Industrial demand for high precision manipulators has lead to heavy, stiff, and therefore, expensive, inefficient, and potentially dangerous serial arm manipulators. The move towards lighter, and therefore, flexible manipulators has not matured, although there have been advancements in data processing capabilities, sensor technology, and control theory in the past couple of decades that could potentially reduce the problems with flexible serial manipulators. Most research in the past has focused on single link manipulators and planar robot arms and less research has been done on spatial multi-link robots. This work presents the derivation for a low-order model for spatial multi-link serial arms. Due to its low number of degrees of freedom this model can be used in real-time systems for control and estimation. The model is then verified by comparing its performance to results from commercial finite element software. Additional tests are performed against a flexible robot arm testbed.

A significant need has been identified for an improved device to assist in transferring mobility limited patients, particularly those who are heavier or bariatric. Typical transfers include moving between a bed, wheelchair, chair/couch, toileting chair or toilet, car, or the floor. Currently, clinicians suffer more disabling workplace injuries than construction workers or firefighters, many of which are attributable to moving patients. A new, cost effective, hydraulically actuated prototype patient transfer assist device has been developed and fabricated; hydraulic actuation has advantages in terms of force density over electrical actuators that are typically used at this power scale. More generally, development of improved methods for control of machines that work collaboratively with humans to maneuver heavy payloads, sharing a task and a workspace, are developed. With this improved control, restrictions requiring two or more caregivers to move a patient can be removed, providing additional savings. A simple, intuitive caregiver interface has been implemented, which provides coordinated rate control using a force sensing handle mounted near the patient for the operator input. The caregiver can interact directly with the patient, while simultaneously controlling the lift device. External interaction forces are managed to keep them in a safe range, in addition to smoothly controlling motion. An impedance control uses redundant sensing of obstacles, with feedback of both external interaction forces and proximity. Operator experiments indicate improvements over current market lifts in terms of operator ratings and speed of completion with reduced forces in case of a collision.

This research aims to improve patient transfers by developing a new type of advanced robotic assist device. It has multiple actuated degrees of freedom and a powered steerable base to maximize maneuverability around obstacles. An intuitive interface and control strategy allows the caregiver to simply push on the machine in the direction of desired patient motion. The control integrates measurements of both force and proximity to mitigate any potential large collision forces and provides operators information about obstacles with a form of haptic feedback. Electro-hydraulic pump controlled actuation provides high force density for the actuation. Nineteen participants performed tests to compare transfer operations (transferring a 250-lb mannequin between a wheelchair, chair, bed, and floor) and interaction control of a prototype device with a commercially available patient lift. The testing included a time study of the transfer operations and subjective rating of device performance. The results show that operators perform transfer tasks significantly faster and rate performance higher using the prototype patient transfer assist device than with a current market patient lift. With further development, features of the new patient lift can help facilitate patient transfers that are safer, easier, and more efficient for caregivers.

For several decades, researchers have known that flexible serial robot arms have several benefits over rigid arms, such as lower costs, increased throughput, and improved safety. However, industry has been reluctant to move towards using flexible robots. There are numerous modeling techniques and controller approaches for flexible robots that have never been validated on hardware. There have been several single link or planar multi-link testbeds created over the years, but only a handful of multi-link spatial serial flexible arm testbeds. This paper presents the development of a 2-link flexible robot testbed. The robot arm is actuated by 3 servo motors and is movable in 3D space. The robot has two accelerometers and three encoders as the sensors. The arm is validated by two sets of experiments: frequency measurements at static positions, and motions along two trajectories.

Finding natural frequencies and mode shapes for flexible structures can be a challenging problem. Although well-known approaches exist for single flexible links, the problem becomes increasingly more complex when dealing with multiple links. Spatial configurations add an additional layer of difficulty. This work presents a systematic method for finding the natural frequencies and mode-shapes for n-link serial structures using a transfer matrix approach. The method is validated by finite element analysis and experiments.

This paper presents theoretical guarantees for the stability and performance for the singular value decomposition (SVD) system. The SVD System uses the SVD to reduce the dimension of the control inputs within a feedback loop. This is used to meet the rank 1 input constraint imposed by the row-column structure. The row-column structure reduces the number of inputs required to control mn subsystems to m + n. The SVD System provides scalable performance for systems coupled using the row-column structure. The results of this paper are the first analytical results for the SVD System when applied to linear subsystems of arbitrary order. The conclusion of all of this is that using the SVD System allows you to reduce the number of inputs while maintaining as close to the same performance as possible.

