Encoders vs. Potentiometers
Interface Technology and History
As applies to the arena of
racing simulation:
This Document:
This
document is written as a historical and technical informative “White Paper” for
racing simulation enthusiast. You will
find documented technical & historical information in relation to the
Sim-Addicts encoder interface technology and a little bit of my experience
concerning the topic of Encoders vs. Potentiometers.
History of the S/A Combo Controller USB
interface:
Around 1999 the Sim-Addicts
Design Group (www.Sim-Addicts.com),
which I am a member of, started a part time endeavor of developing an encoder
& analog based input controller for their simulation racing personal
use. The Sim-Addicts Design Group did not originally perform this
development for profit; it was simply for the passion of simulated racing, to
challenge their development skills and share intellectual ideas. The hardware
developed by S/A was not engineered with retail cost in mind as it was simply
developed based on ideas for an innovative joystick controller interface for
simulation racing. During development
S/A never gave a thought to the expense it would cost to market the end design
and basically for over two years S/A through in all the intellectual bells and
whistles they could dream up no matter the cost within reason. As the project became near completion S/A did
consider marketing the design and for a short time did so, but soon found the
manufacturing cost of such an advanced SMT(Surface Mount Technology) controller
hardware, software and development far exceeded the revenue return possible to
allow Wheel/Pedal manufactures or hobbyist to integrate. S/A was not surprised by the cost downsides
of integration as they new all along the concept was well ahead of its time for
simulation racing and were very happy with the accomplishment of successfully
developing such an advanced interface for their personal use which still rivals
designs of today, roughly 7 years later.
S/A was certainly not the first to use
encoders for positional movement tracking as the majority of early mouse
devices were a encoder based design. You
remember the PC mouse that has the ball in the bottom that we used for years;
yep, that ball spins a set of encoder wheels.
Another interface supporting encoders which is very popular in the arena
of flight simulation is the EPIC interface. The latest interface supporting encoders I’ve
seen is the Beta Innovations
products and is another entity primarily tailored for the flight simulation
market but I have read some threads where simulation racing enthuses are
successfully adapting the Beta Innovations units for encoder steering or pedals
in the simulation racing arena. The
developer of force feedback technology, Immersion,
has supported encoders in their arcade electronics for many years. Although these other interfaces do exist
today to my knowledge the technology of the S/A combo
controller, developed in year 2000, was the first advanced USB programmable
encoder/analog interface designed tailored to simulated racing. I have little knowledge of these other
mentioned interfaces other than that they do exist.
To provide a little insight to the S/A technologies
design history; it all started during the 1999 time frame in which analog
potentiometers and game port hardware had a lot to be desired by racing
simulation enthusiast. Current to the time potentiometers and game port designs
were inadequate and with the release of GPL the S/A team wanted a better
interface for their racing simulation experience. Originally S/A developed an optical
incremental encoder 4 axes, 4 button input controller card which connected via
an 8 or 16 bit ISA PC card bus slot. Click this link for images of
the design history starting with the ISA bus card version. Also more technical detail on optical
incremental encoders will be discussed later in the document. The ISA interface
had four micro-controllers for counting pulses (digital positions of movement)
from each of the 4 unique encoder axes inputs and converted these counts
(positions of movement) to a PWM (Pulse Width Modulation) signal which was
interpreted by the standard PC’s ISA bus legacy joystick interface. This encoder solution worked very well and
motivated the S/A team to continue development using encoder technology.
S/A continued development through the years 1999 - 2001 in which S/A finished
the development of the programmable HID (Human Interface Device) USB combo controller
hardware, a 2.5” x 3.5” PCB footprint using SMT (Surface Mount Technology),
which supports 4 analog potentiometers, 4 Optical incremental encoders, 8
direct programmable joystick buttons, 64 key keyboard emulation matrix, flash firmware
updatable and programmable via USB. Just
as a reference, the Windows Operating System of choice at that time was Windows
98SE and USB hardware was new to most.
Many independent Wheel / Pedal manufactures looked at the S/A combo controller device and S/A even sent demos out of the
controller back in this 2000-2001 time frame.
