encoder/resolver rabbit hole - Control Design

eHANDBOOK

HOW TO NAVIGATE THEencoder/resolver

rabbit hole

For shaft speed and position control, the sensing options are a rabbit hole. Decid-

ing between an encoder and a resolver is simple enough, but it’s just the first step.

Then, there’s the difference between an incremental encoder and an absolute en-

coder. If an incremental rotary encoder is appropriate for the application, there are multiple

choices with optical, magnetic and capacitive. Our panel of experts discusses which encod-

er or resolver to use when and more.

Why are so many different types of solutions available, and is there a checklist that can guide machine builders and help controls engineers to navigate the rabbit hole to find the right solution?

MAHENDRA MULI director of marketing and new business development, dSPACE

(www.dspaceinc.com):

Sensor technology has advanced significantly to generate a wide portfo-

lio of solutions that could match every specific requirement. However, often the speci-

fication of the sensor alone may not be sufficient for engineers to answer if a particular

sensor type, other than the physical attributes, will completely match the requirements

of the application.

The encoder/resolver rabbit holeThe choices seem to go on forever, so when is it best to decide how to manage shaft speed or position control

By Mike Bacidore, editor in chief

www.controldesign.com

eHANDBOOK: Encoders/Resolvers 2

With increasing use of embedded control

systems, the applications can deliver a very

precise and very reliable performance. The

faster and cheaper microcontrollers, pro-

cessors and FPGAs are often used with

complex control system software.

So, how do the engineers choose the sensors

and verify the overall system performance

with a specific sensor before committing to

it? The answer lies in simulation. By simulat-

ing the sensor signals and the overall soft-

ware, engineers can ensure that the system

meets the performance characteristics. Addi-

tionally, through simulation, engineers can

also simulate abnormal behavior and errors

without even having the physical sensors.

DSPACE, in addition to our rapid control-

ler prototyping systems, also provides

hardware to support control system de-

velopment by simulation of both physical

system as well as all of the sensors in the

system. With such a system, referred to as

hardware-in-the-loop (HIL), engineers can

simulate the dynamic behavior of the me-

chanical components of the system, includ-

ing all of the sensors for interfacing to the

controller board. The dynamic behavior of

the mechanical device can be programmed

to simulate real-life normal and abnormal

conditions, as well as failure conditions in

the system.

Tetra Pak used such technology to advance

its milk-packing machine technology (www.

controldesign.com/tetrapak). This approach

for sensor selection and validation of per-

formance can help to reduce risk in the se-

lection of devices for applications (Figure 1).

IVAN MASEK president, Novotechnik U.S.

(www.novotechnik.com):

The reason for so many

types of rotary sensors is the fact that

there are many different applications to

cover regarding precision, mechanical

requirements and electrical output options

plus the different environments. As the

sensor market is huge, it makes a lot of

sense to build exactly what is needed for a

certain application and provide the opti-

mum in cost and performance. In many,

but far from all, cases this requires custom-

ized versions of standard products.



TETRA PAK BEVERAGE-FILLING MACHINEFigure 1: The Tetra Pak simulation environment effectively supports the development of control software for beverage-filling machines long be-fore the first prototype is constructed.(Source: dSPACE)

Use absolute position sensors in applica-

tions where you need to know the position

exactly after a power outage and cannot

move back to an index switch. In most other

applications, incremental can be a more

cost-efficient way to go. For ultra-high pre-

cision in a controlled manufacturing envi-

ronment, use optical encoders, though for

many applications this degree of precision

is not needed.

I recommend talking to an application engi-

neer from a manufacturer that shows appli-

cations similar to yours shown on his ap-

plication Web pages. This person can guide

you very quickly to a tailored solution by

asking detailed questions on performance

and environmental requirements as well as

size and cost.

JONATHAN DOUGHERTY product specialist, Heiden-

hain (www.heidenhain.com):

As the world becomes more and more auto-

mated, there has been a higher demand for

feedback devices suited for a variety of

applications. In order to meet these de-

mands, many encoder companies have

started offering a variety of solutions to

cover all possible applications, which has

produced the introduction of a variety of

scanning technologies and features specifi-

cally suited to handle different environ-

ments. However, the factors for determining

which encoder to use largely remain the

same. Establishing what accuracy you need,

the environmental conditions the encoder

must endure, such as vibration, contamina-

tion and temperature, and the mechanical

restrictions will go a long way toward

narrowing down the right product for you.

And of course, if you ever have any ques-

tions, we always encourage customers to

contact us so that we can help to answer

any of their questions and guide them to

the most effective solution.

CHIP MCDANIEL technical marketing,

AutomationDirect (www.

automationdirect.com): An

encoder is the popular choice for shaft

speed and position control. While a resolver

is popular for motor commutation and is a

robust option, in a servo motor, for exam-

ple, the encoder wins the popularity con-

test. There are a variety of encoder tech-

nologies, as well, but the optical encoder is

the popular choice. Here is a guide to pick

the right product.

