Converting Analog Signals to Digital for Improved Performance

We live in an analog world, where we experience temperatures, pressures, sounds, colors, etc., in seemingly infinite values. There are infinite temperature values between 70-71 degrees, for example, and an infinite number of pressure values between 50-51 psi.

Sensors today continue to use analog circuitry to measure a natural process, but more often, the electrical analog signal is then converted to a digital (binary) signal.

How a signal is converted from analog to digital?

A variety of mechanical and electrical transducer technologies, such as Bourdon tube, piezoresistive, manometers, strain gages, and capacitive can be found in a typical pressure sensor. Any one of these can be used to sense pressure and convert the physical pressure into an analog electrical signal. The analog output continuously varies as the pressure rises and lowers. For many sensors of the past, the story ends here. The sensor works well if certain precautions are met, but enhanced features are limited. This sensor would be comprised of electrical components, such as diodes, capacitors, op-amps, and resistors, with typical signal outputs of 0-5VDC, 0-10VDC, +/- 10V, 4-20mA, 0-20mA, etc.

Analog output sensors provide an infinitely varying signal and converting it to digital cannot improve the accuracy of the measured value. Nor will it increase the amount of information we receive from the natural world. So why do we do it?

Why convert to digital signals?

There are several good reasons for converting analog to digital signals. Analog uses more power than digital and it’s more difficult to encrypt, decode, or synchronize. Analog outputs also have a slow rate of transmission. But typically, the biggest reasons are that analog signals weaken and pick up electrical noise as they traverse, and they’re difficult to process and store.

Noises and transmission rates

Electrical energy from motors, contactors, and other electrical devices can become induced into the sensor’s analog electronics, creating noise on the signal. Analog amplifiers can increase the signal strength to extend transmission distances, but it also amplifies the induced noise. The transmission of digital signals, on the other hand, is faster and has negligible distortion. And although a digital signal may need an amplifier for long lengths, too, digital regeneration can more easily correct any 0/1 errors and amplify the signal without amplifying any noise.

Converting a continuously variable signal into 1s and 0s

An analog-to-digital converter (ADC) is an integrated circuit that performs the conversion. While this process includes many important steps, and there are several popular techniques, each has three main processes: sampling, quantizing, and encoding.

Sampling is a process used to select a subset of values from a larger set. In our case, we are starting with an infinite set of values from the analog signal and want to capture a snapshot of the signal at certain time intervals. With a sampling rate of 500Hz, the ADC will grab and hold a value from the analog signal 500 times per second.

Once the signal is sampled, it is quantized. This involves mapping the sample from a set of infinite signal values down to a finite number of values. If there were 100 available increments for quantizing a 0-5vdc signal, for example, the infinite output would now be reduced to 100 available signal level choices with 0 volts mapping as 0, 2.5 volts mapping as 50, and 5 volts as 99.

Lastly, the quantized signal level is encoded to binary form, where it can benefit from the processing, storage, and transmission advantages that come with a digital signal. A quantized level of 50, encoding with an 8-bit processor, would be 00011001, equating to a 2.5vdc signal.

In actual practice, we do not use 100 increments to quantize. The ADC, which is based on the number of bits within the processor within the ADC chip, determines the amount of quantizing increments or levels. Eight bits provide 256 increments. Twelve bits provide 4096 increments or steps, as it is also referred.

Is 12 bits worth of increments (4096 steps) enough resolution?

5VDC /4096 steps = .00122V/step or 1.22mV/step

In most applications, a small step of 1.22mV is acceptable. The original analog signal is now sampled at a specific time, and an increment closest to the value is chosen as the signal level. The quantizing process in this case will round the infinite analog value that was sampled to the nearest multiple of 1.22mV.

The output signal is now a square wave, rather than the original sinusoidal. The peak of each square wave is always the same amplitude, with the peak of the wave representing a “1” and the trough or zero amplitude being a “0.”

The sensor output, now digitized, is capable of further processing, offering enhanced product features such as faster transmission rates, negligible distortion, and the ability to communicate to advanced systems such as IO-Link.

