Ahmed Joubair

Comparison of two calibration methods for a small industrial robot based on an optical CMM and a laser tracker

The absolute accuracy of a small industrial robot is improved using a 30-parameter calibration model. The error model takes into account a full kinematic calibration and five compliance parameters related to the stiffness in joints 2, 3, 4, 5, and 6. The linearization of the Jacobian is performed to iteratively find the modeled error parameters. Two coordinate measurement systems are used independently: a laser tracker and an optical CMM. An optimized end-effector is developed specifically for each measurement system. The robot is calibrated using fewer than 50 configurations and the calibration efficiency validated in 1000 configurations using either the laser tracker or the optical CMM. A telescopic ballbar is also used for validation. The results show that the optical CMM yields slightly better results, even when used with the simple triangular plate end-effector that was developed mainly for the laser tracker.

Kinematic calibration of a 3‐DOF planar parallel robot

Purpose – The purpose of this paper is to describe a calibration method developed to improve the absolute accuracy of a novel three degrees‐of‐freedom planar parallel robot. The robot is designed for the precise alignment of semiconductor wafers and, even though its complete workspace is slightly larger, the accuracy improvements are performed within a target workspace, in which the positions are on a disc of 170 mm in diameter and the orientations are in the range ±17°.Design/methodology/approach – The calibration method makes use of a single optimization model, based on the direct kinematic calibration approach, while the experimental data are collected from two sources. The first source is a measurement arm from FARO Technologies, and the second is a Mitutoyo coordinate measurement machine (CMM). The two sets of calibration results are compared.Findings – Simulation confirmed that the model proposed is not sensitive to measurement noise. An experimental validation on the CMM shows that the absolute acc...

Absolute accuracy analysis and improvement of a hybrid 6-DOF medical robot

– The purpose of this paper is to describe a calibration method developed to improve the accuracy of a six degrees-of-freedom medical robot. The proposed calibration approach aims to enhance the robot’s accuracy in a specific target workspace. A comparison of five observability indices is also done to choose the most appropriate calibration robot configurations. , – The calibration method is based on the forward kinematic approach, which uses a nonlinear optimization model. The used experimental data are 84 end-effector positions, which are measured using a laser tracker. The calibration configurations are chosen through an observability analysis, while the validation after calibration is carried out in 336 positions within the target workspace. , – Simulations allowed finding the most appropriate observability index for choosing the optimal calibration configurations. They also showed the ability of our calibration model to identify most of the considered robot’s parameters, despite measurement errors. Experimental tests confirmed the simulation findings and showed that the robot’s mean position error is reduced from 3.992 mm before calibration to 0.387 mm after, and the maximum error is reduced from 5.957 to 0.851 mm. , – This paper presents a calibration method which makes it possible to accurately identify the kinematic errors for a novel medical robot. In addition, this paper presents a comparison between the five observability indices proposed in the literature. The proposed method might be applied to any industrial or medical robot similar to the robot studied in this paper.

A comparative evaluation of three industrial robots using three reference measuring techniques

Purpose – The purpose of this paper is to present a technique for assessing and comparing the static and dynamic performance of three different models of small six-axis industrial robots using a Renishaw XL80 laser interferometer system, a FARO ION laser tracker and a Renishaw QC20-W telescoping ballbar. Design/methodology/approach – Specific test methods are proposed in this work, and each robot has been measured in a similar area of its working envelope. The laser interferometer measurement instrument is used to assess the static positioning performance along three linear and orthogonal paths. The laser tracker is used to assess the contouring performance at different tool center point (TCP) speeds along a triangular tool path, whereas the telescoping ballbar is used to assess the dynamic positioning performance for circular paths at different TCP speeds and trajectory radii. Findings – It is found that the tested robots behave differently, and that the static accuracy of these non-calibrated robots var...

Elasto-geometrical calibration of an industrial robot under multidirectional external loads using a laser tracker

This paper presents an elasto-geometrical calibration method for improving the position accuracy of an industrial robot (ABB IRB 1600). Geometric parameter errors and joint stiffness parameters are identified through measuring the position of the robots end-effector in several robot configurations using a laser tracker. Contrary to previous works, robots position errors are measured under a wide range of external forces and torques for each robot configuration. A 6-DOF cable-driven parallel robot is employed to automatically apply the desired load on the end-effector of the ABB robot. Before the experiment, an observability analysis is performed in order to improve the robustness of the calibration process with respect to measurement noise and unmodeled errors. Accordingly, an optimal set of robot configurations and external loads is selected for the calibration process. The measured position errors of the ABB robot for this selected set are used to identify the real robots elasto-geometrical parameters. Finally, the calibration efficiency is evaluated for a number of random combinations of robot configurations and external loads. The experimental results revealed that the proposed elasto-geometrical calibration approach is able to reduce the maximum position error to 0.960 mm, while a customary kinematic calibration can reduce the maximum position error only to 2.571 mm.

