When and Where to Use Continuous Cylinder Position Sensing

The role of smart cylinders — hydraulic or pneumatic cylinders with integrated position detection capability — has increased as manufacturers constantly strive to improve efficiency through automation. Smart cylinders can use either continuous or discrete position sensing, providing manufacturers with options, but possibly leaving them with questions on which is best for their application.

In this post we will review the benefits of continuous position sensors and list the applications where this is the best fit.

Continuous position sensors provide near real-time position feedback throughout the entire stroke of the cylinder making them the ideal choice for applications at the higher end of the control spectrum. Closed-loop servohydraulic systems can achieve sophisticated, dynamic control of motion across the entire cylinder stroke.

Continuous position sensors are commonly used when the application calls for closed-loop servo control, where the position, speed, acceleration, and deceleration of the cylinder must be controlled. Closed-loop servohydraulics have been widely used in industrial applications, such as sawmills, steel processing and tire manufacturing, and more recently in cylinders in off-highway equipment.

Magnetostrictive linear position sensors are the most commonly used continuous position sensors in hydraulic cylinders. These sensors are installed into the back end of the cylinder. The sensor detects the position of a magnet attached to the piston and provides a continuous, absolute position signal.

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Magnetostrictive linear position sensor installed in hydraulic cylinder

The sensor is rated to withstand the full pressure of the hydraulic system. Magnetostrictive technology offers the advantage of being completely non-contact, meaning it requires no mechanical contact between the sensor and the moving cylinder and is not subject to wear and performance degradation. In addition, numerous electrical interface options are available, from simple analog (0 to 10V or 4-20mA) to high-performance industrial fieldbus interfaces that offer advanced functionality.

Continuous position sensors can also be used in pneumatic cylinders. While closed-loop servo control with pneumatics is not as common as it is with hydraulics, there are situations where pneumatic cylinders require continuous position sensing capability. For example, low-pressure pneumatic cylinders are sometimes used as measurement probes, or touch probes, where the cylinder rod is extended until it touches a part to be measured or gaged. In these situations, it is beneficial to be able to get continuous position feedback, especially when there is variability in the measured part.

To learn more about cylinder position sensing, visit www.balluff.com.

Ensure Optimum Performance In Hostile Welding Cell Environments

The image above demonstrates the severity of weld cell hostilities.

Roughly four sensing-related processes occur in a welding cell with regards to parts that are to be joined by MIG, TIG and resistance welding by specialized robotic /automated equipment:

  1. Nesting…usually, inductive proximity sensors with special Weld Field Resistance properties and hopefully, heavy duty mechanical properties (coatings to resist weld debris accumulation, hardened faces to resist parts loading impact and well-guarded cabling) are used to validate the presence of properly seated or “nested” metal components to ensure perfectly assembled products for end customers.
  2. Poke-Yoke Sensing (Feature Validation)…tabs, holes, flanges and other essential details are generally confirmed by photoelectric, inductive proximity or electromechanical sensing devices.
  3. Pneumatic and Hydraulic cylinder clamping indication is vital for proper positioning before the welding occurs. Improper clamping before welding can lead to finished goods that are out of tolerance and ultimately leads to scrap, a costly item in an already profit-tight, volume dependent business.
  4. Several MIB’s covered in weld debris

    Connectivity…all peripheral sensing devices mentioned above are ultimately wired back to the controls architecture of the welding apparatus, by means of junction boxes, passive MIB’s (multiport interface boxes) or bus networked systems. It is important to mention that all of these components and more (valve banks, manifolds, etc.) and must be protected to ensure optimum performance against the extremely hostile rigors of the weld process.

Magnetoresistive (MR), and Giant Magnetoresistive (GMR) sensing technologies provide some very positive attributes in welding cell environments in that they provide exceptionally accurate switching points, have form factors that adapt to all popular “C” slot, “T” slot, band mount, tie rod, trapezoid and cylindrical pneumatic cylinder body shapes regardless of manufacturer. One model family combines two separate sensing elements tied to a common connector, eliminating one wire back to the host control. One or two separate cylinders can be controlled from one set if only one sensor is required for position sensing.

