Zhaoheng Liu

• Ph.D.
• Professor (Full) at École de Technologie Supérieure

Robotic machining is becoming increasingly popular in modern manufacturing owing to its cost and flexibility advantages. Compared with serial robots, a hexapod-based industrial robotic work cell has a much smaller footprint for equivalent rigidity. Thus, a comprehensive understanding of the hexapod’s positioning performance under machining and non-...

This brief addresses the problem of fault-tolerant cooperative control (FTCC) for a group of car-like vehicles experiencing actuator faults. The main feature of this study is the transmission of vehicle’s state information via fading channels. It is challenging to compensate for actuator faults and maintain vehicle’s stability in the presence of un...

In this research, the effects of feed speed, ballbar radius, and pitch as control factors on the measurement outcomes of the helical path ballbar test were investigated in a robotic machining setup. The goal was to provide guidance on selecting setup parameters for the helical path ballbar test in machining operations, aiming to enhance the precisi...

The telescoping ballbar is widely utilized for diagnosing accuracy and identifying faults in machine tools and industrial robots. Currently, there are no established standards for determining the optimal feed rate for ballbar tests. This lack of clear guidelines results in time inefficiency in measurements and inconsistencies in dynamic measurement...

This study addresses the problem of cooperative control design for a group of car-like vehicles encountering fading channels, actuator faults, and external disturbances. It is presumed that certain followers lack direct access to the states of the leader via a directed graph. This arises challenges in maintaining synchronization and coordination wi...

The paper presents a research on the vibration and experimental modal analysis of a tonearm-cartridge system applied to the mechanical system of a turntable. The high fidelity of the music reproduction depends drastically on the dynamical properties of the tonearm-cartridge system, which has only been simplified and studied empirically. The recentl...

Dynamic errors from the robotic machining process can negatively impact the accuracy of manufactured parts. Currently, effectively reducing dynamic errors in robotic machining remains a challenge due to the incomplete understanding of the relationship between machining parameters and dynamic errors, especially for hexapod machining cells. To addres...

The telescoping ballbar is a versatile and easy-to-use error identification instrument for machine tools. The ballbar movement trajectory used while machining, is limited to acquiring a large circular arc for positioning accuracy evaluation. However, it cannot be used to calculate the expected machine tool error parameters. Herein, a helical path i...

This paper proposes a novel method for the modal analysis of slow-varying vibration structures based on vector autoregressive models. The basic idea of this method consists of using a short-time sliding window (STSW) to identify modal parameters for non-stationary vibrations. This method uses the recursive least-squares estimation for multivariable...

Dynamic errors from the robotic machining process can negatively impact the accuracy of manufactured parts. Currently, effectively reducing dynamic errors in robotic machining remains a challenge due to the incomplete understanding of the relations hip between machining parameters and dynamic errors, especially for hexapod machining cell. To addres...

New analytical and experimental methods for modal analysis of a flexible robot are presented. On the analytical side, a linearization of dynamic equation is employed to address the eigenvalue problem of the robot, considering the controlled parameters and the manipulator's configuration. The established dynamic model offers a modal analysis in the...

The paper presents a research on the vibration and experimental modal analysis of a tonearm-cartridge system applied to the mechanical system of a turntable. The high fidelity of the music reproduction depends drastically on the dynamical properties of the tonearm-cartridge system, which has only been simplified and studied empirically. The recentl...

This study proposes a distributed fault-tolerant formation control scheme for a team of car-like vehicles with actuator faults. The proposed method includes a fault diagnostic scheme using a high-order sliding mode observer and a reconfigurable controller based on the super-twisting algorithm. The results showed that by the proposed controller, eac...

This paper proposes a new algorithm for the online monitoring of slow varying modal parameters in vibrating structures subjected to unknown excitations. In this method, an Auto-Regressive (AR) model will be applied in a Short-Time Sliding Window (STSW) on measured signals. The model parameters will be determined and updated through the order and ti...

This paper presents the dynamic model and analytical modal analysis for robotic manipulators with rigid links and flexible joints. Dynamic equations of general robots with both prismatic and revolute joints are firstly developed using the Lagrangian formulation in minimal joint and motor coordinates. Next, linearized dynamic equations taking into a...

There are hundreds of parallel robot designs, of these, some are suitable for fast material transfer operations, others for precision positioning, or like Fanuc's hexapod, for withstanding heavy payloads. Unfortunately, few machining applications using hexapods exist today in the industry due to the complexity and scarcity of practical studies on t...

A new symbolic differentiation algorithm is proposed in this paper to automatically generate the inverse dynamics of flexible joint robots in symbolic form, and results obtained can be used in real-time applications. The proposed method with 𝒪(n) computational complexity is developed based on the recursive Newton-Euler algorithm, the chain rule of...

