Hydraulic Cylinder Position Feedback, Revisited

In a previous Sensortech post entitled “Hydraulic Cylinder Position Feedback“, we discussed the basic concept of hydraulic cylinder position feedback.  In case you might have missed that post, here it is for an encore appearance.

Magnetostrictive linear position transducers are commonly used in conjunction with hydraulic cylinders to provide continuous, absolute position feedback.  Non-contact magnetostrictive technology assures dependable, trouble-free operation.  The brief video below illustrates how magnetostrictive position sensors are used with hydraulic cylinders.

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Back to the Basics: How Do I Wire a DC 2-wire Sensor?

In one of my previous post we covered “How do I wire my 3-wire sensors“. This topic has had a lot of interest so I thought to myself, this would be a great opportunity to add to that subject and talk about DC 2-wire sensors. Typically in factory automation applications 2 or 3 wire sensors are implemented within the process, and as you know from my prior post a 3 wire sensor has the following 3 wires; a power wire, a ground wire and a switch wire.

A 2-wire sensor of course only has 2 wires including a power wire and ground wire with connection options of Polarized and Non-Polarized. A Polarized option requires the power wire to be connected to the positive (+) side and the ground wire to be connected to the negative side (-) of the power supply. The Non-Polarized versions can be wired just as a Polarized sensor however they also have the ability to be wired with the ground wire (-) to the positive side and the power wire (+) to the negative side of the power supply making this a more versatile option as the sensor can be wired with the wires in a non – specific location within the power supply and controls.

In the wiring diagrams below you will notice the different call outs for the Polarized vs. Non-Polarized offerings.

PolarizedDiagramsnon-polarized diagramsNote: (-) Indication of Non-Polarized wiring.

While 3-wire sensors are a more common option as they offer very low leakage current, 2 wire offerings do have their advantages per application. They can be wired in a sinking (NPN) or sourcing (PNP) configuration depending on the selected load location. Also keep in mind they only have 2 wires simplifying connection processes.

For more information on DC 2- Wire sensors click here.

When is a Weld Field Immune Sensor Needed?

When the topic of welding comes up we know that our application is going to be more challenging for sensor selection. Today’s weld cells typically found in tier 1 and tier 2 automotive plants are known to have hostile environments that the standard sensor cannot withstand and can fail regularly. There are many sensor offerings that are designed for welding including special features like Weld Field Immune Circuitry, High Temperature Weld Spatter Coatings and SteelFace Housings.

For this SENSORTECH topic I would like to review Weld Field Immune (WFI) sensors. Many welding application areas can generate strong magnetic fields. When this magnetic field is present a typical standard sensor cannot tolerate the magnetic field and is subject to intermittent behavior that can cause unnecessary downtime by providing a false signal when there is no target present. WFI sensors have special filtering properties with robust circuitry that will enable them to withstand the influence of strong magnetic fields.

WFIWFI sensors are typically needed at the weld gun side of the welding procedure when MIG welding is performed. This location is subject to Arc Blow that can cause a strong magnetic field at the weld wire tip location. This is the hottest location in the weld cell and typically there is an Inductive Sensor located at the end of this weld tooling.

So as you can see if a WFI sensor is not selected where there is a magnetic field present it can cause multiple cycle time problems and unnecessary downtime. For more information on WFI sensors click here.

There’s more than just one miniature sensor technology

As I discussed in my last blog post, there is a need for miniature, precision sensors. However, finding the right solution for a particular application can be a difficult process. Since every sensor technology has its own strengths and weaknesses, it is vital to have a variety of different sensor options to choose from.

The good news is that there are several different technologies to consider in the miniature, precision sensor world. Here we will briefly look at three technologies: photoelectric, capacitive, and inductive. Together these three technologies have the ability to cover a wide range of applications.

Photoelectric Sensors

MiniPhotoelectricPhotoelectric sensors use a light emitter and receiver to detect the presence or absence of an object. This type of sensor comes in different styles for flexibility in sensing. A through-beam photoelectric is ideal for long range detection and small part detection. Whereas a diffuse photoelectric is ideal for applications where space is limited or in applications where sensing is only possible from one side.

Miniature photoelectric sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Capacitive Sensors

MiniCapacitiveCapacitive sensors use the electrical property of capacitance and work by measuring changes in this electrical property as an object enters its sensing field. Capacitive sensors detect the presence or absence of virtually any object with any material, from metals to powders to liquids. It also has the ability to sense through a plastic or glass container wall to detect proper fill level of the material inside the container.

