sensing Archives - AUTOMATION INSIGHTS

Waterways: the Many Routes of Water Detection

Water is everywhere, in most things living and not, and the amount of this precious resource is always important. The simplest form of monitoring water is if it is there or not. In your body, you feel the effects of dehydration, in your car the motor overheats, and on your lawn, you see the dryness of the grass. What about your specialty machine or your assembly process? Water and other liquids are inherently clear so how do you see them, especially small amounts of it possibly stored in a tank or moving fast? Well, there are several correct answers to that question. Let’s dive into this slippery topic together, pun intended.

While mechanical float and flow switches have been around the longest, capacitive, photoelectric, and ultrasonic sensors are the most modern forms of electronic water detection. These three sensing technologies all have their strong points. Let’s cover a few comparisons that might help you find your path to the best solution for your application.

Capacitive sensors

Capacitive sensors are designed to detect nonferrous materials, but really anything that can break the capacitive field the sensor creates, including water, can do this. This technology allows for adjustment to the threshold of what it takes to break this field. These sensors are a great solution for through tank level detection and direct-contact sensing.

Ultrasonic sensors

Want to view your level from above? Ultrasonic sensors give you that view. They use sound to bounce off the media and return to the sensor, calculating the time it takes to measure distance. Their strong point is that they can overcome foam and can bounce off the water where light struggles when there is a large distance from the target to the receiver. Using the liquid from above, ultrasonics can monitor large tanks without contact.

Photoelectric sensors

Use photoelectric sensors when you’re looking at a solution for small scale. Now, this might require a site tube if you are monitoring the level on a large tank, however, if you want to detect small amounts of water or even bubbles within that water, photoelectric sensors are ideal. Using optical head remote photoelectric sensors tied to an amplifier, the detail and speed are unmatched. Photoelectric sensors are also great at detecting liquid levels on transparent bottles. In these applications with short distances, you need speed. Photoelectric sensors are as fast as light.

So, have you made up your mind yet? No matter which technology you choose, you will have a sensor that gives you accurate detail and digital outputs and is easy on the budget. Capacitive, ultrasonic, and photoelectric sensors provide all this and they grow with your application with adjustability.

Liquids are everywhere and not going away in manufacturing. They will continue to be an important resource for manufacturing. Cherish them and ensure you account for every drop.

AMR and GMR: Better Methods to End of Travel Sensing

Today’s pneumatic cylinders are compact, reliable, and cost-effective prime movers for automated equipment. Unfortunately, they are often provided with unreliable reed or Hall Effect switches that fail well before the service life of the cylinder itself is expended. Too often, life with pneumatic cylinders involves continuous effort and mounting costs to replace failed cylinder position switches. As a result, some OEMs and end users have abandoned magnetic cylinder switches altogether in favor of more reliable — yet more costly and cumbersome — external inductive proximity sensors, brackets, and fixed or adjustable metal targets. There must be a better way!

One position sensing technique is to install external electro-mechanical limit switches or inductive proximity switches that detect metal flags on the moving parts of the machine. The disadvantages of this approach include the cost and complexity of the brackets and associated hardware, the difficulty of making adjustments, and the increased physical size of the overall assembly.

A more popular and widely used method is to attach magnetically actuated switches or sensors to the sides of the cylinder, or into a slot extruded into the body of the cylinder. Through the aluminum wall of the pneumatic cylinder, magnetic field sensors detect an internal magnet that is mounted on the moving piston. In most applications, magnetic sensors provide end-of-stroke detection in either direction; however, installation of multiple sensors along the length of a cylinder allows detection of several discrete positions.

The simplest magnetic field sensor is the reed switch. This device consists of two flattened ferromagnetic nickel and iron reed elements enclosed in a hermetically sealed glass tube. The glass tube is evacuated to a high vacuum to minimize contact arcing. As an axially aligned magnet approaches, the reed elements attract the magnetic flux lines and draw together by magnetic force, thus completing an electrical circuit. The magnet must have a strong enough Gauss rating, usually in excess of 50 Gauss, to overcome the return force, i.e. spring memory, of the reed elements.

The benefits of reed switches are that they are low cost, they require no standby power, and they can function with both AC and DC electrical loads. However, reed switches are relatively slow to operate, therefore they may not respond fast enough for some high-speed applications. Since they are mechanical devices with moving parts, they have a finite number of operating cycles before they eventually fail. Switching high current electrical loads can further cut into their life expectancy.

Hall Effect sensors are solid-state electronic devices. They consist of a voltage amplifier and a comparator circuit that drives a switching output. It might seem like an easy solution to simply replace reed switches with Hall Effect sensors, however, the magnetic field orientation of a cylinder designed for reed switches may be axial, whereas the orientation for a Hall Effect sensor is radial. The result? There is a chance that a Hall Effect sensor will not operate properly when activated by an axially oriented magnet. Finally, some inexpensive Hall Effect sensors are susceptible to double switching, which occurs because the sensor will detect both poles of the magnet, not simply one or the other.

Today, solid-state magnetic field sensors are available either using magnetoresistive (AMR) or giant magnetoresistive (GMR) technology. Compared to AMR technology, GMR sensors have an even more robust reaction to the presence of a magnetic field, at least 10 percent.

The operating principle of AMR magnetoresistive sensors is simple: the sensor element undergoes a change in resistance when a magnetic field is present, changing the flow of a bias current running through the sensing element. A comparator circuit detects the change in current and switches the output of the sensor.

In addition to the benefits of rugged, solid-state construction, the magnetoresistive sensor offers better noise immunity, smaller physical size, less susceptibility to false tripping, speed and lower mechanical hysteresis (the difference in switch point when approaching the sensor from opposite directions). Quality manufacturers of magnetoresistive sensors incorporate additional output protection circuits to improve overall electrical robustness, such as overload protection, short-circuit protection, and reverse-connection protection. Some manufacturers also offer lifetime warranty of the sensors.

Over the years, many users have abandoned the use of reed switches due to their failure rate and have utilized mechanical or inductive sensors to detect pneumatic cylinder position. AMR and GMR sensors are smaller, faster, and easer to integrate and are much more reliable however; they must overcome the stigma left by their predecessors. With the vast improvements in sensor technology, AMR and GMR sensors should now be considered the primary solution for detecting cylinder position.

Capacitive Sensors – Part III

Written by: Bjoern Schaefer

The general sensing principle across this myriad of applications is nearly the same. As seen in last months post, the total amount of capacitance, as we remember, the ability to store a charge within an electrostatic field, depends on mainly three factors. Those factors are the ones which determine the success of your application.

Continue reading “Capacitive Sensors – Part III”