Most industrial serial robots use decentralized joint controllers assuming rigid body dynamics. To prevent exciting the flexible mode, gains are kept low, resulting in poor control bandwidth and disturbance rejection. In this paper, a two-stage flexible joint discrete controller is presented, in which the decentralized approach is extended with a s...

Digital manufacturing technology promotes the application of digital information to manufacturing processes and machine tools. However, the high entry cost and complexity of existing offerings limit the wide application of digital manufacturing in small and medium-sized enterprises (SMEs). For CNC machine tools, precision parameters can be acquired...

Tool wear monitoring is a major concern in securing surface quality of workpieces and performance of the machining process. Most existing tool wear monitoring techniques seem to have looked at places other than the tool itself for solution or simply inapplicable in the highly automated industries. With insights from these techniques, this paper thu...

It has been fully demonstrated that the regenerative chatter theory is applicable for predicting chatter-free milling parameters for computer numerical control machine tools, but researchers are still arguing whether it is effective for robotic milling processes. The main reason is that the robot’s modes greatly shift, depending on its varying dyna...

Joint flexibility introduces additional degrees of freedom and vibration modes, thus limiting the performance of the manipulator. To improve control bandwidth, this paper proposes an enhanced two-stage state feedback (SFB) controller, which combines two parts. The first is a SFB loop, which considers the motor position as a virtual control input fo...

The use of industrial robots in modern manufacturing technologies receives more attention from researchers and engineers in recent years. However, the robotic machining systems have generally low rigidity than the traditional CNC machines due to the presence of compliant transmission elements that cause poor performance. To overcome this drawback a...

This paper thus proposes a new method combining Empirical Mode Decomposition (EMD) and Singular Value Decomposition (SVD) for bearing fault diagnosis. The method includes three steps. First, the signal is decomposed using EMD. Second, the instantaneous amplitudes are computed for each component using the Hilbert Transform (HT). Lastly, the Singular...

The use of Force Limited Vibration (FLV) for reducing over-testing during vibration tests has proved to be one very simple and powerful means of mass reduction. For several reasons, the semi-empirical method of FLV has become by far the preferred approach for deriving the force limits which dictate the resulted amount of over-testing reduction. The...

Analytical modal analysis of flexible joint manipulators

Advanced monitoring requires automatic diagnosis of machines operating under variable conditions. In this paper, an intelligent method is introduced in order to enhance the classification and achieves a higher precision for the diagnosis of degradation of rolling bearings operating under condition variations. The method uses the coefficients of aut...

This paper presents a method for the extraction of modal parameters for identification of time-varying systems using Data-Driven Stochastic Subspace Identification (SSI-DATA). In practical applications of SSI-DATA, both the modal parameters and computational ones are mixed together in the identified results. In order to differentiate the structural...

Robotic belt grinding systems can be used not only to replace low efficiency, high pollution manual finishing operations but also to improve production rate and manufacturing flexibility, especially for grinding small batches of workpieces with complicated features. The contact wheel is made from soft material with significant elasticity and is ten...

In the fabrication and maintenance of hydroelectric turbines, the reconstruction by grinding of certain high-curvature surfaces such as junctions has not yet been robotized and must now be done manually. The problem is related to the very fast grinding wheel wear and the difficulty in controlling the position and orientation of the robot's grinder...

Austenitic stainless steel E309L is frequently used for welded repairs on 13 %Cr-4 %Ni martensitic stainless steel blade runners. However, very high tensile residual stresses are found in the upper layers of these repairs, thus reducing the fatigue resistance of the repaired area in comparison with the free of residual stress base material. The use...

This paper presents a method for extraction of real modal parameters for identification of time-varying systems using adaptable functional series vector time-dependent autoregressive (AFS-VTAR) model. In the practical application of AFS-VTAR model, both real modal parameters and computational ones may mix together in the identified results. In orde...

Smoothed Particle Hydrodynamics (SPH) is a meshless technique used for simulating large deformation processes. Due to its particle nature, SPH is capable of modeling metal cutting processes without any need of remeshing methods. This paper presents a hybrid SPH-Lagrangian model to study cutting forces in ball-end milling process. One flute cutting...

The Frequency Response Functions (FRF) are identified for a robot during normal grinding operation using three different identification approaches, namely the spectral method, the Autoregressive with exogenous excitation (ARX) model and the State Space (SS) model. A comparison is made to determine the performance of each approach. The results show...

