This post continues our discussion of the various classifications of encoders identified previously

Form of Output

One of the most common classifications used for encoders is whether their architecture is incremental or absolute in design. This refers to the type of output the encoder emits, or what information is being provided by the encoder. This post will begin our discussion of incremental output and our future post will continue that discussion. Later, we'll have a discussion of absolute encoder output and make some comparisons between incremental and absolute encoders.

Incremental Output

The most common encoders are incremental encoders, for two main reasons. First, the information supplied by an incremental encoder is sufficient for most applications. The second reason is just a matter of economics and simplicity: it is much more cost effective to manufacture an incremental encoder than an absolute encoder.

A very rudimentary form of an incremental encoder is shown above. It only involves a disk with one slot, an LED, and a photo detector. The detector provides an output each time it "sees" the LED.

The first piece of information that can be determined by an incremental encoder's output is distance. As the above disk rotates, each time the slot is aligned with the LED and detector, the detector produces an output. The disk has rotated through an angle of 360 mechanical degrees. A controller can use this information to calculate distance traveled in a system.

The "encoder & controller" represented by the picture shown above is a reconstruction of an odometer made over 2,200 years ago. Each time the cart's wheels rotate, a pin on the axle engages a cog on a system of cogwheels that keep track of total distance traveled.

A second piece of information which can be determined by an incremental encoder's output is velocity, or speed of movement - the encoder essentially acts as a tachometer. As the disk in our first drawing above rotates, each time the slot is aligned with the LED and detector, the detector produces an output. The number of outputs in a minute would be the RPM or speed of the disk.

Although in the rudimentary drawing of an encoder at the beginning of this post you will be able to determine if a full rotation has taken place, for most of the 360 degrees, movement can take place without that movement being reported. The disk rotates without a change in output until the slot is reached.

To resolve that issue, a disk can be used with multiple slots as is shown in the picture above from a blog post by Aditya Prasad. Please note in this specific design, it would work as shown with an optical sensor or with a magnetic sensor as described earlier in the same post.

Now with 15 slots and 15 teeth, the optical sensor will provide an output while the disk turns through 12 mechanical degrees and a slot is in front of the sensor. During the next 12 degrees, a tooth will block the light, and the optical sensor will provide no output.

Most applications require an output when the movement is much less than 12 degrees so disks are made with much finer increments. The technical term used to define the size of increments used is resolution. We will discuss resolution in more detail in a future post but for now, we will define it as the number of outputs provided by the encoder based on the number of lines or windows on the disk.

To illustrate the resolution in everyday (or at least Thanksgiving Day) terms, think of resolution as the size of the pieces of a pie. Neither high resolution nor low resolution is better but the resolution should match the need. Speaking of pies, if one is extremely hungry, the pie on the left would be the best choice. However, if one is trying to limit their caloric intake, other than not eating the pie at all, a smaller piece would be the best choice.

All of the illustrations and examples have been focused on a single output as is further illustrated in the above drawing. The drawing is indicative of what that single output might look like on an oscilloscope. The bottom of the drawing represents an output of zero volts. The top of the drawing represents an output of five volts. The complete drawing is showing the changes in output or cycles as the disk rotates. When the sensor sees the LED, the output goes high (5 volts); when the disk via the line prevents the sensor from seeing the LED, the output goes low (0 volts).

One complete electrical cycle starts when the output goes high and ends just before it goes high again. One electrical cycle is 360 electrical degrees. For every mechanical revolution, the number of electrical cycles will be equivalent to the resolution or lines and windows on the disk.

The specification for resolution is CPR which stands for Cycles Per Revolution. An encoder with 512 CPR will have 512 lines and windows on the disk and, of course, produce a high and low output 512 times for each rotation of the disk.

NOTE: Unfortunately, there are some vendors who use the term CPR to mean counts per revolution which, depending on the vendor, that number can be twice as many or four times as many as the resolution. This will be discussed in more detail in a future post.

One drawback with incremental encoders is that with power cycling, there is no memory as to where the disk is, as all of the lines and windows on the disk are identical. If you have ever been lost, you know this feeling where your surroundings might look the same in every direction. Essentially the position of the disk in relation to the sensor is lost. We will show in our next post how we can use a search operation, like the search dogs shown above, to figure out where the disk is in its rotation.

Although we are able to calculate both distance moved and velocity from a single encoder output, one other piece of information provided by incremental encoders is direction of travel. Our future post will continue this discussion and explain how direction can be determined.

