---
publication_id: "OT-2026-000000025"
title: "Robotic Tool Centre Point Verification Using Three-Plane Reference Artefact"
published_at: "2026-09-15T19:01:26.244279Z"
canonical_url: "https://opentechnical.org/p/OT-2026-000000025"
categories:
  - "tools-tooling/robotic-tools/tool-centre-point-systems"
  - "sensors-controls-machine-vision/position-and-presence-sensing/inductive-sensors"
keywords:
  - "robotics"
  - "tool center point"
  - "calibration"
  - "metrology"
  - "touch probe"
  - "reference artefact"
  - "deviation detection"
  - "kinematics"
  - "tool offset"
---

# Robotic Tool Centre Point Verification Using Three-Plane Reference Artefact

## Abstract

This disclosure describes an arrangement for verifying the tool centre point (TCP) of a robotic manipulator using a three-plane touch routine against a fixed reference artefact. The method addresses inaccuracies arising from tool changes, damage, or mounting variations, which can silently shift the TCP and compromise process precision. By probing multiple defined surfaces of a precisely manufactured artefact, the system determines the current TCP location relative to a known datum. Deviation from a pre-defined nominal TCP triggers an alarm, and the routine is configured to differentiate between tool deformation and shifts in the tool's mounting base, enabling targeted corrective actions.

## 1. Technical Field

[0001] The present disclosure relates to robotic systems, and more specifically to methods and apparatus for verifying the accuracy of a tool centre point (TCP) associated with a robotic manipulator's end effector.

## 2. Technical Context

[0002] Industrial robotic manipulators are widely deployed for tasks requiring high precision, such as welding, machining, assembly, and inspection. In these applications, the accuracy with which the robot's end effector positions a tool (14) at a designated tool centre point (TCP) is critical. The TCP is typically defined as a specific point on the tool (14) relative to the robot's mounting flange. However, factors such as routine tool changes, accidental damage to the tool (14) or its mounting, or even thermal expansion can cause a shift in the physical TCP relative to its programmed nominal position. Such shifts, if undetected, lead to cumulative errors in subsequent operations, resulting in out-of-tolerance workpieces, material waste, or compromised process quality. Current methods for re-calibration can be time-consuming and disruptive. There is a need for an automated, repeatable, and rapid method to verify the TCP's accuracy and to identify the nature of any observed deviation.

## 3. System Overview

[0003] The verification system comprises a robotic manipulator (10) equipped with an end effector (12) that holds a tool (14). A touch probe (16) is integrated with or attached to the tool (14) or end effector (12). A precisely machined reference artefact (18) is rigidly fixed within the robot's workspace, maintaining a known, stable position. A control system (20) manages the robot's motion, interprets signals from the touch probe (16), and performs calculations to determine the actual TCP position. The control system (20) compares this actual position to a stored nominal TCP model (34) and evaluates any deviation against predefined tolerances. An alert is generated if deviations exceed these tolerances. The system is designed to execute a specific three-plane touch sequence on the reference artefact (18) to gather the necessary data. This process, as depicted in FIG. 1, allows for the detection and characterization of TCP deviations.

## 4. Components

- **Robotic Manipulator (10):** A multi-axis industrial robot arm capable of precise and repeatable positioning. The manipulator's kinematic model is stored within the control system (20), allowing for accurate computation of end effector positions in global coordinates. It typically offers a positional repeatability of ±0.05 mm or better.
- **End Effector (12):** The component attached to the robot's mounting flange, designed to hold and manipulate various tools (14). It provides the mechanical interface between the robot and the tool (14).
- **Tool (14):** The active element used to perform the robot's task, such as a welding torch, a dispensing nozzle, a grinding wheel, or a gripper. The TCP is defined relative to a feature on this tool.
- **Touch Probe (16):** A precision sensing device, typically employing a stylus, that generates an electrical signal upon contact with a surface. The probe's triggering mechanism can be mechanical (e.g., microswitch) or optical. The probe (16) is either mounted directly onto the tool (14), or it is the tool (14) itself if the tool has a suitable probing tip. Its repeatability is typically within 0.002 mm.
- **Reference Artefact (18):** A solid block or structure manufactured from a dimensionally stable material, such as hardened steel or ceramic, with precisely defined and machined planar surfaces. As illustrated in FIG. 2, it features at least three orthogonal or near-orthogonal reference planes (22, 24, 26) that the touch probe (16) can contact. The exact location of these planes relative to a global datum is precisely known. The flatness of these planes is typically maintained to within 0.005 mm.
- **Control System (20):** A programmable logic controller (PLC) or industrial PC that manages the robot's motion, processes sensor inputs, and executes the TCP verification algorithm. It incorporates a motion planner (30), kinematics module (32), TCP model (34), measurement processor (36), and deviation monitor (38), as shown in FIG. 3.
- **Motion Planner (30):** A software module within the control system (20) responsible for generating precise robot trajectories to guide the touch probe (16) to the reference artefact (18) surfaces.
- **Kinematics Module (32):** A software component that performs forward and inverse kinematic calculations, translating joint angles into Cartesian coordinates and vice versa. It accounts for the robot's physical dimensions and joint configurations.
- **TCP Model (34):** A data structure within the control system (20) storing the nominal, pre-calibrated coordinates of the TCP relative to the robot's mounting flange or end effector (12). This model includes X, Y, Z offsets and potentially orientation data.
- **Measurement Processor (36):** A software module that receives raw touch probe (16) trigger points from the control system (20), filters any noise, and calculates the actual TCP position based on these measurements and the known geometry of the reference artefact (18).
- **Deviation Monitor (38):** A software module that compares the calculated actual TCP position from the measurement processor (36) against the nominal TCP model (34). It determines the magnitude and direction of any deviation and compares it to predefined tolerance limits.

