Why Robot Collisions Are More Than Just Dents

When two robots share a workcell — or when a robot operates near fixtures, conveyors, or operator stations — collisions represent one of the most disruptive failure modes on the production floor. A single collision event can damage end-of-arm tooling, bend robot arms, crack fixtures, and shut down a line for hours or even days while maintenance teams assess damage and recalibrate.

The root causes are rarely mysterious. They typically fall into a handful of categories: overlapping motion envelopes, timing errors in handoff sequences, incorrect tool center point (TCP) definitions, drifting home positions, or simply inadequate interference zone management during initial programming. The good news is that every one of these causes has a proven engineering solution.

Understanding Interference Zones

An interference zone is any volume of space where two or more moving elements can physically occupy the same location at different times during a cycle. In a single-robot cell, interference typically involves the robot arm interacting with fixtures, part feeders, or conveyor mechanisms. In multi-robot cells — which are increasingly common in assembly systems — the problem compounds because you now have two or more robots whose work envelopes overlap.

The first step in solving collision problems is mapping every interference zone in the cell. This means going beyond the nominal programmed paths and accounting for:

  • Acceleration and deceleration profiles — Robots don't stop instantly. The swept volume during a deceleration event extends well beyond the programmed waypoint.
  • Tool geometry changes — If the robot picks up a part, its effective envelope changes. A gripper carrying a 300mm bracket sweeps a very different volume than an empty gripper.
  • Recovery paths — When a fault occurs and the robot executes a recovery or home routine, it may traverse paths that differ from the normal production cycle.
  • Manual mode movements — During teach pendant operations, operators may jog robots through unexpected positions.

Documenting interference zones should happen during the simulation phase of a project, ideally using offline programming tools like RoboDK, DELMIA, or the robot OEM's own simulation suite. But simulation alone is not sufficient — physical validation during commissioning catches the geometry errors that simulations miss.

Interlocking Strategies for Multi-Robot Cells

The most reliable method for preventing collisions between robots sharing workspace is signal-based interlocking. This approach uses digital I/O signals — typically managed through the PLC — to establish a handshake protocol between robots entering and leaving shared zones.

The basic logic works like this:

  1. Robot A approaches the shared zone and requests access by setting a "zone request" signal.
  2. The PLC checks whether Robot B is currently occupying or has reserved that zone.
  3. If the zone is clear, the PLC grants access and Robot A proceeds.
  4. When Robot A exits the zone, it releases the reservation signal.
  5. Robot B can now request and receive access.

This sounds straightforward, but the implementation details matter enormously. Common mistakes include:

  • Not accounting for the full tool envelope when defining zone boundaries, leading to collisions at the edges of the reserved space.
  • Setting zone boundaries too conservatively, which kills cycle time because robots spend excessive time waiting for zone clearance.
  • Missing the release signal on fault recovery paths, which causes permanent lockout conditions where neither robot can enter the zone.
  • Using time-based delays instead of position-based signals, which breaks down whenever cycle times shift due to part variation or upstream disruptions.

A well-designed interlock system uses position-based zone entry and exit signals, has clearly defined fault recovery behavior, and includes timeout monitoring so the system alerts operators rather than hanging indefinitely when something unexpected occurs.

Path Planning to Minimize Risk

Beyond interlocking, thoughtful path planning reduces collision risk at the design level. Several principles apply:

Separate approach and retreat axes. When two robots must access the same fixture location, program them to approach from different directions. If Robot A approaches from the X+ direction, have Robot B approach from Y+ or Z+. This geometric separation means the robots' paths diverge rather than converge near the workpiece.

Minimize time in shared zones. Restructure the motion sequence so the robot performs its work and exits the interference zone as quickly as possible. Pre-position the tool before requesting zone access, and defer any non-critical motions until after exiting the shared space.

Use via points to control the swept volume. Adding intermediate waypoints along a path allows you to keep the robot arm tucked in and away from interference-prone areas, even if it means a slightly longer travel distance. The cycle time cost of an extra via point is almost always less than the cost of a collision event.

