External Position Feedback for Hydraulic Cylinders

The classic linear position feedback solution for hydraulic cylinders is the rod-style magnetostrictive sensor installed from the back end of the cylinder. The cylinder rod is gun-drilled to accept the length of the sensor probe, and a target magnet is installed on the face of the piston. A hydraulic port on the end cap provides installation access to thread-in the pressure-rated sensor tube. This type of installation carries several advantages but also some potential disadvantages depending on the application.

EDITED Image_HF
Position Sensor Mounted Internally in a Hydraulic Cylinder
(Image credit: Cowan Dynamics)

Advantages of in-cylinder sensor mounting include:

  • Simplicity. The cylinder manufacturer “preps” the cylinder for the sensor and may install it as an extra-cost option.
  • Ruggedness. The sensor element is protected inside the cylinder. Only the electronics head is exposed to the rigors of the industrial environment.
  • Compactness. The sensor is contained inside the cylinder, so it does not add to the cross-sectional area occupied by the cylinder.
  • Direct Position Measurement. Because the target magnet is mounted on the piston, the sensor is directly monitoring the motion of the cylinder without any interposing linkages that might introduce some position error, especially in highly dynamic, high-acceleration / deceleration applications.

Potential disadvantages of in-cylinder sensor mounting may include:

  • Sensor Cost. Cylinder-mounted position sensors require a rugged, fully-sealed stainless-steel sensor probe to withstand the dynamic pressures inside a cylinder. This adds some manufacturing cost.
  • Cylinder Cost. The procedure of gun-drilling a cylinder rod consumes machine time and depletes tooling, adding manufacturing cost over a standard cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
  • Cylinder Delivery Time. Prepping a new cylinder for a sensor adds manufacturing time due to additional processing steps, some of which may be outsourced by the cylinder manufacturer, increasing overall shipping and handling time.
  • Overall Installed Length. Because the sensor electronics and cabling protrude from the back end of the cylinder, this adds to the overall length of the installed cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
  • Service Access. In case sensor repair is required, there must be sufficient clearance or access behind the cylinder to pull out the full length of the sensor probe.
  • Small Cylinder Bores / Rods. Some cylinder bores and rod diameters are too small to allow for gun-drilling a hole large enough to install the ~10.2 mm diameter sensor tube and allow for proper fluid flow around it. In tie rod cylinders, the distance between the rod nuts may be too small to allow the flange of the position sensor to fully seat against the O-ring. In these cases, a mounting boss must be provided to move the mounting position back past the tie rods. This adds cost as well as increases overall installed length.

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In cases where the advantages of in-cylinder mounting are outweighed or rendered impractical by some of the disadvantages, an externally-mounted position sensor can be considered. The list of advantages and disadvantages looks similar, but reversed.

FINAL Profile transducer - hydraulic frame piercing press_HF
Position Sensor Mounted Externally on Hydraulically-Actuated Equipment

Advantages of external sensor mounting include:

  • Sensor Cost. Externally-mounted magnetostrictive position sensors are typically made from an aluminum extrusion and die-cast end caps with gaskets, saving cost compared to all-stainless-steel welded and pressure-rated construction.
  • Cylinder Cost. The cylinder can be a standard type with no special machining work needed to accommodate installation of the sensor.
  • Cylinder Delivery Time. Since no additional machine work is needed, the cylinder manufacturer can deliver within their standard lead time for standard cylinders.
  • Overall Installed Length. Typically, the external sensor is mounted in parallel to the cylinder, so overall length is not increased.
  • Service Access. The externally-mounted sensor is easily accessible for service by simply unbolting its mounting brackets and pulling it off the equipment.

