This invention relates to hydraulic actuators for use in, e.g., robotic applications, and more particularly relates to force control of hydraulic actuators.
An actuator is generally defined as a device or mechanism that converts some form of energy into mechanical force or torque and linear or rotary velocity. A hydraulic actuator typically is connected to a high pressure fluid source and a flow control valve, e.g., a spool valve. Application of a small signal to the valve deflects the valve, allowing the fluid to flow, e.g., into one or more chambers driving a mechanical mechanism such as a piston provided in one or more of the chambers. With this action, the hydraulic actuator converts fluid flow into mechanical piston velocity, and provides the ability to control this velocity and corresponding mechanical position.
Hydraulic actuators are particularly well-suited for velocity and position control of robots and heavy equipment. Hydraulic systems also are generally characterized by the highest power density of modern controllable actuation systems because they are often operated at a pressure of as much as 3000 psi or greater. Hydraulic systems can also support large loads indefinitely while consuming minimal power. Given these attributes, hydraulic actuation systems are frequently the optimum choice for high force, high power density motion control applications such as automobile steering systems, airplane control surfaces, and heavy equipment operations employing, e.g., construction machinery.
While hydraulic systems are in many respects optimal for velocity and position control, a number of inherent hydraulic system limitations constrain their applicability for force control. For most applications, force control requires an ability to sense and correspondingly control the forces of interaction between an actuator and the actuation environment. But in hydraulic systems, a measurement of the primary system variable, hydraulic pressure, does not fully enable such. Specifically, the pressure in a hydraulic chamber, e.g., a piston chamber, is not in general a good representation of the force at the actuator output. Hydraulic systems are in general very sensitive to contamination, such as foreign particles, in the hydraulic fluid. In order to limit such contamination, it is preferable to employ tight fluidic seals at the hydraulic piston and cylinder. Tight seals are found, however, to typically produce substantial stiction and coulomb friction during sliding, and to require a very high breakaway force, all of which contribute to force noise at the hydraulic actuator output and thereby limit the ability to accurately estimate output force. Dynamically, a range of factors, including non-linear flow characteristics, can be very difficult to control.
There have been attempts to reduce the sliding friction and stiction characteristic of tight hydraulic seals by, e.g., reducing the piston seal tolerance. In one example alternative, two or more sets of loose seals are employed, the first seal allowing leakage from the supply fluid pressure chamber and the second and following seals scavenging the leakage. Although this configuration can improve sliding characteristics, it is not cost effective for most applications and in practice can be very prone to leaks. As a result, for most applications only tight hydraulic seals can be employed.
Given this fundamental difficulty in estimating the output force of a hydraulic actuator as function of hydraulic pressure, hydraulic actuators have been largely limited to velocity and position control applications. Implementation of force control for robotic and other applications in a manner that exploits the high power density of hydraulic actuation has heretofore not been fully practical.
The invention provides the ability to effectively and precisely implement closed-loop force control of a hydraulic actuator, provided in accordance with the invention as a hydro-elastic actuator. The hydro-elastic actuator of the invention includes a hydraulic actuator, having a connection to hydraulic fluid and including a mechanical displacement member positioned to be mechanically displaced by fluid flow at the actuator. A valve is connected at the hydraulic actuator connection and has a port for input and output of fluid to and from the valve. At least one elastic element is provided in series with the mechanical displacement member of the hydraulic actuator and is positioned to deliver, to a load, force generated by the hydraulic actuator. A transducer is positioned to measure a physical parameter indicative of the force delivered by the elastic element and to generate a corresponding transducer signal. A force controller is connected between the transducer and the valve to control the valve, based on the transducer signal, for correspondingly actuating the hydraulic actuator and deflecting the elastic element.
The hydro-elastic actuator of the invention can be configured such that the force controller is connected to accept an input indicative of a desired actuator output force to be delivered to the load. Here the force controller is connected between the transducer and the valve to control the valve based on the transducer signal and the input, for correspondingly actuating the hydraulic actuator by an amount that delivers to the load the a desired actuator output force.
The hydro-elastic actuator of the invention provides the ability to make a high-fidelity measurement of the output force of a hydraulic system without measuring pressure or flow characteristics of the hydraulic system. The feedback control loop enables precise hydraulic system force control and control stability to a level not previously achievable without complicated control schemes to accommodate hydraulic characteristics. The high power and high force generation capabilities of the hydraulic actuator are preserved while providing shock tolerance and low system output impedance.
The hydro-elastic actuator of the invention is well-suited for an extremely broad range of applications, and is particularly effective at addressing high-force, high-power density applications. Robotics applications and heavy equipment operations, such as robotic fire fighting and earth moving, as well as telerobotic and haptic systems, are particularly well-addressed. Further, the important and growing class of biomimetic robots, and particularly dynamically-stable legged robots, which primarily rely on force control-based locomotion algorithms, are enabled by the invention to take on mass and scale not previously attainable.
In accordance with the invention, the hydraulic actuator can be provided as a hydraulic actuation chamber in which the mechanical displacement member is disposed with respect to the fluid connection. Here the fluid connection preferably consists of a fluid inlet and a fluid outlet of the chamber. This enables control of displacement of the displacement member by fluid flow into and out of the chamber. The valve can be connected to the fluid inlet and fluid outlet, and preferably is provided as a flow control valve. Whatever connection is employed between the valve and the fluid inlet and outlet, it preferably is dimensionally fixed. The valve port can include a connection for receiving fluid pumped by a fluidic pump.
In embodiments of the invention, the valve control signal is based on proportional or proportional-integral control of actuator output force. The valve control signal is in one embodiment an electrical current. This electrical control current is directed to the valve and is indicative of a controlled fluid flow to be produced through the valve. The electrical control current can be indicative of a controlled bi-state valve operation between a state of zero fluid flow and a state of maximum fluid flow through the valve. The force controller can further be connected to produce a fluid source control signal directed to a fluid source connected to the valve port. Here the fluid source control signal can be indicative of a controlled pulsed delivery of fluid to the valve in synchrony with the bi-state valve operation.
In accordance with the invention, the hydraulic actuator, when provided as a chamber, can consist of a linear piston chamber having a linear piston and a piston push rod extending out of the chamber, a rotary piston chamber having a rotary vane and a rotary shaft extending out of the chamber, or other suitable chamber configuration. When employed, a piston can be double- or single-acting. Preferably either a substantially non-leaky seal is provided at a location where the piston push rod or shaft extends out of the chamber or alternatively, at least one leaky seal and at least one leakage scavenger seal can here be employed.
In embodiments of the invention, the elastic element can be provided as a linear or a nonlinear elastic element. The elastic element can be provided specifically as at least one spring disposed in series with the hydraulic actuator displacement member, or as a plurality of springs positioned to together result in an elasticity provided in series with the hydraulic actuator displacement member. One or more coupling elements can be provided in series with and between the elastic element and the hydraulic actuator displacement member, as well as in series with and between the elastic element and the load.
The transducer can be provided as a potentiometer, a strain gauge, or other suitable sensing configuration, e.g., as a magnetic position sensor or an optical position sensor. In one embodiment, the transducer signal is based on deflection of the elastic element.
The hydro-elastic actuator of the invention can be produced of lightweight, low-cost, easily manufactured components. The elastic element force feedback control of the system preserves the high power and high force or torque generation of the system while providing precise force control and good force control stability. These characteristics are ideal for robotic and other mechanistic systems that interact with their environment. Other applications, features, and advantages of the invention will be apparent from the following description and associated drawings, and from the claims.