TECH TIP
ELECTRO-HYDRAULIC FEEDBACK CYLINDERS
One of the most basic fluid power components, the cylinder, has evolved into an almost endless array of configurations, sizes, and special designs in today’s marketplace. This versatility makes more innovative designs possible and also makes many applications that would not have been practical or possible a few years back a reality. The ever changing demands on today’s equipment have created the need for faster, more flexible, more accurate and more energy efficient linear positioning systems. The hydraulic industry has responded to meet these requirements with the development of the electro-hydraulic feedback cylinder and its associated closed-loop hydraulic systems.
WHAT IS AN ELECTRO-HYDRAULIC FEEDBACK CYLINDER?
Similar to standard hydraulic cylinders, electro-hydraulic cylinders are actuation devices that use pressurized hydraulic fluid to produce linear motion and force. Added to these cylinders is a feedback device, which senses the cylinder’s rod position. A feedback cylinder can provide greatly enhance velocity, force, and positional control in a closed-loop system.
POSITION SENSORS
A position transducer typically consists of two fundamental parts: one part remains fixed, in relationship to the cylinder body, while the other part moves with the cylinder rod whose position is being measured. Some transducers are intended to be mounted integrally to the cylinder rod, while others are designed to be mounted externally. There are advantages and disadvantages to each type of transducer:
Internally Mounted
If a transducer is configured for mounting inside a cylinder, a hole usually is drilled down the center of the piston rod, and the cylinder end cap is machined to accommodate the transducer. This method usually consumes the least amount of space, but makes the transducer less accessible than mounting it externally. The initial cost of gun-drilling the rod and the replacement cost can be the limiting factor in the usage of internally mounted transducers.
Externally Mounted
Mounting the transducer alongside the cylinder eliminates the time, effort, and expense of machining, but exposes the transducer to the environment. A protective enclosure can be used to shield the transducer from certain hostile conditions, but severe applications generally necessitate integrating the transducer into the relatively safe confines of the cylinder.
ACCURACY
Accuracy can vary from ± 0.125” to less than ± 0.0001” depending on the type of feedback device selected. The accuracy of the feedback device is only one of the controlling factors to consider when applying a feedback cylinder. Control valve response, electrical system accuracy and update time, length of distribution lines, the type of fluid being used, the size of the cylinder, and the weight / force of the application also must be considered.
HOW DO ELECTRO-HYDRAULIC FEEDBACK CYLINDERS WORK?
Linear Resistive Transducer / LRT
A linear resistive transducer (LRT) is basically a linear potentiometer used to detect the position of the rod. This can vary from as simple as an external cable connected to a rotary potentiometer to an internally mounted linear (flat) potentiometer contained within the cylinder rod. The distance of the pickup element along the distance (length) of the cylinder’s stroke varies the control voltage from a minimum or home position, to a maximum or fully extended position. An LRT type feedback is generally a lower accuracy device used when lower speeds and a lower cost are required.
Linear Displacement Transducer / LDT
In-cylinder linear displacement transducers (LDTs) have been used widely throughout the fluid power industry. A limitation to most in-cylinder LDTs is that the hydraulic cylinder piston rod must be bored through its center to accommodate certain elements of the LDT, usually the waveguide tube of a magnetostrictive transducer. The placement of a permanent magnet within piston provides a pickup element and the transducer, mounted through the rear of the cylinder, provides the varying distance feedback required. The machining and additional production steps associated with “gun drilling” the rod add substantial cost to the finished cylinder. And although a magnetostrictive LDT provides a higher level of accuracy than it’s little brother the LRT, it is considered to be in the middle of possible accuracies available.
Linear Variable-Differential Transformer / LVDT
The highest accuracy feedback device is a linear variable-differential transformer (LVDT). It is a non-contact transducer that converts linear displacement into an analog electrical output signal. LVDTs consist of primary and secondary transformer coils wound on non-magnetic cylindrical-coil forms. This coil assembly is then installed in the cylinder’s piston. The other major component is a ferromagnetic core moving inside the coil form. An alternating current in the primary winding creates an axial magnetic flux field that is concentrated in the core. This flux is coupled to the secondary windings through the core, inducing an output voltage in each secondary winding. The position of the core, in relationship to the secondary windings, produces an induced voltage; relating to position. An LVDT feedback device can be provided in an analog or digital output and generally is programmable for 0-10VDC or 4-20ma outputs.
APPLYING ELECTRO-HYDRAULIC FEEDBACK CYLINDERS
- Load control (pressure / force)
- Acceleration / deceleration control
- Velocity profiling
- Sequencing or synchronization of multiple cylinders
WHERE ARE ELECTRO-HYDRAULIC FEEDBACK CYLINDERS USED?
- Electro-hydraulic systems are used to control valves in a variety of pumps, compressors and steam turbines that are used in hydropower generation. They can also be combined with a pilot valve, power cylinder, and electronic feedback component designed to operate in mechanically-driven or generator-driven steam turbines.
- An electro hydraulic actuator may be used for linear or rotary applications in the machine tool field. Advances in transducers and the logic used to program them make electro-hydraulic systems highly accurate, efficient, and precise. This includes plastics, machine tools, pressed metal machinery, metal working and motion control systems.
- Electro-hydraulic actuators are used in the forestry and wood products fields, reducing waste and better utilizing the product, as well as reducing the impact on the environment.
- Electro-hydraulic control systems are used in steel and rolling mills for better control of material thickness and faster line speeds.
- The agriculture industry utilizes feedback systems to control tillage depth, seed control and a variety of components to reduce waste of both product and fuel.
- Off-road machines are utilizing electro-hydraulic cylinders in mining, construction, building services, shipbuilding, and offshore drilling applications.
Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.
Experts in fluid power, electrical and mechanical technologies.