Cylinder and sensor under attack.

Unlike reed switches that are very inexpensive (up front purchase price; these generally come from cylinder manufacturers attached to their products) but are prone to premature failure.  Hall Effect switches are solid state, yet generally have their own set of weaknesses such as a tendency to drift over time and are generally not short circuit protected or reverse polarity protected, something to consider when a performance-oriented cylinder sensing device is desired.  VERY GOOD MR and GMR cylinder position sensors are guaranteed for lifetime performance, something of significance as well when unparalleled performance is expected in high production welding operations.

But!!!!! Yes, there is indeed a caveat in that aluminum bodied cylinders (they must be aluminum in order for its piston-attached magnet must permit magnetic gauss to pass through the non-ferrous cylinder body in order to be detected by the sensor to recognize position) are prone to weld hostility as well. And connection wires on ALL of these devices are prone to welding hostilities such as weld spatter (especially MIG or Resistance welding), heat, over flex, cable cuts made by sharp metal components and impact from direct parts impact. Some inexpensive, effective, off-the-shelf protective silicone cable cover tubing, self-fusing Weld Repel Wrap and silicone sheet material cut to fit particular protective needs go far in protecting all of these components and guarantees positive sensor performance, machine up-time and significantly reduces nuisance maintenance issues.

To learn more about high durability solutions visit www.balluff.com.

5 Tips on Making End-of-Arm Tooling Smarter

Example of a Flexible EOA Tool with 8 sensors connected with an Inductive Coupling System.

Over the years I’ve interviewed many customers regarding End-Of-Arm (EOA) tooling. Most of the improvements revolve around making the EOA tooling smarter. Smarter tools mean more reliability, faster change out and more in-tool error proofing.

#5: Go Analog…in flexible manufacturing environments, discrete information just does not provide an adequate solution. Analog sensors can change set points based on the product currently being manufactured.

#4: Lose the weight…look at the connectors and cables. M8 and M5 connectorized sensors and cables are readily available. Use field installable connectors to help keep cable runs as short as possible. We see too many long cables simply bundled up.

#3: Go Small…use miniature, precision sensors that do not require separate amplifiers. These miniature sensors not only cut down on size but also have increased precision. With these sensors, you’ll know if a part is not completely seated in the gripper.

#2: Monitor those pneumatic cylinders…monitoring air pressure in one way, but as speeds increase and size is reduced, you really need to know cylinder end of travel position. The best technology for EOA tooling is magnetoresistive such as Balluff’s BMF line. Avoid hall-effects and definitely avoid reed switches. Also, consider dual sensor styles such as Balluff’s V-Twin line.

#1: Go with Couplers…with interchangeable tooling, sensors should be connected with a solid-state, inductive coupling system such as Balluff’s Inductive Coupler (BIC). Avoid the use of pin-based connector systems for low power sensors. They create reliability problems over time.

Control, Monitor, Measure: Three Main Applications for Linear Position Sensors

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It occurred to me recently that, while linear position sensors are used in a wide variety of industries and applications, all of these applications fall into three broad categories:  controlling linear motion, monitoring linear motion, and measuring linear motion.

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Better Alternatives to Pneumatic Cylinder End-of-Stroke Detection

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There are better alternatives to detect pneumatic cylinder end of stroke position than reed switches or proximity switches. By better, I mean they are faster and easier to implement into your control system. In addition, you can realize other benefits such as commonality of spare sensors and lower long-term costs. So what are the better solutions?

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The Pros and Cons of End-of-Stroke Detection with Reed Switches

Pneumatic cylinders are used in many applications as prime movers in machinery, material handling, assembly, robotics, medical, and the list could go on. One of the challenges facing OEM’s integrators and end users is to detect reliably whether the cylinder has been fully extended or retracted before allowing machine movement. Solutions include the use of inductive sensors with some sort of target and internally mounted magnet (by the cylinder manufacturer) on the cylinder piston. In my previous blog, I discussed the two primary magnets, axially and radially magnetized magnets, used by cylinder manufacturers. Now, we will review one of the most commonly used magnetic field sensors to detect extension and retraction of the cylinder…the well-known reed switch.

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