Understandably despite S/A pioneering a USB
encoder solution for simulated racing many manufactures were interested in only
continuing the low cost analog designs as the encoder solution was costly and
complex to implement. At the time S/A
was not interested in pursuing enhancements of their analog design further as
they were very happy with the encoder solution which satisfied their personal
desire for precision control during simulated racing. However, S/A did include and support analog
as a flexible feature set for the S/A combo controller. S/A’s 10 bit analog piece was far behind
superiority compared to the S/A encoder solution as the year 2001 analog design
did perform firmware filtering to aid in the removal of jitter which reduced
resolution during finite slow steering.
The encoders on the other hand were a rock solid truly digital solution
with superb accuracy, high resolution and no jitter at all.
As a side note, today’s (2006) modern IC’s introduce
a superbly better analog solution which are available for simulation racing
using modern interfaces and I’ll cover “today’s analog solutions” in more
detail later in this document.
For a few individuals like George Sandman (a known
entity in the GPL community); he converted his steering product to optical
encoders in year 2002 with an S/A combo controller and loved it. He still uses it as preference above today’s
(2006) analog solutions despite having to integrate it in newly purchased
Steering/Pedal hardware.
Tom Pabst (WWW.Pabst-Racing.com)
is another entity whom adopted the encoder solution in year 2002 and today uses
the S/A combo controller’s encoder solution for his professional racing
simulators and professional coaching services.
In 2005 Thomas
Enterprises integrated optical encoders, using S/A technology, as a
Wheel/Pedal solution for their simulation customers. Recently in 2006 Thomas Enterprises has also
introduced a new enhanced analog solution, also using newer developed S/A analog technology, for a superior analog solution which
has precision 10 bit incremental positions of movement using potentiometers.
Today S/A Design Group only licenses their
technology and performs contracted research and development for Pabst-Racing,
Thomas Enterprises and few other entities outside the racing simulation
arena. S/A Design Group is very proud to
have contributed to both these vary viable input solutions (analog and encoder)
for the simulation community.
Optical Incremental Encoders:
For the best description of Optical encoders please
reference the US
Digital knowledge base glossary web link under the heading “Incremental
Encoder” & Optical Encoder”. US Digital’s S1 model optical
encoder is the better solution for replacing an analog potentiometer as the
S1 will usually mount accordingly with no hardware modification. By using a correct selectable CPR rating US
Digital’s S1 encoder will not need any added gearing and you can directly
couple the S1 encoder to a pedal or wheel shaft. CPR is the quadrature
cycles per full shaft revolution and I’ll talk more in detail later explaining
this. Early on in S/A development of
using encoders S/A had to gear drive the encoders to turn enough rotations to
produce an adequate resolution. The US
Digital S1 encoder resolves the need for gearing as the S1 encoder can be
purchased in a CPR rating which doesn’t require turning full shaft rotation to
produce high resolution positions of movement.
Again I’m going to reference you to the US Digital knowledge
base glossary web link to help you with these terms like CPR, revolution
and resolution. In single quantities US
Digital S1 encoders cost in the $50.00 to $60.00 dollar range each depending on
selected options; the S1 is the cheapest and best performing incremental
optical encoder solution that I’m aware of for direct potentiometer
replacement. The S1 life span for Racing
Simulation use is likely to be life-time, but I can say factually I do know users
of over 5 years without having any S1 encoder failures or problems.
As I mentioned earlier US Digital encoders are rated
in CPR (Cycles per Revolution). A "CYCLE" is 4 positions of
movement. You may also read this
referenced as counts, pulses, square waves or gray code in other
documents. Reference link US digitals
Knowledge base FAQ to help out with these details. With many ways of writing this, I will use
“position of movement” when speaking in terms of Joystick positions recorded by
the PC. With that said, using S/A
technology a 540 CPR S1 encoder will produce 2160 positions of movement in a
single revolution of its shaft (540 CPR X 4 = positions of movement). In
another example a 1024 CPR will produce 4096 positions of movement in a single
revolution turn. In terms of joystick reporting to the PC, if you turn a 540
CPR encoder 1/2 turn then your resolution would be 1080 position of movement
which in this example is your maximum number of positions reported due to your
Min/Max mechanical hardware limits of 1/2 turn.