Selection of an incremental or absolute

encoder output is application-specific and

often hardware-specific. Incremental en-

coders provide count and direction signals

and will work with high-speed PLC counter

modules for accurate position monitoring

and control. The position signal provided is

relative to a zero position where the coun-

ter was reset or powered on. Some absolute

encoders monitor position with gray code



and standard PLC dc inputs. The absolute

position is retentive—it retains its position

data even after a power-off-to-on cycle.

Once the type of encoder—incremental or

absolute—and resolution are selected, the

housing type —duty-level, ingress rating—

must be selected. This is often called duty

level. This duty level includes light, medium

and heavy duty and includes a dustproof

IP40 or IP50 rating to splash-proof IP65 rat-

ing depending on housing construction.

After encoder resolution and housing type

have been determined, users must select

their preferred operating voltage and out-

put signals that they would prefer from the

various options.

The type of output must be carefully select-

ed. Line driver, differential, and NPN open

collector are probably the most common. A

line driver output refers to the fact that each

channel has a complement channel—Chan-

nel A and Channel A not. A differential line

driver is used to help increase noise im-

munity and works with either a sinking or

sourcing circuit. An open collector output is

an NPN transistor. An NPN transistor allows

the sinking of current to common. A voltage

supply through a load must be connected

to the output. In this case, a pull-up resistor

must be used. It is the load which pulls the

encoder output high when the output is off.

The controller input monitoring the encoder

open-collector output must sink the voltage

to the same common. The encoder output

voltages are wide ranging, with typical val-

ues of 12-24 Vdc, 5-30 Vdc and 10-30 Vdc.

NEIL BURGARD electric automation business

development specialist,

Festo (www.festo.com/us):

I recommend using an encoder over a

resolver. It’s smaller and lighter, a more

intelligent device for great flexibility and

capability, has a larger selection of shapes

and sizes, higher resolution for greater

accuracy and improved dynamics, is com-

patible with most controls and has a greater

diversity of network interfaces, as well as

lower cost and better availability.

Encoders are better in every way except

for when temperatures fall within 125 °C

to 220 °C, vibrations range from 20 G to

100 G, or if that is the only interface your

controls will accept.

The first step in selecting any encoder is to

figure out whether you need to know the

position as soon as power is applied. If you

do, then you require an absolute encoder.

Even if you don’t, I would still recommend

an absolute encoder over incremental

because absolute encoders offer higher

resolutions, faster speeds, better immunity

to noise and fewer wires—in some cases a

reduction from 14 wires to two). Start with

optical encoders because they produce the

highest resolutions, the best accuracy, fast



speeds and the most options for voltage,

outputs, mounting, size, communications,

safety, cabling and vendors. Magnetic,

capacitive and inductive encoders have

a larger temperature range and are bet-

ter for shock, vibration and dust. Magnetic

and inductive encoders work best with

moisture and fluids. Magnetic are the most

tolerant of misalignment and can be very

low cost. The downfall is that they offer

lower resolutions and accuracy. Inductive

encoders are even more robust than mag-

netic and provide higher resolutions and

accuracy, but they cost more and require

tighter tolerances. Capacitive encoders

also offer higher resolutions and accuracy

and are lower cost, but they don’t operate

well with moisture and fluids. Capacitive

are also the best choice for battery appli-

cations because they use little power, or if

you need to change the resolution.

SCOTT ORLOSKY global market

segment manager,

Sensata Technologies

(www.sensata.com): I can understand the

frustration and confusion when confronted

with so many options. First you will need to

decide if you need to know absolute posi-

tion, even if the power is turned off and then

turned back on again. If so, you will need to

choose an absolute type of product. Most

resolvers will inherently provide an absolute

position. Optical encoders can be absolute

or incremental. If you only require position

information relative to an index—provided

once per revolution with an incremental

encoder—and your primary concern is speed

control, then an incremental solution would

be the answer. Also, if your application can

use a homing sequence after power shut-

down without any problem, then an incre-

mental solution would work there, as well.

One other consideration is that absolute so-

lutions are generally more complex and ex-

pensive to install than incremental. Beyond

that, it depends on how much accuracy you

need in your application. Do not confuse

resolution with accuracy. High resolution

does not mean high accuracy. Designers

often assume that a higher resolution will

result in better performance of the sys-

tem. The reality is that higher accuracy and

repeatability are more important. Optical

encoders are the most accurate. Next are

resolvers and magnetic encoders. There are

other types of encoders, but they are not as

widely used, generally do not have as large

of an installed base and do not appear to be

any more accurate than optical encoders,

magnetic encoders or resolvers.