A digital to analog converter (DAC) can convert the signal back to analog, but complete restoration is no longer possible due to the samples taken only at specific times, and the quantizing step rounding off to the nearest increment.

So, the next time you see a spec sheet that says “12-bit resolution,” rest assured you are working with a sensor that has some enhanced capabilities.

Detecting Liquid Media and Bubbles Using Optical Sensors

In my line of work in Life Sciences, we often deal with liquid media and bubble detection evaluation through a vessel or a tube. This can be done by using the absorption principle or the refraction principle with through-beam-configured optical sensors. These are commonly embedded in medical devices or lab instruments.

This configuration provides strong benefits:

    • Precise sensing
    • Ability to evaluate liquid media
    • Detect multiple events
    • High reliability

How does it work?

The refraction principle is based on the media’s refraction index. It uses an emitted light source (Tx) that is angled to limit the light falling on the receiver (Rx, Figure 1). When the light passes through a liquid, refraction causes the light to focus on the receiver as a beam (known as a “beam-make” configuration). All liquids and common vessel materials (silicon, plastic, glass, etc.) have a known refraction index. These sensors will detect those refraction differences and output a signal.

The absorption principle is preferred when a media’s absorption index is high. First, a beam is established through a vessel or tube (Figure 2). Light sources in the 1500nm range work best for aqueous-based media such as water. As a high absorption index liquid enters the tube, it will block the light (known as a beam-break configuration). The sensor detects this loss of light.

Discrete on-off signals are easily used by a control system. However, by using the actual light value information (commonly analog), more data can be extracted. This is becoming more popular now and can be done with either sensing principle. By using this light-value information, you can differentiate between types of media, measure concentrations, identify multiple objects (e.g., filter in an IV and the media) and much more.

There is a lot to know about through-beam sensors, so please leave a comment below if you have questions on how you can benefit from this technology.

IO-Link Measurement Sensors Solve Application Challenges

In industrial distance and position measurement applications, one size definitely does not fit all.  Depending on the application, the position or distance to be measured can range from just a few millimeters up to dozens of meters.  No single industrial sensor technology is capable of meeting these diverse requirements.

Fortunately, machine builders, OEM’s and end-users can now choose from a wide variety of IO-Link distance and position measurement sensors to suit nearly any requirement.  In this article, we’ll do a quick rundown of some of the more popular IO-Link measurement sensor types.

(For more information about the advantages of IO-Link versus traditional analog measurement sensors, see the following blog posts, Solving Analog Integration Conundrum, Simplify Your Existing Analog Sensor Connection, and How Do I Make My Analog Sensor Less Complex?)

 

Short Range Inductive Distance Sensors

These sensors, available in tubular and blockScott Image1.JPG style form factors are used to measure very short distances, typically in the 1…5 mm range.  The operating principle is similar to a standard on/off inductive proximity sensor.  However, instead of discrete on/off operation, the distance from the face of the sensor to a steel target is expressed as a continuously variable value.  Their extremely small size makes them ideal for applications in confined spaces.

Inductive Linear Position Sensors

Inductive linear position sensors are available in several block style form factors, and are used for position measurement over stroke lengths up to about 135 mm.  These types of sensors use an array of inductive coils to accurately measure the position of a metal target.  Compact form factors and low stroke-to-overall length factor make them well suited for application with limited space.

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Magnetostrictive Linear Position Sensors

IO-Link Magnetostrictive linear position sensors are available in rod style form factors for hydraulic cylinder position feedback, and in external mount profile form factors for general factory automation position monitoring applications.  These sensors use time-proven, non-contact magnetostrictive technology to provide accurate, absolute position feedback over stroke lengths up to 4.8 meters.

Laser Optical Distance Sensors

 

Scott Image 4.JPGLaser distance sensors use either a time-of-flight measuring principle (for long range) or triangulation measuring principle (for shorter range) to precisely measure sensor to target distance from up to 6 meters away.  Laser distance sensors are especially useful in applications where the sensor must be located away from the target to be measured.