Achieving a precise fault diagnosis for rolling bearings under variable conditions is a problematic challenge. In order to enhance the classification and achieves a higher precision for diagnosing rolling bearing degradation, a hybrid method is proposed. The method combines wavelet packet transform, singular value decomposition and support vector m...

Traditionally, grinding is used as a finishing process in the manufacturing chain. However, in recent years, it has been also used to machine hard or brittle materials. Another application, developed by Hydro-Québec’s research institute, IREQ, is robotic grinding for in situ maintenance of large hydropower equipment. This high material removal rate...

Amplitude demodulation is a key means of diagnosing bearing faults. The quality of demodulation determines the effectiveness of spectrum analysis in detecting defects. However, the quality of the demodulated signal depends on the frequency band selected for demodulation. In this paper, a new method combining the empirical wavelet transform (EWT) an...

A flexible compact robot was developed for grinding. Its varying dynamic properties during motion must be known. An autoregressive model is applied for modal analysis of the flexible robot first under impact conditions and then under operational excitations. The modes are classified with respect to modal power to show the energy distribution. Energ...

This paper investigates vibration of a moving flexible robot through modal analysis and by constructing vibration spectra of operational signals. A vector autoregressive model combined with a sliding window technique is used for signal processing in order to take into account system nonstationarity. Modal decomposition is conducted on the state mat...

The dynamics of a 6 DOF lightweight and compact manipulator is modeled and simulated in this work. Both joint and link flexibilities are considered in order to determine modal properties of the robotic system. The numerical results are then compared with experimental measurements of predefined trajectories through a laser tracker. It is demonstrate...

Knowing the grain geometry in grinding wheels is an asset for better understanding the grinding processes. This study investigates the grain protrusion and rake angles of two self-dressing zirconia-alumina grinding wheels in a robotic grinding process. The topography of the wheel is measured using a confocal scanning laser microscope. An optical im...

In hydro turbine manufacturing and repair, grinding of certain high-curvature surfaces like fillet welds presently cannot be carried out by robots so hand grinding is necessary. During this process, the grinding wheel is moved back and forth over the workpiece and thus the wheel maintains its profile and better control of the material removal rate...

This paper presents a method for constructing spectrum in non-stationary vibration using time series vector autoregressive model. A modal classification criterion and a modal amplification factor are introduced based on the eigen-decomposition of the block data matrix. The classical spectrum computation can be therefore modified to amplify only the...

Robotic machining is emerging as a viable alternative for some conventional machining tasks. Nevertheless, a challenge to cutting performance remains: vibrational instability is reported to be more severe with robotic cutting due to robot compliance. In this paper, the regenerative vibrational instability of a robotic grinding operation is studied....

This paper studies the vibratory dynamics governing material removal performed by a compliant robot. The objective is to understand whether metal removal at a target rate or to a target depth of cut is possible given the unavoidable significant and sustained vibrations inherent to the process. Robotic grinding by the SCOMPI robot developed at Hydro...

This paper presents the experimental characterization and vibration control of a flexible robotic system. For this work, a test bench was built to characterize the harmonic drive (HD) and joint components, while control algorithms were designed and compared to minimize vibration. Encoder accuracy was critical since the difference in the measurement...

Grinding is traditionally categorized as a finishing process in manufacturing. However, more recently it has been used as a machining process as well. Temperature distribution is crucial for investigation of thermal softening effects in the material removal process and thermal damages on the surface of workpiece. One important aspect to be consider...

A flexible robotic grinding system has been used for in situ maintenance of large hydro turbine runners by Hydro-Quebec. Field trials for more than 20 years have proven the reliability and efficiency of the technology for hydropower equipment maintenance and repair. This portable robot named SCOMPI, is developed by IREQ, Research Institute of Hydro...

Grinding processes performed with flexible robotic tool holders are very unlike conventional types of grinding because of low stiffness of the robot's structure. A special flexible robotic grinding process is used for in situ maintenance of large hydroelectric equipment for bulk material removal over large areas rather than as a finishing step, as...

This paper proposes a comparison study of two vehicle stability control methods by direct yaw-moment control (DYC): a PID and a sliding controller. For the purpose of this study, control systems are based solely on vehicle side-slip angle state feedback and the lateral dynamics of the 2 DOF vehicle model are used to establish the desired response....

A nonlinear dynamic system of cylindrical transverse grinding process is studied in this paper. The system consists of a grinding wheel and a workpiece, which are connected to the base by spring-damper elements, interacting with nonlinear normal forces. This two DOF model includes two time delays originated from the regenerative effects of the work...

This paper presents a study on the dynamic behavior of a flexible robot performing a grinding process. The ultimate goal is to understand whether regenerative chatter is the source of divergent vibrations observed when machining with a compliant robot. An important nonlinear characteristic of the dynamic response of the system is found and is inclu...