It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post

Source for photo detector graphic - reviseomatic.org
Tachometer image source - boschperformance.com
Image source for ancient odometer - commons.wikimedia.org
Source for slotted disk on motor - technlab.blogspot.com
Pumpkin pie image source #1 - finecooking.com
Pumpkin pie image source #2 - bettycrocker.com
Image source for search dog team - vsar.org

What do an organ, a black widow spider, and a scale have in common?

Encoders 008 - The Strange History of the Industries from which Encoders Evolved

It has always been intriguing to me how companies came into existence. US Digital's founding is an exclamation to the saying - "necessity is the mother of invention". David Madore, the founder of US Digital, worked at the time as a design engineer for a medical ultrasound imaging company. The equipment had many knobs on the front panel for potentiometers. The company wanted to upgrade the design to use optical encoders to improve operation. In doing research they could not find a good source suitable for the application. The ones they found were overpriced, had long lead times and poor specifications. In 1980, David Madore, made his first encoder to meet this need. And as they say, the rest is history.

BEI's history dates back to the 1800's. Many of us have heard of Baldwin pianos or organs but are unaware of how they fit in with encoders. Dwight Hamilton Baldwin was a minister and a singing teacher in schools who also opened a music store in Cincinnati, Ohio in 1862. Instead of just distributing the keyboard instruments, he set out to make "the best piano that could be built". World War II interrupted their operation as the company participated in making wings and other aircraft parts. A more detailed account of their history is available if you are interested.

After the war, Baldwin started using electronics in the development of keyboard instruments. The goal was to use this technology in an organ to replicate the sound of pipe organs in European cathedrals. The engineers at Baldwin came up with a way to use optically encoded glass discs to reproduce the organ tones. The codewheel transcribed the original organ tones into etched glass - in opaque and transparent segments so that when the disk turned, it created an alternating pattern of light and dark. Photodiodes were used to translate this into an electronic signal (sound familiar?) which was processed and amplified to create the tones and harmonics desired.

Sidebar - this development by Baldwin was not the first use of photo-electricity with a spinning glass disk to create musical tones. This was done as early as the 1920's in France, Austria, Russia, Germany, and the USA.

In 1951 the U.S. Army Signal Corp contracted with Baldwin to develop optical encoders, realizing that the company's technology could help in the pointing and tracking for radar antennas. In 1955 Baldwin made their first experimental optical encoder. In 1962, Baldwin's research resulted in an 18-bit optical encoder which was the first optical encoder used in space. The following year, they produced the first optical encoder with an LED light source which was used in space as was highlighted in our post, "Who Made the First Optical Encoder". That same year the electronics division was incorporated as Baldwin Electronics, Inc., hence the name, BEI.

The Gurley enterprise was established in 1845 but changed to W. & L. E. Gurley in 1852 as the brothers, William and Lewis, both engineering graduates of Prensselaer Polytechnic Institute in New York teamed up to create products with technical innovations. The Gurley brothers had many different interests but most related in one way or another to measuring things - from electrical current to pressure, to weights and distances and angles. In their factory the brothers created different departments, with each department making different components and then final assembly taking place in still another department after all components had been made.

The area of technology by Gurley which is of most interest to those of us in the encoder field is the surveying field and their designing of theodolites - an optical instrument used for measuring angles. The crosshairs used in the late 1800's for these surveying tools was the spider web filament from a black widow spider. The spider web filament was impregnated into the glass of their surveying instrument eyepieces. Gurley and other surveying equipment companies, actually had black widow spiders in their employment to provide them with the material they needed to create the crosshairs.

This technology was also used in the war effort in both world wars. One of the numerous industries developed in the US following the bombing of Pearl Harbor was the special defense plants or spider ranches supplying the spider silk for everything from bomber sights to periscopes and telescopes. The next time you think about complaining about your coworker, remember to be thankful it isn't a black widow spider.

Gurley was an early adopter of photolithography to transition from the use of spider webs. This new method provided for chrome patterns on the glass which not only was a superior method but provided a relief from the literally toxic work environment. This technology was offered as a service to others and in fact, according to Martin Gordinier of Gurley, they created the first encoder disk for Dr. Gray - famously known for the development of the Gray code used on encoder disks. After selling encoder disks to many firms, Gurley started producing their own encoders in the 1950's.

Wilhelm Heidenhain founded his company in 1889. It began as a metal etching factory. The company etched templates, signs, graduations, and scales. Heidenhain's enterprise was destroyed in World War II and the Dr. Johannes Heidenhain Company was founded in Traunreut by Wilhelm Heidenhain's son in 1948.