Design issues, dynamic modeling, trajectory planning, and feedback control problems are presented for robot manipulators having components with mechanical flexibility, either concentrated at the joints or distributed along the links. The chapter is divided accordingly into two main parts. Similarities or differences between the two types of flexibility are pointed out wherever appropriate.For robots with flexible joints, the dynamic model is derived in detail by following a Lagrangian approach and possible simplified versions are discussed. The problem of computing the nominal torques that produce a desired robot motion is then solved. Regulation and trajectory tracking tasks are addressed by means of linear and nonlinear feedback control designs.For robots with flexible links, relevant factors that lead to the consideration of distributed flexibility are analyzed. Dynamic models are presented, based on the treatment of flexibility through lumped elements, transfer matrices, or assumed modes. Several specific issues are then highlighted, including the selection of sensors, the model order used for control design, and the generation of effective commands that reduce or eliminate residual vibrations in rest-to-rest maneuvers. Feedback control alternatives are finally discussed.In each of the two parts of this chapter, a section is devoted to the illustration of the original references and to further readings on the subject.

Pneumatic actuators are frequently selected for use in machines intended for human interaction because of their clean operation and natural compliance. However, the compliance, coupled with friction, can also make motion control difficult, leading to the use of more aggressive controllers, such as high-gain PID or sliding mode control, which result in stiff closed-loop system behavior. Model-based options are needed to obtain behavior that provides a better trade-off of compliance and accurate position control. In particular, Model Predictive Control (MPC) is suggested; through the use of constrained optimal control, it offers a framework for minimizing tracking error while enforcing force constraints that ensure low impedance behavior.This paper assesses the suitability of controllers for pneumatic systems to positioning applications in which human machine interaction is anticipated. MPC is compared against commonly-used alternatives for such scenarios: sliding mode, PID, and impedance control. Results are shown in simulation, and use spectral analysis of the impedance and closed loop tracking to characterize the balance of compliance and accuracy for each of the controllers.

This paper presents a novel user interface (UI) for coordinated rate control (CRC) of an excavator end effector using traditional hardware. Coordinated control of an excavator end effector alleviates the cognitive load created by nonlinear arm dynamics on the excavator operator, allowing the operator to perform tasks more quickly and with fewer errors. A human subject experiment demonstrates the feasibility of excavator CRC using the traditional twin joystick setup, and compares operator performance between a CRC UI and traditional excavator UI. Performance of the CRC UI was statically equivalent to the performance of the traditional UI. When asked to self-report UI preference: 26 participants stated they preferred the CRC UI, 6 preferred the traditional UI, and 14 had no preference. Although the current iteration of the CRC UI offered no measurable performance improvements, a remapping of the CRC joystick inputs to the end effector motion could make the CRC UI more intuitive, lead to better performance metrics, and make hydraulic excavators safer, more efficient, and easier to use.

Systems comprised of many subsystems can benefit from reducing the number of inputs. Pin array shape displays are one example where this is particularly true. Many pin arrays use a row-column structure, as found on LCD screens, to reduce the number of inputs. However, reducing inputs in this manner makes simultaneous control of the pins challenging. Using semi-nonnegative matrix factorization (SNMF) to reduce the rank of the inputs solves this problem, but there has not yet been a method to analyze the stability of the resulting system. Additionally, the application of the SNMF System to pin arrays, particularly those using fluid power, presents many practical challenges. This paper proves that the SNMF rank-one approximation is strictly-output passive, leading to approaches to analyze the stability of the SNMF System. We also address practical challenges by showing that the SNMF System improves performance over the previous approach, line scanning, even in the presence of power limitations and by examining the effects of row and column dynamics and proposing approaches to limit these negative effects.

A significant need has been identified for an improved device for transferring mobility limited people, for example, from a bed to a wheelchair, a wheelchair to a toilet, a floor to a wheelchair, or a wheelchair into a car. A new prototype concept patient transfer assist device has been developed, using electro-hydraulic pump controlled hydraulic actuation. With such a powerful device working in a relatively delicate and unstructured environment with both the patient and caregiver in its workspace, a form of interaction control to manage any external interaction forces is necessary. This paper presents a form of interaction control using redundant sensing of obstacles, using both proximity sensing from an ultrasonic sensor and external force sensing from a load cell. The control strategy is based on impedance control with force feedback. Experiments on one degree of freedom of the patient transfer device show that the control strategy can significantly reduce external interaction forces, as long as one or both of the sensors detects the obstacle. Further investigations will incorporate the redundant interaction control into multiple degrees of freedom of the patient transfer assist device system, and human operator experiments will be performed.