Therefore, when thinking about resolution while reading information
remember a lower CPR value doesn’t mean you have a lower resolution. If you read that a person is using a 540 CPR
S1 encoder, their resolution could be anywhere between 100 or less to 2160 or
even more if their turning the encoder shaft multiple revolutions.
Absolute and relative are other terms you may to need to reference in the
US digital glossary.
You may read that potentiometers are absolute and encoders are relative, which
is true for incremental encoders, but absolute encoders do exist
also and although the S/A technology don’t use absolute encoders other
interface hardware may. Absolute
encoders are more intelligent sensors than relative incremental optical
encoders and more costly. Instead of
just generating pulses for positions of movement they actually have a
communication protocol and communicate absolute position data to an interfaces
micro-processor. Absolute encoders can
even remember axes position after being powered down and back up. Absolute encoders are in many regards a
digital potentiometer.
Another term you may encounter is “Shaft Loading”. This is the amount of load force being
applied to the encoder or potentiometer shaft.
The greater the shaft loading the quicker the encoder or potentiometer’s
shaft bushing or bearing will likely degrade.
It’s my experience that today’s wheel / pedal manufactures have advanced
their hardware designs to minimize shaft loading to a very minimum level
therefore increasing the effective life of the sensors. The S1 US Digital sleeve bushing version
encoder has a shaft loading rating of 2 lbs while in use (rotating). I believe
2 lbs far exceeds any shaft loading you will find from simulation application
use.
S/A Encoder Setup Software &
Hardware Technology:
You may read threads where S/A
software is mentioned. The S/A software are utility tools that allow you to configure the
S/A hardware technology. The S/A hardware encoder technology is fine tuned for
US Digital S1 encoders use in simulated racing but initial setup is required
before use. If you read the Optical
incremental encoder topic above you will remember that I mentioned that
the S/A technology use relative encoders.
Relative means the encoder reports pulses only from a virtual starting
point position. The absolute position of
the encoder is unknown until the encoder is turned from its maximum rotation to
its minimum rotation. The S/A technology
uses the Min / Max physical ranges for fixed positioning as the encoders
position will always be relative to the physical Minimum or Maximum range. The
S/A software is used to easily configure these ranges
which S/A calls the encoders physical Min / Max range. The S/A
software has an encoder setup wizard which makes setting physical
minimum and maximum ranges easy. S/A software encoder clamp options
resolve any issues with mechanical stop variances. Also software invert
options exist to resolve mounting rotation direction issues. The S/A
software technology is debug able, script able, flash
able, and developer application design oriented. HID axes usages and
button usage's can be programmed via a software controlled user
interface. Follow this link to find more
details relating to configuring the S/A technology.
You may encounter statements using phrase like
"Stack over flow", "Loosing wheel center", and
"Loosing calibration" in regards to incremental encoder interface
hardware; all these phrases relate to the anomaly of the encoder interface
missing encoder counts or pulses while in use. Factually this can occur but
also factually with a properly setup encoder solution it is not an issue. The occurrence of this can only be caused
from a poor choice of encoders (CPR rating), faulty Min/Max configuration or
misuse which generates encoder pulses which exceeds normal use. The
"misuse" occurrence is not caused by a limited USB bus speed, or the
inability of the interface to send data via USB to the host PC. It is simply
caused by exceeding the micro-controllers speed ability to scan the pulses
produced by the encoder. A faster micro controller if used would make it harder
to cause this occurrence. However, today, no one with a properly setup S/A
technology solution has reported this as a problem so therefore no efforts have
been made to increase the processors speed. It’s my experience that this
anomaly doesn’t occur during aggressive driving or fast controlled wheel
movement; only a deliberate attempt of slamming the wheel from side to side in
an uncontrolled manner may result in this type of occurrence.
Another common misuse of statements relates to USB
bandwidth in regards to HID joystick interfaces. Concerning USB 1.1 or 2.0; USB
HID Joysticks are interrupt class USB endpoints. That means that the PC
actually polls the HID USB joystick device for position report data. The HID
USB joystick device tells the PC how fast to poll it at enumeration (startup).