One revolutionary advancement was the development of the diadur process in 1950 which enabled them to apply very fine structures of chromium on glass. In 1952 Heidenhain used the diadur process to create optical position measuring devices for machine tools. That same year they introduced their first optical linear and angle encoders for machine tools. It was in 1961 that Heidenhain produced its first photoelectric incremental rotary encoder for position feedback (10,000 lines).

It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post

Organ picture source - 120years.net
Black widow spider image - nationalgeographic.com
Source for scale picture - heidenhain.com
Ultrasound machine image - ob-ultrasound.net
Glass disk imagery - 120years.net
Theodolite image - Etsy.com
Cross hair imagery source - surveyhistory.org
Source for bombsight image - masseyaero.org
Classic Baldwin Organ photo - patternsofink.blogspot.com

Encoders 007 - Encoders: Mechanical Configurations

We will continue in this post going through the various classification of encoders which were identified in my earlier post.

Mechanical Configurations

Mechanical configurations include differences based on whether the encoder comes with a shaft or will be installed onto an existing shaft. There are also different configurations based on the size of the motor, the required IP rating, and other environmental conditions.

Motor feedback encoders may contain their own bearings, or they may use an existing bearing set such as that found on the tail shaft of a servo motor. The best configuration option to use is a function of the stability of the shaft/bearings to which the encoder is attached. Feedback encoders with bearings are typically used when the application shaft has a significant amount of axial or radial run out (eccentricity or vibration). The use of a shafted encoder with a motor will require some sort of flexible member, either a flexible shaft coupling or flexible body mount, to allow mechanical compliance with the application shaft operating irregularities.

Modular encoders, also referred to as encoder kits, don’t contain their own internal shaft. They are assembled from components supplied by the encoder manufacturer and are designed to be attached to the tail shaft and end bell of the motor. These encoders rely on a mechanically stable motor shaft, as the shaft is responsible for holding the encoder’s internal rotating code wheel in a precise location relative to the encoder’s sensing module. For these applications, motor manufacturers put a considerable amount of effort in designing high-performance motors with very stable shaft/bearing assemblies. Because the modular design does not add the expense of the extra set of bearings that a motor feedback encoder does, modular encoders offer one of the most cost effective feedback solutions.

When using a kit style encoder, another mechanical consideration is the size of the motor. Most encoder manufacturers supply a motor/encoder compatibility chart based on the motor size. It is recommended to use such a chart, like the one below, to help identify the appropriate encoder model.

Another mechanical configuration relates to environmental conditions. If the encoder will be installed in an environment that is subject to excessive dust and/or moisture, encoders are available with various IP ratings to meet those requirements. Magnetic and capacitive encoders would also be a potential candidate to meet some environmental concerns.

Finally, there are many applications of encoders where they are inherently protected from physical contact. However, other encoders are located where they might be contacted by a person or object, therefore, requiring more physical protection. The above housing is machined from solid aluminum to provide the kind of protection needed in such an application.

If you are afraid of spiders, you might want to skip our next post which is scheduled for May 28th.

It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post

NASA doesn't just make spaceships!

Who Made the First Optical Encoder?

I have been researching the answer to that question for some time and although I have been able to gather some great information both from encoder manufacturers and my own research, at this point I do not have a definitive answer.

When I initially tried searching for an answer, someone had answered the question online stating that the first optical encoder was made by David Cronin in 1964 - based on the date of his patent application (seen above).

First Encoder Used in Space

According to chapter 14 on encoders in Space Vehicle Mechanisms: Elements of Successful Design (A textbook that has been used by NASA), Tim Malcolm states that optical encoders were developed about 1951 to address a need for higher resolutions than what was available with other kinds of sensors. In 1958 Baldwin Electronics (now BEI Precision Systems and Space Company, Inc.) provided 18-bit optical encoders which were used in the Atlas missile guidance system. Those encoders were still functioning 36 years later when that article was written.

Back at the beginning, the life of the light source limited the encoder life. Many illumination sources were used based on the technology available at the time. Some light sources used include xenon flash tubes, incandescent lamps, and neon lamps. The first encoder used in space incorporated a redundant gas-filled incandescent light source but it only had a life requirement of 12,000 hours.

First LED Optical Encoder Used in Space

LED's were a big step forward by providing extended lifetimes with very little degradation. You may have heard a statement that optical encoders have reduced life due to dimming or burning out of the LED. Statements to that effect are contradicted by the chief scientist at BEI. In this same chapter, Tim Malcolm states as of the writing of the chapter (1996), "the concerns with the quality and life of these devices (LED's) which were common years ago have largely disappeared." He also noted that some encoder manufacturers had more than 10 years of continuous use with no encoder failures. Based on those statements, it would be safe to conclude that if your encoder supplier is using high-quality LED's, this should not be an issue.