KATE SOKOLNICKI product marketing

manager for encoders,

Rockwell Automation

(www.rockwellautomation.com): When

choosing encoders, I recommend creating a

two-part checklist: electrical considerations

and mechanical considerations. With sens-



ing technology—optical versus magnetic,

for example—technological advances lessen

the gap for many applications. Optical

sensors used to be recommended as higher

resolution, and, within that, glass code disks

recommended over metal or plastic, but

increasingly the same resolution can be

achieved in disparate sensing technologies.

Before selecting electrical specifications,

consider the maintenance of position after

power cycle or if the application requires

absolute position. Because they retain posi-

tion even after power loss, absolute encod-

ers tend to be more expensive. If this is not

required, choosing an incremental encoder

is recommended. For an incremental en-

coder, the user need only select the resolu-

tion—pulses per revolution—supply voltage,

output voltage and output signal type—

HTL, TTL, open collector or push-pull.

Absolute encoders offer more choices, with

both single-turn—pulses per revolution—

and multi-turn resolution—total number

of revolutions—options to select. Further,

many absolute encoders are configurable

over a network, such as EtherNet/IP or

DeviceNet. This allows the user to select the

exact resolution or to change the resolution

depending on the application.

Both incremental and absolute encoders

have the same mechanical considerations.

What size housing do you need? Do you

prefer a cable or connector? What size

shaft will the encoder be coupled to? Would

you prefer a hollow shaft or solid shaft?

Hollow-shaft options typically require a sep-

arate tether for mounting, which may have

a few options, while a solid shaft encoder

requires a flexible coupling.

TERRY HADDOCK senior applications engi-

neer, Encoder Products

(www.encoder.com):

There are many types of encoder solutions,

and each has its own benefits. Optical

encoders typically are capable of higher

resolution and accuracy. Magnetic encod-

ers are more resistant to environmental

contamination. Capacitive encoders are

also resistant to contamination, while offer-

ing less power consumption.

While these differences are very important

to encoder selection, there are also several

more factors when it comes to selecting

any electronic device: quality/reliability,

availability/delivery and support. In many

encoder applications, any type of encod-

er—optical, magnetic or capacitive—can

be used without any noticeable difference.

I wish there was a simple checklist to as-

sist with this complex decision. Due to the

many types of encoders and many different

performance specifications I don’t believe

this checklist will ever be simple. I believe

that selecting a supplier who can help you

through this process may be just as impor-

tant as the checklist itself.



MARTY CWACH product manager, sensors,

Turck (www.turck.us):

Having a quick checklist

would be helpful, but the applications

would have to be very clearly defined, as

well. Almost always, the applications are not

clear and there are multiple variables to

solve simultaneously. This is the reason for

the variety of technologies, packages and

configurations.

ROBERT MILLER director, advanced testing

group, IVC Technologies

(www.ivctechnologies.

com): There are so many options because

there are so many individual uses and

applications for specific encoders and

resolvers. Many manufacturers provide very

good selection tools and aids to guide the

customer in the right direction, but that is

based on the assumption that the customer

really knows both what he wants and what

he needs. It is also important to keep in

mind the future needs, as opposed to

simply focusing on the immediate needs to

satisfy a performance spec. For example,

when given the choice of getting an encod-

er with or without an absolute phase mark-

er, I will always opt for getting the encoder

with multi-pulse and z-marker pulse out-

puts. I may only need the marker pulse for

basic vibration and process analysis appli-

cations; however the multi-pulse output

gives me the option for much more ad-

vanced analysis and diagnostic methods, if

and when they are needed. If this isn’t taken

into consideration at the time the encoder

in question is specified and procured, then

it’s too late when the problem that requires

this type of data and this type of analysis

occurs and you don’t have it.

With process data acquisition (PDA) sys-

tems becoming more and more prevalent, it

is also good to make provisions for redun-

dant outputs so one set of signals can be

dedicated for controls only, and one set of

outputs is strictly for process data analytics.

MARK LANGILLE technology planner, Dy-

napar (www.dynapar.com):

This is a complex topic, but

some of the high-level concerns involve

choosing between incremental single

channel and incremental quadrature or

absolute vs. resolver feedback. This chart

has quick guidelines (Figure 2, next page).

CHRISTIAN FELL COO, Posital-Fraba

(www.posital.com): Resolv-

ers can be very useful. They

are physically robust, relatively inexpensive

and have a wide temperature range. How-

ever, they also have significant limitations.

Resolvers are analog devices which require

an A/D converter in the controller interface.

They are also relatively low-accuracy devices

and do not have multi-turn capabilities.



Encoders used to be more expensive but

recent developments in magnetic mea-

surement technologies have made them

more competitive with resolvers. Encod-

ers can offer very high levels of resolution,

accuracy and dynamic response, which

make them suitable for almost all applica-

tions. Incremental encoders can be a good

choice for speed control applications, while

absolute encoders offer advantages for

position control applications. Multi-turn

absolute encoders can also be very useful

for motion control systems with extended

ranges of motion.