This brief presents theoretical guarantees for stability and performance for the singular value decomposition (SVD) system with subsystems that are linear and first order. The SVD system reduces the dimension of the control input. It is used to meet the rank-one input constraint imposed by the row-column structure. The row-column structure reduces the number of inputs required to control mn subsystems to m + n. Although the subsystems are linear and first order, they can be dynamically coupled and are coupled nonlinearly by the SVD of the control input. Thus, the entire system is of order mn and nonlinear. Lyapunov stability and performance analysis demonstrates the effect of the SVD dimension reduction through comparisons to a system with full-rank inputs. The analysis also provides convenient methods for control design. Simulation examples demonstrate the use of the SVD system, theoretical results, and the SVD system's robustness with respect to noise and nonlinearities.

This paper presents a new hand controller interface that addresses practical challenges to implementing coordinated position control (CPC) for an excavator arm. The new interface uses a hybrid control scheme with closed-loop CPC of the excavator arm and open-loop flow control of the swing. CPC is achieved using a joystick that is kinematically similar to the excavator arm. The kinematically similar joystick motion is planar. Thus, it can be mounted vertically, matching the excavator arm, or horizontally, which has many advantages, such as reduced operator fatigue. The new interface is compared to a conventional interface in a human subject experiment using a dynamic excavator simulator. The results demonstrate similar improvements in spoil removed and fuel efficiency as previous CPC approaches while being more practically designed. Although the design is for an excavator, the concepts presented can apply to a range of hydraulic manipulators.

A new, advanced patient transfer device is being developed for moving mobility limited patients, for example, from a wheelchair to a bed or a floor into a chair. Current market patient lift devices are antiquated and insufficient for customer needs, with only one actuated degree of freedom. The high power to size ratio of hydraulic actuation makes it suitable for moving larger, heavier patients.We have developed a prototype pump-controlled hydraulically actuated patient transfer device that is more maneuverable and agile, using multiple actuated degrees of freedom. We are also working toward developing a more intuitive and safe caretaker interface and control strategy. We have performed an extensive needs assessment; these are a few associated key design requirements relevant to this presented work. A compact package is needed for ease of maneuvering the patient around obstacles in an uncertain environment. The device is capable of producing large forces, so it is desirable for the controller to minimize any unintentional large external contact forces, or provide force feedback. In this system, the caretaker and device work together to maneuver a complex payload, a human body; the operator's mental workload must be minimized. With humans in the device workspace, safety and stability are necessary, including environment interactions.This new application presents several challenges related to the hydraulic control. First, we are using a separate bidirectional fixed displacement pump with a reversible brushed DC motor for each degree of freedom. The low level control involves obtaining desirable response from each motor-pump-actuator system, while compensating for significant static friction. At a higher level, we are testing several approaches to attain the desired intuitive control and desired dynamics, and minimize the operator workload. First, we are developing a coordinated control using a force input, such that the operator simply pushes on the device in the desired direction of motion. We are testing several different controllers to attain the desired dynamics. This paper presents the design of the machine, the proposed control structures as applied to this application, operator interface options, some preliminary results, and future work.

The bulk modulus of pure hydraulic oil and its dependency on pressure and temperature has been studied extensively over the past years. A comprehensive review of some of the more common definitions of fluid bulk modulus is conducted and comments on some of the confusion over definitions and different methods of measuring the fluid bulk modulus are presented in this thesis. In practice, it is known that there is always some form of air present in hydraulic systems which substantially decreases the oil bulk modulus. The term effective bulk modulus is used to account for the effect of air and/or the compliance of transmission lines. A summary from the literature of the effective bulk modulus models for a mixture of hydraulic oil and air is presented. Based on the reviews, these models are divided into two groups: “compression only” models and “compression and dissolve” models. A comparison of various “compression only” models, where only the volumetric compression of air is considered, shows that the models do not match each other at the same operating conditions. The reason for this difference is explained and after applying some modifications to the models, a theoretical model of the “compression only” model is suggested. The “compression and dissolve” models, obtained from the literature review, include the effects of the volumetric compression of air and the volumetric reduction of air due to the dissolving of air into the oil. It is found that the existing “compression and dissolve” models have a discontinuity at some critical pressure and as a result do not match the experimental results very well. The reason for the discontinuity is discussed and a new “compression and dissolve” model is proposed by introducing some new parameters to the theoretical model. A new critical pressure (PC) definition is presented based on the saturation limit of oil. In the new definition, the air stops dissolving into the oil after this critical pressure is reached and any remaining air will be only compressed afterwards. An experimental procedure is successfully designed and fabricated to verify the new proposed models and to reproduce the operating conditions that underlie the model assumptions. The pressure range is 0 to 6.9 MPa and the temperature is kept constant at °C. Air is added to the oil in different forms and the amount of air varies from about 1 to 5%. Experiments are conducted in three different phases: baseline (without adding air to the oil), lumped air (air added as a pocket of air to the top of the oil column) and distributed air (air is distributed in the oil in the form of small air bubbles). The effect of different forms and amounts of air and various volume change rates are investigated experimentally and it is shown that the value of PC is strongly affected by the volume change rate, the form, and the amount of air. It is also shown that the new model can represent the experimental data with great accuracy. The new proposed “compression and dissolve” model can be considered as a general model of the effective bulk modulus of a mixture of oil and air where it is applicable to any form of a mixture of hydraulic oil and air. However, it is required to identify model parameters using experimental measurements. A method of identifying the model parameters is introduced and the modeling errors are evaluated. An attempt is also made to verify independently the value of some of the parameters. The new proposed model can be used in analyzing pressure variations and improving the accuracy of the simulations in low pressure hydraulic systems. The new method of modeling the air dissolving into the oil can be also used to improve the modeling of cavitation phenomena in hydraulic systems.