Portable robotic systems are nowadays used as an effective and profitable solution for the automation of maintenance tasks on large hydroelectric equipments. Most utilities perform in-situ inspections and interventions to address issues such as cavitation and cracking. These repair tasks involve automated processes such as robotic welding and grind...

A 6 DOF robot dedicated to repair hydroelectric equipments is analyzed in this study. A generic approach is proposed to conduct dynamic simulation using MD Adams software and to identify modal information at the system level for any given configuration of the robot. Four out of six links of the robotic system are modeled by flexible bodies, pre-pro...

Identifying the dynamics and parameters for an articulated heavy vehicle/driver (tractor semitrailer/driver) system is presented in this paper. The dynamic behaviour of the vehicle system is studied by setting up a closed-loop control model of a vehicle/driver system that included a three degree of freedom (3-DOF) tractor semitrailer lateral dynami...

Robot manipulators play an important role in industrial automation. Various aspects of robotic systems were subject of intensive investigations in the past, but the vibration problems in robotic machining processes have been rarely treated in the available literature. In this paper we present dynamic modeling of an ongoing research to study chatter...

Torsional stiffness or rigidity is a crucial characteristic in the design of transmission devices, including harmonic drives (HDs). Among the various design aspects constituting a reduction mechanism in robotic systems, torsional stiffness is an important factor for positioning accuracy and control issues. One of the major advantages of HDs is thei...

This article presents an algorithm for characterizing and detecting geometric faults in a wooden board. It is developed for integration into a quality control system in factories. The industrial environment where this system will be implemented is characterized by high levels of variation stemming from measurement noise and operating conditions.Geo...

A general simulation model is developed to study the dynamic process of roll plate bending using finite element method (FEM). In this work, the continuous three-roll pyramidal bending configuration with conical rolls is used to bend a thick plate into a conical tabular shape. We present the kinematic relationship existing between rolls and workpiec...

A two‐dimensional model with eight Degrees Of Freedom (DOF) is developed to simulate and animate the response of a vehicle to different road, traction, braking and wind conditions. An algorithm is developed to compute the normal force acting on each tyre depending on its position with the road, allowing multiple contact regions along the tyre circu...

This paper investigates the global stability behavior present near a bifurcation point of a nonlinear road vehicle system. The nonlinear behavior of the system is determined by reducing its dimensions according to the center manifold theory applied to a nongeneric case. A generalized Hopf bifurcation is analyzed by unfolding the limit cycle mean am...

This paper investigates the global stability behavior present near a bifurcation point of a nonlinear road vehicle system. The nonlinear behavior of the system is determined by reducing its dimensions according to the center manifold theory applied to a nongeneric case. A generalized Hopf bifurcation is analyzed by unfolding the limit cycle mean am...

In this paper, a vehicle/driver close-loop system is studied in order to characterise the inherent model parameters of an optimal human controller for a regulation task (e.g. stabilisation after a wind gust) in articulated vehicle motions. The tractor-semitrailer vehicle model consists of two articulated rigid bodies moving on a horizontal plane wi...

In this paper, we investigate the stability properties of a cylindrical grinding process. The dynamical model of the process includes two inherent delayed forcing terms, one from workpiece regeneration and the other from grinding wheel regeneration. The prediction of chatter onset is carried out by computing the spectrum of the doubly delayed diffe...

The multiobjective ant colony system (ACS) meta-heuristic has been developed to provide solutions for the reliability optimization problem of series-parallel systems. This type of problems involves selection of components with multiple choices and redundancy levels that produce maximum benefits, and is subject to the cost and weight constraints at...

Best student paper award

The Ant Colony System is well suited to reliability allocation problem (RAP) of gear train system, and offers distinct advantages compared to alternative optimization methods. While the redundancy apportionment problem does indeed exist in many forms, it generally involves the selection of components and redundancy levels that produce maximum benef...

A new dynamic model for interconnected rigid bodies is proposed here. The model formulation makes it possible to treat any physical system with finite number of degrees of freedom in a unified framework. This new model is a non minimal realization of the system dynamics since it contains more state variables than is needed. A useful discussion show...

In this paper, linear stability and chaotic motion of a time-delayed nonlinear vehicle system are studied. The stability is determined by computing the spectrum associated with a system of linear retarded functional differential equations, which reveals that a loss of stability occurs following a Hopf bifurcation. Beyond the critical value for line...

We present a novel point-based rendering approach based on object-space point interpolation. We introduce the concept of a transformation-invariant covariance matrix of a set of points to efficiently determine splat sizes in a multiresolution hierarchy. ...