 

Magnetic Linear Encoders

IO-Link magnetic linear encoders use an absolute-codedScott Image 5 flexible magnet tape and a compact sensing head to provide extremely accurate position, absolute position feedback over stroke lengths up to 8 meters.  Flexible installation, compact overall size, and extremely fast response time make magnetic linear encoders an excellent choice for demanding, fast moving applications.

IO-Link Measurement Sensor Trends

The proliferation of available IO-Link measurement sensors is made possible, in large part, due to the implementation of IO-Link specification 1.1, which allows faster data transmission and parameter server functionality.  The higher data transfer speed is especially important for measurement sensors because continuous distance or position values require much more data compared to discrete on/off data.  The server parameter function allows device settings to be stored in the sensor and backed up in the IO-Link master.  That means that a sensor can be replaced, and all relevant settings can be downloaded from master to sensor automatically.

To learn about IO-Link in general and IO-Link measurement sensors in particular, visit www.balluff.com.

Linear Measurement Sensors for Short Stroke Applications

We’ve posted numerous articles here on the Sensortech blog about linear position sensors used for applications such as hydraulic cylinder position feedback, plastic injection molding machinery, tire manufacturing machinery, etc.  What all of the applications have in common is that we’re generally talking about fairly long linear travels, usually longer than 12″, sometimes up to 300″.spindle

But in applications such as spindle clamp positioning on machine tools or positioning of
linear movements on automated assembly machinery, travels are sometimes only a couple of inches, and the available space to mount a position sensor is extremely limited.  Fortunately, there are highly capable linear position sensors that are perfectly suited for such applications.

For example, there are sensors that use an array of inductive coils to detect the bips
precise linear position of a simple metal target.  These sensors, with working strokes ranging from < 1″ up to around 5″ have are extremely compact, with very little dead zone.  That means they fit into very tight spaces, where other type of linear position sensors simply couldn’t.

Typically, these types of sensors provide a position signal in the form of an analog voltage (0-10V) or current (4-20 mA).  Increasingly though, IO-Link interfaces are gaining in popularity, offering simplified wiring, better noise immunity, built-in diagnostics, and the ability to easily get the position data into virtually any industrial field-bus architecture.

For more information, visit www.balluff.com

What’s best for integrating Poka-yoke or Mistake Proofing sensors?

Teams considering poka-yoke or mistake proofing applications typically contact us with a problem in hand.  “Can you help us detect this problem?”

We spend a lot of time:

  • talking about the product and the mistakes being made
  • identifying the error and how to contain it
  • and attempting to select the best sensing technology to solve the application.

However this can sometimes be the easy part of the project.  Many times a great sensor solution is identified but the proper controls inputs are not available or the control architecture doesn’t support analog inputs or network connections.  The amount of time and dollar investments to integrate the sensor solution dramatically increases and sometimes the best poka-yoke solutions go un-implemented!”

“Sometimes the best poka-yoke solutions go un-implemented!”

Many of our customers are finding that the best controls architecture for their continuous improvement processes involves the use of IO-Link integrated with their existing architectures.  It can be very quickly integrated into the existing controls and has a wide variety of technologies available.  Both of these factors make it the best for integrating Poka-yoke or Mistake Proofing due to the great flexibility and easy integration.

Download this whitepaper and read about how a continuous improvement technician installed and integrated an error-proofing sensor in 20 minutes!

For Industrial Controls, What’s Next After Analog?

Analog signals have been part of industrial control systems for a very long time.  The two most common signals are 0-10V (“voltage”) and 4-20mA (“current”), although there are a wide variety of other voltage and current protocols.  These signals are called “analog” because they vary continuously and have theoretically infinite resolution (although practical resolution is limited by the level of residual electrical noise in the circuit).

Measurement sensors typically provide analog output signals, because these electronic circuits are well-understood and the designs are relatively economical to produce.  But that doesn’t mean it’s easy to design and build a good-quality analog sensor: in fact it is very difficult to engineer an analog signal that is highly linear over its measuring range, has low noise (for high-resolution), is thermally stable, (doesn’t drift as temperature changes), and is repeatable from sample to sample.  It takes a lot of careful engineering, testing, and tweaking to deliver a good analog sensor to the market.