The wide range of encoder options, while

confusing at the first glance, is an advan-

tage for users, since it makes it possible to

obtain a device that meets an application’s

full gamut of requirements. This included

performance specs (resolution, dynamic

response and measurement range) and

extends to system integration needs such

as preferred communication interface, level

of environmental protection and mechani-

cal interface features. We recommend that

users begin by defining their full require-

ments and then go to the market to find

no-compromise solutions. Product-finder

tools are designed to allow users to select

the functional characteristics that they

need and then zero in on the products that

meet their requirements.



Incremental Absolute

Single-channel Quadrature

Complexity Simpler More complex

OutputSpeed,

displacementVelocity and

directionVelocity and

absolute position

Needs homing on startup? Yes No

Resolution Up to 10k PPR (direct read)Up to 22 bit (ST)/12 bit

(MT)

Communication via protocol? No No Yes

Cost Generally lower Generally higher

Resolution Accuracy IP Rating Shock Vibration

Optical Incremental 10,000 PPR1 Up to 2.5 arc-min IP67 Up to 400 G 20 G

Magnetic Incremental 4,096 PPR N/A3 IP67 Up to 200 G 18 G

Optical Absolute 22-bit CPR2 36 arc-sec IP67 100 G 10 G

Magnetic Absolute 4,096 PPR 0.6º IP69K Up to 200 G 20 G

Resolver 2 arc-min IP654 200 G5 40 G

HOW TO CHOOSE AN ENCODER/RESOLVERFigure 2: Some of the high-level concerns involve choosing between incremental single channel and incremental quadrature or absolute vs. resolver feedback.(Source: Dynapar)

For shaft speed and position control, the sensing options are almost limitless. Just as

infinite are the possibilities of applications that resolvers and encoders can be used

for, some of them more interesting than others. Our panel of experts shares some of

the more interesting applications they’ve encountered.

Can you share details about the most interesting or innovative use of an encoder or a resolver that you’ve ever seen?

KATE SOKOLNICKI product marketing manager for encoders, Rockwell Automation

(www.rockwellautomation.com): Some of the more interesting use cases

I’ve seen are for monitoring speed of a vehicle, either on a track for enter-

tainment applications or autonomous for AGVs.

NEIL BURGARD electric automation business development specialist,

Festo (www.festo.com/us): Using an inductive encoder on top

of a profile bearing rail is unusual.

11 most interesting encoder/resolver applicationsThe options seem unlimited and so are the ways encoders and resolvers are being used; here are 11 that are noteworthy






RICHARD HALSTEAD president, Empire Magnet-

ics (www.empiremagnetics.

com): Beam lines are inter-

esting. The need for centralized data over a

large facility, while at the same time sup-

porting closed-loop functions, makes it

technically challenging.

Research centers are now built around

beam lines. Typically beam lines are in a

vacuum, and many produce radiation. In

addition, the facilities are large. Motion

systems that are involved with the control

and direction of the beam are typically

high-precision and require feedback to

provide the desired functions. One exam-

ple facility is the Advanced Light Source

(ALS) at Lawrence Berkeley National Lab

managed by the University of California.

To address the requirements of motors

and feedback that operate in vacuum

while subject to radiation dose, Empire

Magnetics provides stepper motors with

resolver feedback. Micro-stepping drives

combine with 200 step motors to provide

resolutions of 25,000, 36,000 and 50,000

counts per revolution. Such resolution

supports submicron positioning of devices

around the facility.

However, these microstep motor drives

are not typically resolver-compatible, and

encoders are not typically suitable for the

environment; in addition the facility opera-

tors would like to have information in their

centralized control room, which can be a

significant distance from the device. To

accommodate these requirements, Empire

Magnetics designed a set of electronics

that reads the resolver information, makes

it available in incremental encoder format

compatible with closed-loop drives, but

also makes positional information available

over Ethernet. Each of the products has

customer set hardware addresses, so many

units can be on one network. This approach

allows for the information to be presented

in the control room, but the use of Ethernet

avoids the cost of multiple cable runs.

Microstep motor drives are not typically resolver-compatible, and encoders are not

typically suitable for the environment.




(www.posital.com): No

single example comes to

mind, just the sheer spectrum of applica-

tions, from medical to off-highway and from

simple speed control to complex multi-axis

positioning. Also, declining costs and the

rise of IoT are driving the trend toward the

use of such sensors in more applications.

JONATHAN DOUGHERTY product specialist,

Heidenhain (www.heiden-

hain.com): As a company providing feed-

back, we are in a lot of interesting and

emerging technologies that are exciting to

see, such as collaborative robots, automat-

ed vehicles, and smart factories but one of

the coolest and most satisfying projects we

have been a part of is with a team develop-

ing an exoskeleton suit to assist paraplegics

to walk again. Beyond just the appreciation

for the amount of intelligence and technol-

ogy involved in this endeavor, it feels

rewarding to be a part of such a project,

which can help aid a person to walk again.