Previous research has shown that operator performance of industrial machines is superior with position control rather than rate control, except for large-workspace and dynamically slow manipulators, which includes most hydraulic machinery. This paper describes an investigation to determine why position control leads to better performance than rate control except for with dynamically slow manipulators, in an effort to increase operator performance of mobile hydraulic equipment. It examines why dynamically slow systems are an exception to the general rule, and proposes a human-machine interface (HMI), called command feedback, that leads to position control having superior performance, even in these exceptional situations.Thirty participants performed five tasks six times using one of five HMIs. A rate and a position controller were used to manipulate a dynamically fast system and a dynamically slow system that was designed to mimic the motion of hydraulic cylinders. A new HMI that provided real-time position feedback to the operator of his/her commanded position was applied to the position controller for the dynamically slow system. Task performance was measured and comparisons were made between position and rate control. The addition of the real-time position feedback to the dynamically slow system resulted in nearly identical performance with both controllers. From these results we conclude that position control is more intuitive for fast systems when human operators do not have the physical capability to control the velocity well with rate control, and that the intuitiveness of rate control for dynamically slow systems results from the lack of position feedback because of the machine's speed of response. Command feedback can be used to elevate operator performance of hydraulic machinery.

This paper presents a new systematic algorithm to symbolically derive the full nonlinear dynamic equations of motion of multi-link flexible manipulators. Lagrange's-Assumed modes method is the basis of the new algorithm and adapted in a way suitable for symbolic manipulation by digital computers. It is aaplied to model a two-link flexible arm via a commercially available symbolic manipulation program. The advantages of the algorithm and simulation results are discussed.

Abstract The compact rescue robot (CRR), a quadruped pneumatically-actuated walking robot, seeks to use the benefits garnered from pneumatic power: force & power density, lightweight, inexpensive actuators, and inherent compliance. A simulation has been developed that models the dynamics of the robot and its interaction with the environment. However, development of an entirely new dynamic simulation specific to the system is not practical. Instead, the simulation combines a MATLAB/Simulink actuator simulation with a readily available C++ dynamics engine. The actuator simulation places particular emphasis on accuracy near the neutral position (the region most active during closed loop control), modeling this regime with a higher level of detail than observed in past literature. Because the multi-platform approach results in additional incurred challenges due to the transfer of data between the platforms, it is valuable to analyze the necessity of detail in the actuator model. The goal of this analysis is to find a balance of realistic behavior, model integrity, and practicality.

Pneumatic actuators possess several attractive qualities: high power and force density, potentially adaptable compliance, and clean, safe, and low cost actuation. However, control of pneumatic actuators has proven difficult, limited by inherent compliance of the actuator, nonlinear and discontinuous third order dynamics, and friction. Stiction and compliance lead to a sandwiched nonlinearity that causes stick-slip and can cause significant tracking error and even instability. A broadly applicable method of friction compensation is addition of a feedforward term updated from a friction estimate at each time step. Since pneumatic dynamics are slow, achievable compensation can be insufficient. In this work, friction is estimated over a prediction horizon and then input into a model-based predictive controller as an offset term, so that compensation is planned and optimal over the prediction horizon. The controller is tested in simulation. Results are compared to control using instantaneous compensation and are characterized based on performance.