Continue reading “For Industrial Controls, What’s Next After Analog?”

Cable Length for Analog Sensors

A question came in recently concerning the maximum recommended cable length for analog sensors.  Even as digital interfaces gain popularity, sensors with analog interfaces (0-10V, 4-20 mA, etc.) still represent the overwhelming majority of continuous position sensors used in industrial applications.

The question about maximum cable length for analog sensors comes up pretty frequently.  Generally speaking, the issue is that electrical conductors, even good ones, have some resistance to the flow of current (signals).  If the resistance of the conductor (the cable) gets high enough, the sensor’s signal can be degraded to the point where accuracy suffers, or even to the point where it becomes unusable.  Unfortunately, there is no hard and fast answer to the question.  Variables such as wire gauge, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, and other factors come into play, and need to be considered.  A discussion about all of these variables could fill a book, but we can make some general recommendations:

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E = IR: It’s Not Just a Good Idea, it’s the Law

I recently had a conversation with a customer that resulted in one of those forehead-slapping “duh” moments for me, and I thought it might be worth passing along. Here’s the story:

The customer had an application that required an analog linear feedback sensor that provided an output of 1 volt to 5 volts over the linear stroke range. Now, a 1-5V output is not very common, and the particular sensor he was interested in was only available with either a 0-10V or a 4-20 mA output. What to do? Perhaps the answer should have been obvious to me, but it was the customer who provided the solution this time: “couldn’t I use a 4-20 mA output and 250 ohm resistor to get my 1-5V output?” Why, yes….yes you could (smack…..duh!). And I know it will work, because we have the law on our side. Ohm’s Law, that is: E = IR, or voltage equals current x resistance.

Let’s check it:

4 (mA) x 250 (ohms) = 1 (volt)

20 (mA) x 250 (ohms) = 5 (volts)

So there you have it. Take a very common 4-20 mA output and drop it across a 250 ohm resistor and, lo and behold, you have your less common 1-5V signal. And, if you do this conversion right at the input to the controller, you get the added benefit of increased noise immunity of the 4-20 mA signal.

And, yes, I’m sure I knew of this little trick at one time. Maybe the part of my brain where this information was stored got overwritten by the names of the contestants on The Amazing Race or by the rollout plans for my million dollar consumer product idea: Dehydrated Water (just add water). But let’s keep that just between us, ok?

To learn more about analog feedback sensors visit www.balluff.us

Analog Inductive Sensors

In his post, When Do You Specify An Inductive Sensor?, Shawn Day (Market Manager, Inductive Sensors) discusses selection criteria and application for inductive proximity sensors.  In that article, Shawn focuses on what are sometimes referred to as discrete sensors – sensors that detect the presence of a metal target, and then turn on (or turn off).  As Shawn points out, there are many, many applications for this type of discrete sensing.

But what if just indicating the presence or absence of a part is not enough?  What if you need to know not only if a part is in a particular position or not, but rather you need to know exactly where the part is at any given point along its entire range of travel?  That’s where analog, or continuous, inductive position sensors come into play.

Analog inductive sensors employ basically the same technology as discrete proximity sensors.  That is, they use inductive coils to generate eddy currents that respond to a metal target.  But, unlike discrete sensors, analog inductive sensors provide a continuously variable output, not just an on/off change of state.

Tubular Analog Inductive Sensors

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Intelligent Interfaces and IO-Link Innovation

I recently had the opportunity to attend Hannover Fair in Germany and was blown away by the experience… buildings upon buildings of automation companies doing amazing things and helping us build our products faster, smarter and cheaper.  One shining topic for me at the fair was the continued growth of new products being developed with IO-Link communications in them.

All in all, the growth of IO-Link products is being driven by the need of customers to know more about their facility, their process and their production.  IO-Link devices are intelligent and utilize a master device to communicate their specific information over an industrial network back to the controller.  To learn more about IO-Link, read my previous entry, 5 Things You Need to Know about IO-Link.

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