CHIP MCDANIEL technical marketing, Auto-

mationDirect (www.automa-

tiondirect.com): Check out

the video, “Triple Pendulum on a Cart,” (www.

controldesign.com/triplependulum). This is

my favorite application for encoder and one

of my favorite Youtube videos of all time. I

think anyone with some technical knowledge

will realize just how hard this is from a con-

trols standpoint. Although it is not really in

the industrial arena, there are industrial

applications where this type of positioning

technology can be used to great advantage.

The following is a quote from Andreas

Eder’s and Tobias Glück’s scientific paper

on the project (www.controldesign.com/tri-

plependulumpaper). “Incremental encoders

at each joint measure the different angles

between two adjacent arms and between

the first arm and the cart, respectively, with

a resolution of 4.395 × 10−2°. In order to

keep the friction low, the signals are trans-

mitted contactless via an optical connection

from the joints to the control unit.”

I’m reminded of Arthur C Clarke’s Third

Law, “Any sufficiently advanced technology

is indistinguishable from magic.”

MAHENDRA MULI director of marketing and

new business development,

dSPACE (www.dspaceinc.

com): We often see our customers do

amazing innovative control system applica-

tions using the encoder or resolver sensors

across the industries, from automotive and

aerospace to commercial on- and off-high-

way vehicles and medical devices.

Cleveland State University recently de-

signed a dynamic, adaptable exercise ma-

chine for enhanced physical training using



motors and control systems to generate

continuously adjustable mechanical imped-

ances for better user interactivity. More

information can be found at www.contr-

oldesign.com/smarttraining (Figure 1).



Some of the coolest applica-

tions for sensors we have worked on very

successfully in the past are the lean-steering

of the Segway i2, the wakeboard wave-con-

trol-surface angle measurement on pleasure

crafts from Malibu Boats and the position

measurement of the visitor glass platform

on the Willis Tower in Chicago. They are all

featured at www.controldesign.com/novo-

technikapplications.



(www.encoder.com): One

interesting encoder application involved

“Wheel of Fortune.” The plan was to incor-

porate high resolution screens in place of the

cardboard cutouts placed on the wheel. An

encoder would be used to track position and

allow the show to know on which prize the

spin had landed. This would allow for more

involved prize images and quicker change-

over during commercials. This design was

quickly scrapped, as producers were con-

cerned that they would lose viewers if any

suspicion arose that the game was rigged.

SCOTT ORLOSKY global market segment

manager, Sensata Technol-

ogies (www.sensata.com):

Perhaps the most visually stunning are

these elaborate stage acts, with lots of

moving stage platforms, traveling trapeze

artists, sets that spring up out of nowhere

and disappear and so on. They really are

quite spectacular to watch and rely heavily

on a carefully choreographed set of ma-

chine instructions and feedback devices

working with split-second timing.

EMG DATAFigure 1: During a trial, a control-system development platform collects 16-channel electromyography (EMG) data and machine mechanical data at a sampling rate of 1 kHz. A separate system collects motion data using visual markers (shiny spots) and metabolic data. Data from all systems is synchronized by off-line post-processing.(Source: dSPACE)



MARTY CWACH product manager, sensors,

Turck (www.turck.us): Turck

helped to provide sensor

selection and support for position feedback

on vehicles used to gather data to map

streets/roads, which is ultimately used in

navigation software. The customer had

difficulty with longevity of the encoders, as

they were applied to the wheel of the

vehicle. The encoders happened to be

failing during the vehicle’s cleaning, or car

wash, operation. Once we saw photos of

the installation, we could quickly under-

stand that the selection and placement of

the encoder needed to be reviewed to help

to prevent premature failure. The original

encoder did not have adequate ingress

protection and the cable routing allowed for

a drip line/water path directly into the

electrical connection of the encoder. We

improved the encoder life by offering a

solution with higher IP and recommended a

drip loop for the connection cable.

The engineer involved had said that he was

given a set of electrical requirements that

the original encoder satisfied very well.

Even when testing the vehicle for the first

few weeks, all was well with the original de-

sign, as it had not rained and there was no

reason to wash the vehicle. As soon as the

customer had a few vehicles wrapped with

logos/decals and advertising, they would

wash it nearly every day to keep it clean

and presentable. Those operations and

requirements to survive washdown changed

the durability requirements of many sensors

from the original specification.




com): The most interesting application that

I have been involved in is torsional analysis

using encoders. Traditionally it is necessary

to mount strain-gage-based torque telem-

etry on rotating shafts of a drive assembly

to observe and measure the torsional

characteristics. This often requires signifi-

cant downtime, and, because the transmit-

ters are often battery-powered, thus the

effective measurement window can be

extremely limited. In contrast, by taking the

encoder pulse train and running it through a

torsional frequency-demodulation routine

available in off-the-shelf packages such as

ibaRotate, the torsional frequencies within

the range of the specific encoder can be

evaluated at any time during operation

without requiring downtime. In addition,

since the encoder pulse count effectively

becomes the sampling rate for spectrum

analysis in the angular domain; the higher

the pulse count, the higher the torsional

frequency range that can be evaluated.