Define safe home positions. Every robot in the cell should have a clearly defined home position that is outside all interference zones. This is the position the robot retreats to during faults, e-stops, and mode changes. If your home position sits inside a shared zone, you are setting yourself up for collisions during recovery.

Sensor-Based Collision Detection and Response

Modern robot controllers include built-in collision detection that monitors motor torque and compares it against expected values for the programmed path. When the measured torque exceeds the threshold — indicating the robot has contacted something unexpected — the controller triggers a protective stop.

This is a valuable safety layer, but it has limitations. By the time torque-based detection triggers, contact has already occurred. For sensitive applications, the forces involved may already be enough to damage parts or tooling.

More proactive approaches include:

  • Presence-sensing devices such as laser scanners or light curtains that detect objects entering a zone before the robot arrives.
  • Proximity sensors on end-of-arm tooling that detect approaching surfaces within 10-50mm, allowing the controller to decelerate before contact.
  • Vision-based monitoring systems that track robot positions in real-time and compare them against expected envelopes, flagging deviations before they become collisions.

For collaborative applications where robots operate near human workers, these sensor-based approaches are not just helpful — they are required by safety standards like ISO 10218 and ISO/TS 15066.

Diagnosing Recurring Collision Events

If you are experiencing intermittent collisions in an existing cell, systematic diagnosis usually reveals the cause. Start with these steps:

Review the fault log. Most robot controllers log the axis positions at the time of each fault event. Plotting these positions across multiple events often reveals a pattern — collisions always occur at the same location in the cycle, or only when a specific product variant is running.

Check TCP calibration. A miscalibrated tool center point means the robot thinks its tool tip is somewhere it is not. Even a 5mm TCP error can cause collisions when clearances are tight. Recalibrate the TCP using a multi-point method and verify against a known reference.

Inspect mechanical components. Worn gearboxes, loose mounting bolts, or damaged cable carriers can cause the robot to deviate from its programmed path. This type of drift is gradual and often goes unnoticed until a collision occurs.

Verify part consistency. If incoming parts vary in size, shape, or orientation beyond what the cell was designed to accommodate, the robot may collide with out-of-spec components. This is particularly common in applications that lack upstream part inspection or vision and quality control systems.

Audit recent program changes. Many collision events trace back to a recent modification — a new point taught, a speed override changed, or an interlock signal inadvertently disabled. Version control for robot programs is just as important as it is for PLC code.

Prevention Through Simulation and Virtual Commissioning

The most cost-effective time to solve collision problems is before the cell is built. Modern offline programming and simulation tools allow engineers to verify robot reach, check for interference, optimize cycle times, and validate interlock logic in a virtual environment.

Virtual commissioning takes this further by connecting the simulation to the actual PLC program, allowing you to test the control logic against the simulated mechanical system. This catches timing-related collision scenarios that static reach studies miss — situations where the robots' paths are geometrically clear but the timing of their movements creates transient overlaps.

Investing in simulation upfront pays dividends not only in avoiding collisions during commissioning but also in reducing the time required for on-site integration. When robots arrive at the customer's facility with programs that have already been validated in simulation, the path from installation to production runs far more smoothly.

Building Collision-Resistant Systems

Collision avoidance is not a single feature — it is a design philosophy that spans path planning, control architecture, sensor integration, and operational procedures. The most reliable systems layer multiple safeguards: well-planned paths that minimize interference, PLC-managed zone interlocking, sensor-based detection as a backup, and clear recovery procedures that prevent secondary collisions during fault handling.

At AMD Machines, our engineers design robotic systems with these principles built in from the concept phase. With over 2,500 machines delivered across decades of integration work, we have encountered — and solved — virtually every collision scenario that multi-robot workcells can produce. Contact us to discuss how we can help you eliminate collision and interference problems in your automation systems.