Matthew E. Kontz

Electronic Control of Pump Pressure for a Small Haptic Backhoe

Abstract The use of haptic interfaces to control mobile hydraulic machinery has several enhancing features over traditional human-machine interfaces comprised of joysticks/levers. This paper presents and analyzes schemes for controlling pump pressure designed for coordinated haptic control. Typical of many small backhoes and excavators, the hydraulic system used on the test-bed is comprised of a constant displacement pump and proportional directional control valves. In this type of system, a main pressure regulator is needed to supply the other closed-centre valves with pressure and to dump the additional flow generated by the pump to tank. An energy-saving solution has a load-sensing pressure regulator that maintains the system pressure to a preset margin above the highest active load pressure. Using these valves for haptic applications requires closed-loop control. Applying closed-loop control to these valves can excite instabilities in the valve assembly due to complex interactions and nonlinearities in the load-sensing pressure regulator and proportional valves. On this setup, the hydro-mechanical pressure regulator has been replaced with one that is electronically controlled, and a non-linear filter is utilized to decouple oscillations in port pressure from the pump pressure input signal. This filter does not slow down the build up of pump pressure. Experimental results with multiple degrees-of-freedom are presented.

Impedance Shaping for Improved Feel in Hydraulic Systems

Applying haptic control to mobile hydraulic equipment presents a practical yet challenging application. One criticism of newer electro-hydraulic system is a lack of “feel.” To a haptics researcher this sounds like a call for haptic feedback in the human-machine interface. However, for an operator the “feel” of the system likely has more to do with how the actual system responds to forces or higher work port pressures. At some point, the high pressures slow down the system or naturally redirect flow to lower pressure circuits in a hydro-mechanical system. How this is done plays a large part in the “feel” of the system. In this paper, a paradigm is presented that tries to merge these two concepts of “feel.” Instead of trying to make the system transparent, the goal is to make the system react to forces acting on the system then use haptic feedback to help alert the operator to these forces. This is done by shaping this impedance so that the system provides a response or “feel” that is closer to a typical excavator. A haptic interface is used to enhance the haptic feel. Performance is evaluated using data from human-in-the-loop testing.Copyright

Teleoperation of a Hydraulic Forklift from a Haptic Manipulator over the Internet

Abstract Remote operation of material handling equipment is of potential value when the proximity of the user is inconvenient or dangerous. Haptic feedback makes operation of this equipment more effective. The pervasive presence of the Internet makes it attractive to use it as a means of communication between the human operator and the remote device. This paper describes research underway to facilitate haptic feedback to the human operator in spite of the significant and variable delay of the Internet. An additional novelty of these experiments is the configuration of the remote device as a vehicle rather than a manipulator. The nature and role of the haptic interface, the material handling device and the wave variable treatment of the communication signal are summarized.

Flow Control for Coordinated Motion and Haptic Feedback

Abstract The use of haptic interfaces to control mobile hydraulic machinery has several enhancing features over traditional human-machine interfaces comprised of joysticks/levers. This paper presents pressure and flow control schemes designed for coordinated haptic control. Typical of many small backhoes and excavators, the hydraulic system used on the test-bed is comprised of constant displacement pumps and proportional directional control valves. In this type of system, a main pressure regulator is needed to supply the other closed-centre valves with pressure and dumps the additional flow from the pump to tank. Using these valves for haptic applications requires accurate closed-loop control. The focus of this paper is on the flow control scheme. Performance of the proportional directional control valves are compared with and without pressure compensation. A velocity feedback loop is present in order to improve the accuracy of the control system. A small input dead-zone function is proposed in order to prevent a limit cycle around zero velocity caused by the valve dead-band. Coordinated motion with multiple cylinders is demonstrated.

Kinematic analysis of backhoes/excavators for closed-loop coordinated control

Abstract Backhoes and excavators are an application for robotic technology. This can be done in two ways: autonomous excavation and coordinated motion control and/or haptic feedback. This analysis is motivated by haptic coordinated control of the bucket using position and velocity signals from a dissimilar master device. One issue that must be addressed is the effect of joint constraints imposed by the hydraulic cylinders. The result of this not being addressed is demonstrated. Two solutions are presented: a closed form graphically derived method based on the position of the buckets wrist and a more general numerical optimization approach that also penalizes bucket angle error.

Pressure Based Exogenous Force Estimation

Using pressure transducers to estimate the exogenous force acting on an end effector could be useful for a number of applications. These might include load estimation or diagnostics. In this case, it is motivated by haptic control of backhoes/excavators. Using pressure transducers is attractive from a cost point of view because they are relatively inexpensive and more importantly already present on many electro-hydraulic systems. Measuring the cylinder pressure and multiplying by the respective areas give the total force on each cylinder that is produced by the hydraulic pressure. Embedded in this force is cylinder and structural friction, gravitational forces, inertial/Coriolis/centrifugal forces and exogenous forces acting on the end effector. Particularly challenging is the friction inside the cylinders which is not only velocity dependent, but also pressure dependent. The friction increases as the pressure inside the cylinders push harder on the seal. This paper discusses how to remove the unwanted force components which result in an estimation of the exogenous force on the end effector. Experimental results are presented from a backhoe test-bed and the new method is compared with previous work related to this topic.Copyright

Improved Control of Open-Center Systems for Haptic Applications

The use of haptic interfaces to control mobile hydraulic machinery has several enhancing features over traditional human-machine-interfaces compromising of joysticks/levers. A prohibitive attribute of these systems is cost due to added complexity. One way to limit this cost is to use lower cost hydraulic components such as a constant displacement pumps and proportional directional control valves. In this type of system, a main pressure regulator is needed to supply the other closed-center valves with pressure and dump the additional flow from the pump to tank. An energy-saving solution has a load-sensing pressure regulator that maintains the system pressure a preset margin above the highest active load pressure. With the addition of pressure compensators on each proportional valve, these valves can provide steady-state flow proportional to an input voltage generated from a joystick. Using these valves for haptic applications requires closed-loop control. Applying closed-loop control to these valves can excite instabilities in the valve assembly due to complex interactions and nonlinearities in the load-sensing pressure regulator. This paper describes and analyzes these interactions. It also shows through experimental examples of how these characteristics limit closed-loop performance and destabilize the system. A possible valve modification is presented and closed-loop performance is compared using the original and modified system.Copyright