If pulses can be obtained from both ends

of a given shaft, you can use variations of

these same routines to look at shaft twist/

windup, from which an approximation of



the actual shaft torque can also be derived.

Simply put, these two routines allow not

only the evaluation of conditions such as

torsional resonances, but also the torsional

influence of key mechanical components

such as couplings, bearings and gearing.

This is in comparison to merely looking at

the shaft rotation speed. There also exist

calibration routines that allow for the usage

of low-cost encoders with uneven/jittery

angular spacing.



Dynapar produced large

bore magnetic encoders for an engineering

services company that installed cables on

the largest suspension bridges in the world.

The encoder wheel adapter IDs were 6

inches in diameter. It is always impressive to

see how the mechanical power of motor,

gearbox and electronic control can be

synchronized together to move mountains

or in this case cross them. The encoder

provided the link between the mechanical

motion and the control.

The output from an encoder needs to match the input of the controller, but don’t

be fooled by some of the tricky hurdles. Resolution has some stumbling blocks of

its own, as well. Our panel of experts makes you aware of the potential hazards to

keep your eyes peeled for.

When matching the output to the controller’s input, what tricky hurdles do machine builders sometimes overlook? And what about resolution—any potential stumbling blocks to note?

CHRISTIAN FELL COO, Posital-Fraba (www.posital.com): Not all interfaces are well-standard-

ized. This can be especially tricky in the MRO business when supposedly

“drop-in” substitutes turn out to be incompatible.

When it comes to resolution we often see that products are overspecced by the customer—

for example, 16-bit ST for a concrete boom pump. To keep costs low, encoder performance

should meet a realistic set of mechanical and performance requirements of the application.

Potential problems await your encoder/resolver implementationWhat to look for and how to avoid them when designing machine that in-cludes encoders or resolvers






JONATHAN DOUGHERTY product specialist, Heiden-

hain (www.heidenhain.com):

One major hurdle we see is when machine

builders try to choose from the many

different encoder protocols available on the

market today. Often many machine builders

aren’t aware of all the different terminology

used by different encoder companies, and it

can become quite confusing when looking

through protocol options. The best way to

solve this is to have direct conversations

with your encoder suppliers and push for

apples-to-apples comparisons on the

specifications required.



Regarding the use of poten-

tiometer-based sensors on, for example,

injection-molding machines, presses, valve

control and dancer arm applications, it is

important to use high-impedance inputs:

greater than 10 MegOhm on the PLC.

Use shielded cables on potentiometers and

Hall-effect sensors to minimize noise and con-

nect the shield only to measurement ground

on the PLC, and do not connect the shield to

the sensor housing. This will prevent ground

loops and ensure a noise-free signal.

A common error is mistaking resolution for

accuracy and repeatability. For processes

that must come to the same position re-

petitively—for example, indexing tables,

assembly processes—repeatability is gen-

erally the more important consideration.

For the same sensor output, how close are

you to arriving at the same position every

time? Higher resolution does not always

mean higher repeatability.

Specific interfaces have their own unique

potential stumbling blocks. Serial periph-

eral interface (SPI), synchronous serial

interface (SSI) and CANopen each has its

own specific considerations. In general,

take care to make sure the manufacturer’s

device matches the requirements of the

master controller.

Use shielded cables on potentiometers and Hall-effect sensors to minimize noise

and connect the shield only to measurement ground on the PLC, and do not connect the

shield to the sensor housing.



Avoid installing sensor wiring in the same

conduits as power lines for other devices,

such as motors, to avoid noise problems.

TERRY HADDOCK senior applications engineer,

Encoder Products (www.

encoder.com): Matching the

encoder output to controller input can be a

hurdle. Most of the new PLCs, high-speed

counters and controllers have the ability to

accept many different encoder outputs. The

tricky part is configuring the device to match

the encoder. Referring to the manual or

contacting the controller manufacturer can

help to configure the device properly.

If a user does not know which encoder out-

put to select, I recommend differential line

driver. The encoder output line driver IC has

the ability to either sink or source current,

as well as supply differential signals. This

gives the user the most options for match-

ing the controller input.

Encoder resolution is extremely important

to system control. In a perfect system,

higher resolution gives you tighter control.

Unfortunately, no system is perfect. It is im-

portant to know the maximum error within

a system before selecting encoder resolu-

tion. Ideally a user will select a resolution

slightly better than the system error. Select-

ing a resolution that is much larger than the

system error will cost more and not result in

improved control.