Metrological Evaluation of a Novel Medical Robot and Its Kinematic Calibration

The vessels are twisted in a longitudinal 3D space in the lower limbs of humans. Thus, it is difficult to perform an ultrasound scanning examination in this area. In this paper, a new medical parallel robot is introduced to effectively diagnose vessel disease in the lower limbs. The robots position repeatability and accuracy are evaluated. Furthermore, the robots accuracy is improved through a calibration process in which the kinematic parameters are identified through a simple identification approach.

Local and closed-loop calibration of an industrial serial robot using a new low-cost 3D measuring device

We propose an automated, closed-loop, and local calibration method for serial robots that uses a new, low-cost, 3D measuring device. The device consists of three Mitutoyo digital indicators, arranged in an orthogonal manner, and a mastering fixture based on kinematic coupling. The indicators communicate, via wireless connection, with a PC that controls the movements of the robot. To measure absolute Cartesian coordinates, the device is positioned incrementally over each of several 0.5-inch precision balls until all indicators are at zero, at which time the robot joint encoders are read. The balls are fixed with respect to the robots base. The precise relative positions of the centers of these balls must be known in advance. In this study, the measuring device is mounted on the flange of an ABB IRB 120 robot. Only three precision balls are used, spaced 300 mm apart, and the distances between these balls are measured with a Renishaw telescoping ballbar. The absolute accuracy of the robot was enhanced by minimizing its position errors, using the least squares method. The feasibility of the calibration approach was demonstrated through a simulation study. Finally, an experimental validation showed that our calibration method caused the maximum position error of the robot, inside a sphere of 400 mm in diameter, to be reduced to 0.491 mm.

Performances of observability indices for industrial robot calibration

This work presents a comparison of the five observability indices used for robot calibration. The comparison is realized in order to determine the most appropriate observability index, which allows for the best parameter identification of a calibrated robot, and therefore leading to the best improvement of the robot accuracy. In this study, the accuracy analysis is based on the robot end-effector errors, which are expressed in term of Euclidean errors. The parameter identification process is based on minimizing the residual of the position errors. The actual values of these positions are usually measured by an external measurement device and have measurement noise. The position residuals are calculated in all the calibration configurations, which are selected by using observability indices. An optimal set of configurations is the one reducing the impact of the measurement noise on the parameter identification efficacy. Our study is carried out for the calibration of four robots: two degrees of freedom (DOF) and 6-DOF serial robots, and 2-DOF and 3-DOF planar parallel robots. The comparison of the observability indices was achieved through a Monte Carlo simulation, using 100 different cases for each of the four robots considered. The position measurement noise was assumed to be within a range of ± 200 μm. Investigations led to conclude that there is a specific index that may be considered the best observability index for robot calibration. Finally, an experimental study has been applied to a LR Mate 200ic FANUC robot and confirms the simulated results.

Accuracy enhancement of industrial robots by on-line pose correction

This paper presents a novel, cost-effective dynamic pose correction (DPC) strategy to address the issues on the accuracy enhancement of industrial robots. This strategy, also known as visual-servoing, uses a photogrammetry based 6D measurement device to track the position and orientation of the robots end-effector in real-time. To realize this strategy, we first propose a root mean square (RMS) method to filter the noise from the pose measurements. The estimated pose from the sensor serves as a feedback for visual-servoing system. Next, a DPC controller is designed and integrated with a FANUC robot controller through FANUCs dynamic path modification (DPM) software package. As a result, the robot is guided to the desired pose in real-time and hence the positioning accuracy is enhanced. Extensive experimental tests of the proposed algorithm have been carried out. The experimental results demonstrate that the pose accuracy of the robot (a FANUC M-20iA) for stationary tasks has been improved to 0.050 mm and 0.050° for position and orientation respectively.

Use of a force-torque sensor for self-calibration of a 6-DOF medical robot

The aim of this paper is to improve the position accuracy of a six degree of freedom medical robot. The improvement in accuracy is achieved without the use of any external measurement device. Instead, this work presents a novel calibration approach based on using an embedded force-torque sensor to identify the robot’s kinematic parameters and thereby enhance the positioning accuracy. A simulation study demonstrated that our calibration approach is effective, whether or not any measurement noise is present: the position error is improved, inside the robot target workspace, from 12 mm to 0.320 mm, for the maximum values, and from 9 mm to 0.2771 mm, for the mean errors.