## 5. Operation

[0005] The TCP verification process initiates with a command from the control system (20).

[0006] **Start-up Sequence:**

- The control system (20) retrieves the nominal TCP model (34) and the known geometry of the reference artefact (18) from memory.
- The motion planner (30) generates a sequence of robot movements to position the touch probe (16) for contact with the first reference plane (22) of the artefact (18). This typically involves moving the probe (16) to a safe approach position, then slowly advancing it towards the surface.

[0008] **Steady Running (Touch Sequence):**

- **First Plane Measurement:** The robot (10) moves the touch probe (16) to contact the first reference plane (22) of the artefact (18) along a trajectory normal to the plane. Upon contact, the touch probe (16) sends a trigger signal to the control system (20). The robot's (10) position at the moment of trigger is recorded by the measurement processor (36). Multiple touches (e.g., 3 to 5) are typically performed on this plane, with the robot (10) retracting slightly between each touch, to average out minor positional variations and improve accuracy.
- **Second Plane Measurement:** The robot (10) retracts the probe (16) to a safe intermediate position and then approaches the second reference plane (24) of the artefact (18), again along a trajectory normal to the plane. Upon contact, the probe (16) triggers, and the robot's (10) position is recorded. This is also repeated multiple times.
- **Third Plane Measurement:** The robot (10) repeats the process for the third reference plane (26), acquiring multiple touch points.
- **TCP Calculation:** The measurement processor (36) uses the recorded robot positions for each plane and the known offsets of the reference planes (22, 24, 26) from the artefact's (18) datum to calculate the current physical location of the touch probe's (16) tip in the robot's global coordinate system. From this, the actual TCP position relative to the robot's mounting flange or end effector (12) is determined by subtracting the known probe (16) tip length and any fixed offsets.
- **Deviation Analysis:** The deviation monitor (38) compares the calculated actual TCP position with the stored nominal TCP model (34). It computes the translational (ΔX, ΔY, ΔZ) and potentially rotational deviations.
- **Tolerance Check:** The computed deviations are compared against predefined alarm tolerances. For instance, a translational deviation exceeding 0.15 mm or a rotational deviation exceeding 0.5 degrees could trigger an alarm.

[0010] **Fault or Interruption:**

- If any deviation exceeds the set tolerances, the deviation monitor (38) generates an alarm or warning message through the control system (20). This alert can halt further robot operations, illuminate a warning light, or notify an operator.
- The control system (20) may log the deviation data, including magnitude, direction, and timestamp, for diagnostic purposes.
- For deviations indicating a bent tool (14), the system typically suggests tool replacement and re-calibration. For deviations indicating a shifted mounting base (e.g., the end effector (12) loosened), the system may indicate a need for mechanical inspection and re-tightening, followed by a full re-calibration of the TCP.
- If the touch probe (16) fails to trigger within an expected range during an approach, or if an unexpected obstacle is detected, the robot (10) can be configured to stop motion safely and report an error.

## 6. Example Embodiments

[0012] **Embodiment 1: Standard Three-Plane Rectangular Artefact** This embodiment utilizes a reference artefact (18) that is a solid rectangular block with three precisely machined, mutually orthogonal planar surfaces (22, 24, 26). The touch probe (16) sequentially contacts the top surface (22), then a side surface (24), and finally an adjacent side surface (26). The known distances between these planes, and their position relative to a fixed datum in the robot's work cell, allow the measurement processor (36) to establish three coordinates of the probe's (16) tip. This configuration is effective for determining translational shifts in the TCP. For example, the artefact (18) might present a top face at Z=100mm, a front face at Y=200mm, and a side face at X=300mm in the robot's base coordinate system. The touch points on these surfaces directly map to X, Y, Z coordinates of the probe (16) tip.

[0013] **Embodiment 2: Artefact with Feature for Rotational Verification** In addition to the three orthogonal planes, this embodiment incorporates a fourth geometric feature on the reference artefact (18), such as a precisely machined bore or a conical pin. After the initial three-plane touches establish the translational TCP, the robot (10) attempts to insert the tool (14) (or a specific feature of the touch probe (16)) into the bore or mate with the pin. Sensors, such as additional force/torque sensors on the end effector (12) or proximity sensors on the tool (14), detect any misalignment during this attempted engagement. This additional step allows the deviation monitor (38) to detect and quantify rotational deviations of the TCP, providing a more comprehensive verification for tools (14) sensitive to orientation, such as welding torches or assembly tools.

