Level Sensing in Machine Tools

Certainly the main focus in machine tools is on metal cutting or metal forming processes.

To achieve optimum results in cutting processes coolants and lubricants are applied. In both metal cutting and metal forming processes hydraulic equipment is used (as hydraulics create high forces in compact designs). For coolant, lubricant and hydraulic tanks the usage of level sensors to monitor the tank level of these liquids is required.

Point Level Sensing

For point level sensing (switching output) in many cases capacitive sensors are used. These sensors detect the change of the relative electric permittivity (typically a change of factor 10 from gas to liquid). The capacitive sensors may be mounted at the outside of the tank wall if the tank material is non metallic like e.g. plastic or glass. The installation may even be in retrofit applications yet limited to non metallic tanks up to a certain wall thickness.

When using metal tanks the capacitive sensors enter the inner area of the tank via a thread and a sealing component. Common thread sizes are: M12x1, M18x1, M30x1,5, G 1/4″, NPT 1/4″ etc. For conductive liquids specially designed capacitive level sensors may be used which ignore build up at the sensing surface.

Continuous Level Sensing

Advanced process control uses continuous level sensing principles. The continuous sensor signals e.g. 0..10V, 4…20mA or increasingly IO-Link deliver more information to better control the liquid level, especially relevant in dynamic or precise applications.

When using floats the magnetostrictive sensing principle offers very high resolution of the level value. Tank heights vary from typically 200 mm up to several meters. Another advantage of this sensor principle is the high update rate (supporting fast closed loop systems for level sensing)

In many applications the  requirements for the level control solutions are not too demanding. In these cases the ultrasonic principle has gained significant market share within the last years. Ultrasonic sensors do not need a float, installation on the top of the tank is pretty easy, there are even sensor types available which may be used in pressurized tanks (typically up to 6 bar). As ultrasonic sensors quite often are used in special applications, field tests during the design in process are recommended.

Finally hydrostatic pressure transducers are an option for level sensing when using non pressurized tanks (typically  connected to ambient pressure through a bore in the upper area of the tank). With the sensor mounted at the bottom of the tank the level is indirectly measured through the pressure of the liquid column above the sensor (e.g. 10m of water level resembles 1 bar).

Summary

Concerning level sensing in metalworking applications in the first step it should be decided whether point level sensing is sufficient or continuous level sensing is required. Having chosen continuous level sensing there are several sensor principles available (selection depending on the application needs and features of the liquids and tank properties). It is always a good engineering practice to prove the preselected sensing concept with field tests.

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Recap of our top 5 posts of 2015

goodbye-2015-hello-2016As we wrap up the old and begin to open up the new, let’s take a moment to reflect on what this past year has brought us.  Apart from the triumphs and the hard lost battles, we want to bring you some of our top posts from 2015.  These posts are as follows:

#5: 5 Tips on Making End-of-Arm Tooling Smarter

Everyone wants their robot to work faster, smarter, and more efficiently.  In this post we review five easy tips to help you improve the efficiency of your end-of-arm tooling.

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

#4: Liquid Level Sensing: Detect or Monitor

Who doesn’t like complicated concepts broken down into easy to understand terminology? In this post we break down the differences between point level detection and continuous position sensing as well as provide you with technologies to put into practice.

#3: How Can I Convince My Boss to Send Me to Training?

As Aristotle once said “All men (and women) by nature desire knowledge.”  Here we are giving you the tools needed to break down the barriers your boss (or you) might have against investing in training.

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

So you just got a brand spanking new 2-wire sensor for the holidays but you realize you don’t know exactly what wire goes where.  In this post we make wiring that bad boy easy and even break down what polarized and non-polarized mean.

So we have covered four of the top posts from 2015, are you ready for the number one post from the past year? So are we! And we will have it for you right after a quick message from our sponsors! (just kidding!)

power&dataexchange#1: Inductive Coupling – Simple Concept for Complex Automation

Through the use of magnetic induction, we are able to reduce the downtime of a machine due to the failure of a slip ring.  Inductive couplers pass power and data over an air gap creating a maintenance free, non-contact environment to operate a variety of machinery.

We want to thank you for the wonderful year that is behind us and be sure to be on the look-out for even more exciting news to come this year!

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.