Miniature capacitive sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Inductive Sensors

MiniInductiveInductive sensors use a coil and oscillator to create a magnetic field to detect the presence or absence of any metal object. The presence of a metal object in the sensing field dampens the oscillation amplitude. This type of sensor is, of course, ideal for detecting metal objects.

Miniature inductive sensors come with the electronics fully integrated into the sensor.

One sensor technology isn’t enough since there isn’t a single technology that will work across all applications. It’s good to have options when looking for an application solution.

To learn more about these technologies, visit www.balluff.us

Liquid Level Sensing: Detect or Monitor?

Pages upon pages of information could be devoted to exploring the various products and technologies used for liquid level sensing and monitoring.  But we’re not going to do that in this article.  Instead, as a starting point, we’re going to provide a brief overview of the concepts of discrete (or point) level detection and continuous position sensing.

 Discrete (or Point) Level Detection

Example of discrete sensors used to detect tank level
Example of discrete sensors used to detect tank level

In many applications, the level in a tank or vessel doesn’t need to be absolutely known.  Instead, we just need to be able to determine if the level inside the tank is here or there.  Is it nearly full, or is it nearly empty?  When it’s nearly full, STOP the pump that pumps more liquid into the tank.  When it’s nearly empty, START the pump that pumps liquid into the tank.

This is discrete, or point, level detection.  Products and technologies used for point level detection are varied and diverse, but typical technologies include, capacitive, optical, and magnetic sensors.  These sensors could live inside the tank outside the tank.  Each of these technologies has its own strengths and weaknesses, depending on the specific application requirements.  Again, that’s a topic for another day.

In practice, there may be more than just two (empty and full) detection points.  Additional point detection sensors could be used, for example, to detect ¼ full, ½ full, ¾ full, etc.  But at some point, adding more detection points stops making sense.  This is where continuous level sensing comes into play.

Continuous Level Sensing

Example of in-tank continuous level sensor
Example of in-tank continuous level sensor

If more precise information about level in the tank is needed, sensors that provide precise, continuous feedback – from empty to full, and everywhere in between – can be used.  This is continuous level sensing.

In some cases, not only does the level need to be known continuously, but it needs to be known with extremely high precision, as is the case with many dispensing applications.  In these applications, the changing level in the tank corresponds to the amount of liquid pumped out of the tank, which needs to be precisely measured.

Again, various technologies and form factors are employed for continuous level sensing applications.  Commonly-used continuous position sensing technologies include ultrasonic, sonic, and magnetostrictive.  The correct technology is the one that satisfies the application requirements, including form factor, whether it can be inside the tank, and what level of precision is needed.

At the end of the day, every application is different, but there is most likely a sensor that’s up for the task.

Ultrasonic Sensor Reflection Targets

In my previous posts (Ultrasonic Sensors with Analog Output, Error-proofing in Window Mode, and The Other Retro-Reflective Sensors) we covered the Ultrasonic sensor modes and how they benefit in many different types of applications. It is also important to understand the reflection properties of various materials and how they interface with the sensor selected. For example some Photoelectric sensors will have a very difficult time detecting clear materials such as glass or clear films as they will simply detect directly through the clear material detecting what is on the other side giving a false positive target reading. As we know, this is not an issue with an Ultrasonic sensor as they detect targets via a sound wave so clear objects do not affect the sensors function. When looking at sensor technologies it is import to understand the material target before selecting the correct sensor for the applied application such as an Inductive sensor would be selected if we are looking at a ferrous (metal) target at short range. Below are some examples of good and poor reflective materials when Ultrasonic sensors are used.

Good Reflective MaterialsUltrasonicApplication

  • Water
  • Paint
  • Wood
  • Metal
  • Plastic
  • Concrete/Stone
  • Glass
  • Hard Rubber
  • Hard Foam

Challenging Relective Materials

  • Cotton Wool
  • Soft Carpet
  • Soap Foams
  • Powders With Air
  • Soft Foam
  • Soft Rubber

So as you can see materials that are hard or solid have good reflective properties whereas soft materials will absorb the sound wave provided from the sensor making it much more challenging to detect our target. For more information on Ultrasonic sensors click here.