SCOTT ORLOSKY global market segment

manager, Sensata Technolo-

gies (www.sensata.com):

Designers often try to push for higher resolu-

tions, but there is a trade-off between

resolution and speed. Also, at some point in

increasing resolution, you reach the accuracy

limit of the device and further increases to

resolution are counterproductive. Look at all

of the physical errors in your system—wind-

up, backlash, deflection, timing, propagation

delays in control—to figure your total error

budget and use that to determine the accu-

racy your feedback device will require. Pick

your resolution accordingly.

NEIL BURGARD electric automation

business development

specialist, Festo

(www.festo.com/us): For incremental

encoders, you need to make sure the

pulses from the encoder don’t exceed the

max frequency of your encoder input.

For very long cable runs you need to make

sure you don’t have too much voltage drop.

Watch out for electrical noise on your incre-

mental encoders. This can cause havoc with

your controller. Make sure your cable shields

are grounded, and keep the encoder lines

away from things like motor power lines.

Not all network encoders, such as Modbus

TCP, work well at high-speed controls.



MAHENDRA MULI director of marketing and

new business development,

dSPACE (www.dspaceinc.

com): Engineers always consider signal

conditioning and filtering requirements

while matching the sensor output to the

control device. However, the impact of such

additional hardware, such as delay or drop

in resolution, should be considered on the

overall system performance. It is easier to

evaluate such impacts using simulation

technology. With dSPACE systems, engi-

neers can easily and quickly simulate the

controller software, as well as the additional

delays, noise and sensor signal distortion

effects, using model-based design. This

ensures that all potential stumbling blocks

that could potentially happen are discov-

ered early in the machine design stage.




com): As a diagnostics guy, when it comes

to resolution I always want more. That

being said, I also need to be able to objec-

tively look at the application and determine

how much is enough and how much is too

much. If I have too many pulses or too

much resolution, it will probably result in

additional requirements with regard to data

acquisition such as dedicated counter-timer

channels sampling at much higher rates that

will still need to synchronize with the lower

speed channels handling the process data

acquisition. If things are too snappy and too

responsive it can also be an issue from a

controls stability standpoint since you can

end up chasing the noise. The resolution

should be high enough to allow accurate

evaluation and diagnosis of any of the

known or anticipated issues that may be

encountered and leave a little bit of margin

for Murphy.



com): For resolvers with

long cables, the excitation frequency be-

comes a factor. The higher the frequency,

the higher the cable impedance, or the

shorter the cable that can be supported.

The lower the frequency, the lower the

rotational speed that the system can read.

For radiation areas, the cables must be

routed around shielding to protect people.

It is a real challenge to get a cable that is

made of radiation-resistant materials and

still has the reactance low enough to sup-

port long cable runs.

KATE SOKOLNICKI product marketing manager

for encoders, Rockwell

Automation (www.rock-

wellautomation.com): The output signal

type will depend on where you are taking

the encoder feedback—to a high-speed

counter card, to a relay or to a drive?



Networked encoders, such as the Allen-

Bradley Bulletin 842E EtherNet/IP Encoder

from Rockwell Automation, simplify this

decision by allowing the user to bring speed

and position feedback directly into the

controller, with no need for a high-speed

counter card.



For incremental encoders,

it’s beneficial to use twisted pair, with foil

and braided shield whenever possible.

Selecting a cable with less than 35 picoFar-

ads/ft is important in applications more

than 30 ft to reduce the capacitance in the

cable. As the rpm and pulses per revolution

(PPR) increase this becomes a critical

factor. This is crucial in applications such

as servo motors that may have both high-

speed and low/zero-speed motion profiles.

An absolute encoder with an all-digital

interface can alleviate equipment techni-

cians from having to chase ghosts in the

equipment due to a host of analog artifacts

that are common in industrial applications.

The latest protocols offer a single cable for

motor power and encoder signal. This

offers the machine builder the best of both

worlds—high-bandwidth 20-bit encoders

with reduced cabling and plug-and-play

capability.

When you include encoders or resolvers to sense shaft speed or position con-

trol, it’s important to know where and how to mount them and equally im-

portant to have a plan for connecting them, especially when conditions are

hazardous. Our panel of experts shares some tricks of the trade for making sure the sensory

data you’re receiving is exactly what you need, even under extreme conditions.

Do you have any recommendations on mounting or for cables and connec-tors, especially in hazardous locations or extreme conditions?

SCOTT ORLOSKY global market segment manager, Sensata Technologies

(www.sensata.com): Absolutely. You should also check with cable manufac-

turers, as well, but in general you would want the following attributes.

Twisted pair data lines used with a differential signal to minimize noise are critical. The

power and ground lines should have extra copper to reduce their impedance and make

them more noise-immune, as well. The cable bundle should have an overall shield, either

braid or foil, and the jacket should be rugged material that is resistant to chemical attack

and UV exposure; PVC does a remarkably good job at a reasonable price.