[0014] **Embodiment 3: Multiple Artefacts for Distinguishing Tool Bending from Mount Shift** This embodiment employs two or more reference artefacts (18) placed at different, known locations within the robot's workspace. The robot (10) performs the three-plane touch routine on a first artefact (18a), then moves to a second artefact (18b) and repeats the routine. If the observed TCP deviation is consistent in magnitude and direction relative to the robot's base frame across both artefact locations, it suggests a uniform shift of the entire end effector (12) and tool (14) assembly, likely due to a loose mount. If, however, the observed deviation varies significantly between the two artefact locations, it indicates a deformation or bend in the tool (14) itself, where the TCP's position relative to the end effector (12) has changed. This approach leverages the differential measurement to diagnose the root cause of the TCP error.

## 7. Alternative Configurations

- **Integrated Probe:** The touch probe (16) can be permanently integrated into the tool (14) itself, for example, as a spring-loaded tip on a dispensing nozzle, eliminating the need for a separate probe attachment.
- **External Vision System:** Instead of a contact probe (16), a high-resolution 2D or 3D vision system could be used to observe the tool (14) tip relative to the reference artefact (18). The vision system would capture images from known viewpoints and apply image processing algorithms to determine the TCP's position. This eliminates physical contact but requires precise camera calibration and lighting control.
- **Non-orthogonal Reference Planes:** While orthogonal planes simplify calculations, the reference artefact (18) could be designed with non-orthogonal planes, provided their precise angles and positions are known and accounted for in the measurement processor (36). This might be useful in confined spaces.
- **Single-axis Touch Probe:** For applications where only deviation along a primary axis (e.g., Z-axis) is critical, a simpler single-axis touch probe (16) and a single reference plane (22) could be employed, though this would provide less comprehensive TCP verification.

## 8. Parameter Variations

| Parameter | Range | Effect of Moving Towards Lower End | Effect of Moving Towards Upper End |
| --- | --- | --- | --- |
| **Tolerance Threshold (mm)** | 0.05 to 0.5 | Higher sensitivity, more frequent alarms, reduced false negatives | Lower sensitivity, fewer alarms, increased risk of undetected errors |
| **Touch Speed (mm/s)** | 5 to 50 | Increased measurement accuracy, longer cycle time | Reduced measurement accuracy, shorter cycle time |
| **Number of Touches per Plane** | 1 to 10 | Faster measurement, lower statistical confidence | Slower measurement, higher statistical confidence, reduced noise |
| **Artefact Material Hardness (HB)** | 200 to 600 | Increased risk of wear/damage to artefact surfaces | Reduced wear, extended artefact lifespan, higher cost |
| **Artefact Plane Flatness (mm)** | 0.002 to 0.02 | Higher measurement precision, higher manufacturing cost | Lower measurement precision, lower manufacturing cost |
| **Probe Stylus Diameter (mm)** | 0.5 to 6 | Enables smaller feature probing, increased fragility | Increased robustness, less precise for small features |
| **Robot Repeatability (mm)** | 0.01 to 0.1 | Higher overall verification system accuracy | Lower overall verification system accuracy |

## 9. Additional Implementations

- **Automatic TCP Compensation:** Upon detecting a deviation within a defined minor tolerance band (e.g., 0.05 mm to 0.15 mm), the control system (20) could automatically update the TCP model (34) with the newly measured offsets, without operator intervention. This would allow the robot (10) to continue operation with corrected kinematics, extending the time between full re-calibrations.
- **Predictive Maintenance Integration:** The deviation monitor (38) could track trends in TCP deviations over time. A gradual, consistent drift in a particular direction might indicate impending mechanical wear in specific robot joints or a loosening of mounting hardware, enabling proactive maintenance scheduling before critical tolerances are exceeded.
- **Tool Identification System:** An integrated radio-frequency identification (RFID) or barcode scanner could identify the specific tool (14) currently mounted. This allows the system to store and retrieve unique nominal TCP data for each tool (14), accommodating variations between individual tools of the same type and ensuring the correct reference is used during verification.
- **Environmental Compensation:** Temperature sensors placed on the robot (10), end effector (12), and reference artefact (18) could feed data to the control system (20). The measurement processor (36) could then apply thermal expansion compensation algorithms to adjust the nominal TCP model (34) and artefact (18) dimensions, mitigating errors due to ambient temperature fluctuations.

## 10. Figures

![FIG. 1](figures/fig-001.svg) This diagram illustrates the primary components of the tool centre point verification system.

![FIG. 2](figures/fig-002.svg) This drawing shows the relevant features and dimensions of a representative reference artefact.

![FIG. 3](figures/fig-003.svg) This diagram outlines the flow of data within the control system during the verification process.