Do’s and Don’ts For Applying Inductive Prox Sensors

Inductive proximity sensor face damage
Example of proximity sensor face damage

In my last post (We Don’t Make Axes Out of Bronze Anymore) we discussed the evolution of technologies which brought up the question, can a prox always replace a limit switch?  Not always.  Note that most proxes cannot directly switch large values of current, for example enough to start a motor, operate a large relay, or power up a high-wattage incandescent light.   Being electronic devices, most standard proxes cannot handle very high temperatures, although specialized hi-temp versions are available.

A prox is designed to be a non-contact device.  That is, it should be installed so that the target does not slam into or rub across the sensing face.  If the application is very rough and the spacing difficult to control, a prox with more sensing range should be selected.  Alternately, the prox could be “bunkered” or flush-mounted inside a heavy, protective bracket.  The target can pound on the bunker continuously, but the sensor remains safely out of harm’s way.

If direct contact with a sensor absolutely cannot be avoided, ruggedized metal-faced sensors are available that are specifically designed to handle impacts on the active surface.

Be sure to consider ambient conditions of the operating environment.  High temperature was mentioned earlier, but other harsh conditions such as disruptive electrical welding fields or high-pressure wash-down can be overcome by selecting proxes specially designed to survive and thrive in these environments.

Operationally, another thing to consider is the target material.  Common mild carbon steel is the ideal target for an inductive prox and will yield the longest sensing ranges with standard proxes.  Other metals such as aluminum, brass, copper, and stainless steel have different material properties that reduce the sensing range of a standard prox.  In these cases be sure to select a Factor 1 type proximity sensor, which can sense all metals at the same range.

Aw, yiss…BiSS!

BiSSThere’s a cool new serial data interface coming on the scene.

It’s called BiSS (Bi-Directional Synchronous Serial), and is an open-source, free-to-license digital interface for sensors and actuators. BiSS is hardware compatible to the industrial standard SSI (Serial Synchronous Interface) but offers additional features and options. Here are a few highlights:

  • Serial Synchronous Data Communication
  • 2 unidirectional lines Clock and Data
    o Cyclic at high-speed (up to 10 MHz with RS422 and 100 MHz with LVDS)
    o Line delay compensation for high-speed data transfer
    o Request processing times for data generation at slaves
    o Safety capable: CRC, Errors, Warnings
    o Bus capability for multiple slaves and devices in a chain
  • Unidirectional
    o BiSS C (unidirectional) protocol: Unidirectional use of BiSS C
  • Bidirectional
    o BiSS C protocol: Continuous mode
  • Actuators
    o Operate actuators via two additional lines

bmlFor more information and FAQs, visit http://www.biss-interface.com/

Click here to learn about absolute, magnetically coded position measurement systems for large measurement sections with BiSS interface.

 

Optical Window Sensors

Optical window sensors are utilized where reliable part counting is needed. This type of sensor technology is based on an array of LEDs on one side, opposite an array of phototransistors on the other side. This array covers the whole area of the window’s opening with an evenly as possible distribution of light. The more evenly distributed the light is throughout the window, the higher the resolution.

Optical window sensors are usually assigned a particular term to reveal their specific functionality type. The two typical functionality types for an optical window sensor are either static or dynamic. The differences between the two functionality types are briefly outlined here.

Static functionality looks for unchanging events. In the case for an optical window sensor, static means it detects the percentage of signal blocked by an object present in or passing through the window. Dynamic functionality looks for changing events. In the case for an optical window sensor, dynamic means it detects moving objects in the window and ignores non-moving objects. Still, in either case whether static or dynamic, the sensor detects objects as they pass through the window.

A common follow-up question is: what are the pros and cons for using either functionality over the other? This is a good question, because there are definite benefits and disadvantages to both approaches. A few of these benefits and disadvantages are briefly outlined below.

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Asset Tracking – Top 10

The goal of plant-based asset tracking is to reduce non-productive time and asset losses, while increasing overall productivity and utilization by accurately tracking assets. Bar code and RFID technologies track changes to an asset’s location, condition, conformity status, and availability.

Balluff has been in this business for over 25 years. Based on that experience, we have compiled the top 10 list of commonly tracked plant-based assets:

1. Dunnage containers
2. Machine tools
3. Plant-floor Equipment
4. Stamping dies
5. Torque Wrenches
6. Plastic Molds
7. Storage tanks and vessels
8. IT equipment
9. Automated Guided Vehicle (AGV)
10. Modular automation sub-systems

If you are looking to gain tighter control of your assets, visit www.balluff.us

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