Mounting, cables and connectors in hazardous locationsEncoders and resolvers require special connections and mounting in extreme conditions






If possible, use a cable/connector assembly

with an overmolded backshell, which will

be more resistant to water intrusion. The

cable termination should be done in an en-

closure that cannot get wet. Any moisture

that enters an open cable end will eventu-

ally work its way down to the connector

and cause corrosion. Also make sure that

the cable is installed with a generous radi-

us to prevent mechanical damage through

flexing and be sure to control the geom-

etry of the cable lead-in to the encoder, so

that it is not loose and subject to snagging.

In general, the cable should lead down and

away from the encoder so that any mois-

ture will drip off the cable, rather than pool

around the connector.

NEIL BURGARD electric automation business

development specialist,

Festo (www.festo.com/us):

For extreme conditions, use grommet with

cable instead of a connector on the encoder.




com): The more protection, the better, and

it will always be money well spent. Often

times the pencil gets sharpened during the

specification and procurement phase to

keep costs down, but these costs are typi-

cally insignificant when compared to the

equipment damage, production downtime

and general mayhem that can result when a

critical-speed reference signal fails. In these

situations it’s hard to justify the $50 savings

on a cheaper cable when the failure just

resulted in a $1 million loss due to un-

planned downtime and equipment damage.

There are some environments that are just

plain bad for instrumentation, and, in these

cases, it is also good to plan in redundan-

cies as an additional layer of protection.

In addition to cables and connectors, simple

things like shaft alignment and proper cou-

pling selection are crucial. Improper align-

ment results in the encoder shaft speed ac-

celerating and decelerating each revolution.

A worn coupling insert allows for an erratic

pulse train—speed/position error.



(www.encoder.com): To

extend the life of any encoder, I recom-

mend incorporating some secondary

protection beyond the housing included in

the encoder design. It may be as complex

as a hermetically sealed enclosure or as

simple as a shield to redirect liquid con-

tamination. I have even seen applications

where plastic bags have been the explana-

tion for encoder survival.

This type of protection can also help to

eliminate physical damage, as well. Cables

and connectors can easily be damaged



when used as a step or handle on large

automated equipment. Installing a guard or

shield is much less expensive than replacing

a broken encoder or cable assembly, espe-

cially when you include machine downtime.



com): In areas with explo-

sive hazards, a low-cost approach is to put

the cables inside pressurized conduits.

Following the NFPA 496 safety code, it is

possible to safely install systems without

going to expensive equipment.

Instead of installing long cables on the motors

that will be pulled into conduits and that must

later be cut or pulled out of those conduits

if there is a maintenance problem, the motor

is set up with a junction box located close to

the motor, and then a plumbing union is used

to allow for easy connect/disconnect from

the conduit. Removal of the motor without

removing long cable runs is a snap.

JONATHAN DOUGHERTY

product specialist,

Heidenhain (www.heiden-

hain.com): This is a great

question because it does feel at times that

the cabling and connectors are sometimes

left as an afterthought when, in fact, they

are such a critical part to ensuring that the

feedback loop works effectively and

efficiently. We here at Heidenhain take

great pride in the amount of research and

development we put in to ensuring that

the cables and connectors we provide are

tested to the extreme and are capable of

operating in the hazardous conditions we

see our customers encountering. This

helps to ease some of the burden from the

customer and allows them to feel com-

fortable that they are receiving a pre-ap-

proved solution with their feedback de-

vice. Some keys to ensuring a clean signal

are to verify the correct shielding concept

for the encoder and cable is used, as not

every encoder is the same, and to avoid

Some keys to ensuring a clean signal are to verify the correct shielding concept for the

encoder and cable is used, as not every encoder is the same, and to avoid installing signal cables near possible interference sources—contactors,

frequency inverters, solenoids.



installing signal cables near possible

interference sources—contactors, frequen-

cy inverters, solenoids.


(www.posital.com): Even

though most of our encod-

ers can handle high shaft loads, the use of

proper flexible couplings reduces bearing

wear and improves encoder service lives.

This is too often neglected. We have also

found that buyers look primarily at direct

encoder costs, without paying adequate

attention to system costs on the mechani-

cal or electromechanical side. Total system

costs can often be significantly reduced by

using an encoder with the right features.

For example, blind hollow shaft versions

can save space and eliminate the need for

external couplers.

There are often alternative mechanical

configurations and connector accessories

available that would make the job of inte-

grating an encoder into a design simpler

and less costly, if only the customer knew

that they existed. We have specifically de-

signed the Posital Product Finder so that

our customers can see the full range of

mechanical and performance options that

are available, without having to talk to a

product specialist.

encoder/resolver rabbit hole - Control Design

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