's end-effector goes from unloaded to holding a weight, the DOB quickly estimates the extra load torque and adds it into the current command, leaving position error largely unaffected. Subtracting the known gravity and friction terms from the estimate leaves an external-force estimate, a common way to do force control and collision detection without a force sensor.

Also called

DOB, DOb, DOBC, Disturbance-Observer-Based Control

Related

State Observer · Generalized Momentum Observer · Active Disturbance Rejection Control · Sensorless Force Estimation · Robust Control · Friction Compensation

Sources

Sariyildiz, Oboe, Ohnishi: Disturbance Observer-based Robust Control and Its Applications: 35th Anniversary Overview (arXiv:1902.09032)
Hyungbo Shim: Disturbance Observer (arXiv:2101.02859)