Emergency Stop Categories (IEC 60204-1)

Stop Category (IEC 60204-1)

Operating Conditions

Sweep speed 1.20 rad/s

Braking deceleration 4.0 rad/s²

Payload mass 6 kg

Status

RUNNING

Angular speed

0.00 rad/s

Time since stop

— s

Stopping angle travelled

— °

Post-stop drift (holding torque)

— °

How it works

IEC 60204-1 defines three emergency-stop categories for machinery, differing only in when motor power is removed relative to braking:

Category 0 — uncontrolled stop: power to the actuator is removed immediately. The arm coasts on inertia and gravity alone until friction and gravity settle it — fastest to de-energise, but the least predictable stopping angle.
Category 1 — controlled stop: the drive brakes at a bounded deceleration until velocity reaches zero, then power is removed. Motion stops in a known distance, but once power is off the arm can still sag under gravity with no holding torque.
Category 2 — controlled stop with power maintained: same bounded braking to zero velocity, but the drive stays energised afterward and supplies a gravity-compensating holding torque, so the arm stays exactly where it stopped.

The arm is a single revolute joint carrying a rod plus tip payload, so its motion obeys:

I·θ'' = τ_motor(t) − b·θ' − m_eff·g·(L/2)·cos(θ)
I = (1/3)·m_rod·L² + m_payload·L²   (rod + point mass about the pivot)

During normal sweeping, τ_motor tracks the target angular speed. On E-stop: Cat 0 sets τ_motor = 0 instantly; Cat 1 and Cat 2 apply τ_motor = ∓I·a_dec (the chosen braking deceleration, sign opposing motion) until θ' ≈ 0, then Cat 1 also sets τ_motor = 0 while Cat 2 sets τ_motor = m_eff·g·(L/2)·cos(θ) to hold position. Heavier payloads increase both I and the gravity torque, which is why the same braking deceleration travels a larger angle and why an un-held Cat 1 stop drifts further before Cat 0 was even used.

Robot arm + payload E-stop trigger point

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