Disadvantages of external sensor mounting may include:

  • Complexity. The machine designer or end user must provide the means to mount the sensor brackets and the means to position a floating magnet target over the sensor housing. Alternatively, a captive sliding magnet target may be used with a length of operating rod and swivel attachment hardware.
  • Exposure to Damage. Unless guarded or installed in a protected area, an externally mounted position sensor is subject to being mechanically damaged.
  • Space Requirements. There must be enough empty space around the cylinder or on the machine to accommodate the sensor housing and operating envelope of the moving magnetic target.
  • Indirect Position Measurement. Any time a floating target magnet is mounted to a bracket, there is the potential for position error due to the bracket getting bent, flexing under acceleration / deceleration, mounting bolts loosening, etc. In the case of operating rods for captive sliding magnets, there will be some mechanical take-up in the swivel joints upon change of direction, adding to position hysteresis. There is also the potential for rod flexing under heavy acceleration / deceleration – particularly when the rod is acting under compression vs. tension. Take note of the amount of sliding friction of the captive magnet on the sensor rails; some sensor magnet designs offer high friction and stiff resistance to movement that can increase operating rod deflection and resultant position error.

In conclusion, be sure to consider all aspects of an application requiring cylinder position feedback and choose the approach that maximizes the most important advantages and eliminates or minimizes any potential disadvantages. It may be that an externally-mounted position sensor will solve some of the challenges being faced with implementing a traditional in-cylinder application.

For more information about internally- and externally-mounted cylinder position sensors, visit www.balluff.com.

Stop the Scrap

steelmanufacturingIn the current era of steel production, steel manufacturers employ a continuous process during the casting phase of production. The molten steel is solidified during this process by a continuous casting machine. The processes include feeding the liquid steel through a series of rollers to cool the material and slowly form into the next shape of production (e.g. slabs, round, etc…). In this process, the rollers are positioned using hydraulic cylinders that include linear position sensors as closed loop feedback devices. The outputs of these sensors are closely monitored and are critical to the steel quality. Because of the harsh environment of the continuous casting process, the life span of these sensors can be cut short. If the sensor’s output becomes unstable and begins to fail, the continuous casting process cannot simply stop quickly. The steel quality during this sensor failure mode will most likely become scrap, costing the steel mill tens of thousands of dollars.

btl7-t-redundant-seriesFor maximum reliability, a linear position sensor with 2 or 3 times redundancy can be utilized to provide position feedback of hydraulic systems. Such sensors employ 2 or 3 independently-operating sensing elements and processing circuitry . The extra feedback signals can be monitored through an automation system. When the outputs are compared, a failure could be identified early and the automation system could switch over to the reliable output maintaining the quality of steel. No scrap! During the next possible scheduled stoppage in the manufacturing process, the sensor could be replaced.

For more information on Balluff solutions for the metallurgy industry, start here.

For more information, visit www.balluff.com.

Quick field replacement for linear sensor electronics

Micropulse Transducers BTL 7 Rod-style with Rapid Replacement Module
Micropulse Transducers BTL 7
Rod-style with Rapid Replacement Module

When maintenance technicians replace linear position sensors (also known as probes or wands) from hydraulic cylinders, it can leave a terrible mess, waste hydraulic oils, and expose the individual to harmful hot fluids.  Also, the change out process can expose the hydraulic system to unwanted contaminants. After the sensor replacement has been completed, there can also be more work yet to do during the outage such as replacing fluids and air-bleeding cylinders.

Hydraulic linear position sensors with field-replaceable electronics/sensing elements eliminate these concerns.  Such sensors, so-called Rapid Replacement Module (RRM) sensors, allow the “guts” of the sensor to be replaced, while the stainless steel pressure tube remains in the cylinder.  The hydraulic seal is never compromised.  That means that during the replacement process there is no danger of oil spillage and no need for environmental containment procedures. There is also no need to bleed air from the hydraulic system and no danger of dirt or wood debris entering the open hydraulic port. Finally, there is no danger of repair personnel getting burned by hot oil.

The RRM is an option for Balluff’s BTL7 Z/B Rod Series used in applications for the lumber industry, plastic injection and blow molding, tire and rubber manufacturing, stamping presses, die casting, and all types of automated machinery where a continuous, absolute position signal is required.  Applications in industries such as Oil & Gas and Process Control are especially critical when it comes to downtime.  For these applications, this Rapid Replacement Module capability is especially advantageous.

You can learn more about linear position sensors with hazardous area approvals, by visting http://www.balluff.com/local/us/products/sensors/magnetostrictive-linear-position-sensors/

The video below shows a demonstration of the Rapid Replacement Module in action.