I was fortunate enough to be able to get in touch with Timothy Malcolm and found out that the first optical encoders were actually made near the end of WWII. Although war is very costly, it has always been a great driver for the the development of new technologies. I will plan to share more about that in our post on May 28th.

Heidenhain's Digital Optical Measuring Instrument

In a future post, we will provide information on how several of the oldest encoder companies came into being. The above photograph shows an optical counter made by Heidenhain, in the early 1960's, for accurate, reliable and easy positioning of slides and carriages on machine tools, gauges, and other instruments. It could actually measure measurements as small at 0.0001" - which is remarkable for that time period.

Encoders have been a key ingredient to space exploration in general from its inception. This is definitely true for NASA. I recommend checking out NASA’s website and articles like this one which mentions how they are used on the International Space Station on cameras to view earth. Here is also a video which gives some background on Valkyrie, the humanoid robot at the top of this post.

It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post

Heidenhaim DOM Source - Heidenhain.de
Early space encoder pictures were provided by Timothy Malcolm.

Encoders Have Been Used in Space Exploration for Almost 70 Years

In our March 12th post, we identified different ways by which encoders can be categorized. In that post we focused on the first category - type of movement which encoders are able to monitor. Today we will discuss the second category: sensing technology used in most encoders.

Note: The descriptions and drawings in this post are intentionally simplified to provide an overview of the technologies used in these three kinds of encoders. Future posts will delve more deeply into actual construction and function of encoders and address more of the advantages of each

Optical encoders have been the most common encoder technology used for many years. To this day they provide the highest levels of precision, accuracy and resolution. Rotary encoders use an optical sensor to detect light that is transmitted through a disk (transmissive) or reflected from a disk (reflective). The disk is also sometimes called a code wheel. The transmissive optical encoder disk has alternating transparent and opaque lines. When the light is received by the sensor, the encoder puts out a high signal. Conversely, when the light is blocked by a line on the code wheel, the sensor puts out a low signal. On a reflective encoder disk the alternating lines are reflective and non-reflective. With a known pattern on the disk, the distance moved and speed of movement can be obtained using the signal information. Linear encoders use the same method with the only difference being a strip is used in place of rotary disk.

Magnetic encoders use code wheels with alternating magnetic poles or unique patterns distributed around the wheel according to the resolution required. A magnetic sensor in the encoder detects the change in the magnetic field as the wheel rotates and produces a digital pulse train. Magnetic encoders have an advantage over optical encoders in that they are can be used in areas which have higher humidity, dirt and dust. Magnetic encoders may also operate in various fluid environments. Magnetic encoders use less power than their optical counterparts, but typically do not provide the same resolution or positional accuracy as optical encoders due to inherent non-linearities in the magnetic field.

Use of a magnetic encoder may be preferable to an optical encoder when there is a chance for an optical disk to become fogged as moisture condenses on the code wheel. Consider an application where an optical encoder is held at a very low temperature and then the ambient temperature quickly increases. This quick temperature change can cause condensation on all surfaces of the encoder, including the optical code wheel. When the code wheel surface collects droplets of moisture, the light transfer of the code wheel image to the optical sensor can become distorted and a false or missing signal may occur on the output. With a magnetic encoder design, condensation of moisture is not an issue with the rotating magnet and magnetic sensor.

Capacitive encoders are a newer technology but inherently possess all of the same environmental advantages as the magnetic encoders. A capacitive encoder detects the changes in capacitance using a high frequency reference signal. The rotor has either a pattern etched into it or uses a specially shaped design as in the above drawings. When this rotor moves between the transmitter and receiver, that pattern modulates the high frequency signal of the transmitter. The receiver reads the modulations and those changes are translated into increments of rotary motion. Although capacitive encoders can be more susceptible to noise and electrical interference—the manufacturer of the encoder can mitigate that potential issue with appropriate engineering. Another benefit of using a capacitive encoder is the typically lower current draw - 10 milliamps is not uncommon.

There are advantages and disadvantages of each technology used. My suggestion is to determine your requirements and then compare specifications to ensure the encoder you are looking at will meet your needs. In a future post we will discuss how to choose an encoder. If you have any additional questions, please feel free to contact me via email or LinkedIn.

Have you ever seen the first encoder used in space? We will have that and more in our next post.

It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post

Mars Lander picture source - Space.com