Mostly HID Joysticks commonly use 10ms interrupt endpoints, that's 100 polls
from the PC a second. The HID S/A encoder technology
is setup for 1ms interrupt endpoints which is 1000 polls from the PC a second.
At 10ms, for common HID Joysticks interfaces, the PC will poll for HID Joystick
report position data 100 times a second.
With the S/A technology the PC is going to poll for joystick position
data roughly 1000 times a second, as quickly as the PC's hardware and software
can. If frame rates ever do exceed 100
frames a second, with S/A encoder technology you are
insured of getting current position data at frame rates above 100. The
bandwidth of the USB 1.1 or 2.0 makes very little difference as HID joystick
reports consist of very small packets of data which utilize very little USB
bandwidth.
Today’s Analog Potentiometers & Interfaces:
Today’s analog solutions are far better than 6 years
ago when the S/A’s encoder & analog hardware were developed. I myself have
recently revisited analog solutions performing development work using current
modern micro-controllers for TSW that comes closer to encoder performance but
still does not exceed the digital precision of optical encoders. However, the
analog solution is jitter free and while using an S/A developed algorithm
provides for a very incremental 10 bit position movement solution for a maximum
of 1023 incremental positions of movement without using any bit filtering. The
current analog solutions are very impressive compared to the older analog
interfaces and are a very viable cost savings solution for the simulation
community. Other manufactured analog interface solutions are available and I
believe are comparable in performance.
Always insure the analog solution performs Analog to Digital conversions
at minimal 10 bit with no bit filtering, verify it operates in incremental
linear counts and is stable with no jitter.
The high grade potentiometers in use today allow for
a longer life span of the sensor.
However, unfortunately potentiometers are by nature a degrading sensor
from the first day of use and life spans for potentiometers do vary depending
on usage conditions. Operating
temperature, humidity, vibration, and shaft loading all are conditions that can
affect a potentiometer’s life expectancy.
The manufactures of steering / pedal devices should always be the best
source as for what you should expect as a life span for the potentiometer being
used. I hear 1 year, sometimes 2 years
or even higher, it just depends on the user, usage and how much the
potentiometer degradation is noticeable or concerns the user. Still, with that said, today potentiometers
can be very adequate sensors for simulation racing and are very cost affective.
Potentiometers Vs
Encoders:
I unfortunately don’t have an independent study of
data compiled to determine the factual advantages or disadvantages of using encoders
over potentiometers or vise versa as applied to actual racing performance. However, now that you know a little history
and have a better understanding of encoders I can offer my experience and
technical view concerning the advantages or disadvantages of each type of
sensor for simulation racing in each sensors overall usage to sustain accuracy
& precision for positional movement input over time. Since I have engineered and have experience
in both sensor technologies I’m going to answer some of the most direct and
common questions I encounter related to encoders; the reader will have to use
his/her own judgment to deduct whether my statements based on my experience are
justifiable, factual or just an opinion.
Question
#1 – Aside from cost; are encoders a better sensor & interface solution for
positional tracking than potentiometer based interfaces?
Answer
#1 – In my experience the answer is yes.
Encoders provide a true digital solution; they have a longer life span
and are not susceptible to inference, noise or signal degradation. Encoders have a lower percent of error in the
position of movement reporting process and no signal conversion overhead. Encoders have a more cleanly direct method
for digital position reporting of data to the Host PC. Encoder interfaces are likely to have higher
resolution capabilities than potentiometer interfaces and more advanced feature
sets for programmability of axes and firmware updates.
Question
#2 – What are the downsides of encoders?
Answer
#2 – Cost and complexity are the only two items in my experience which may be
considered downsides. However, TSW has resolved the complexity of
integrating encoders into a wheel / pedal device, therefore, TSW customers can obtain a fully
configured ready to go encoder steering / pedal solution. Taking consideration that the complexity of
integration may already be done, I’m going to limit this answer to just
cost. The use of encoders being more
costly is the only downside in my experience.
Question
#3 – Will encoders improve my driving or give me a competitive edge?
Answer
#3 - No, maybe or yes are all good answers.
I have no data to support whether a user drives better or worse while
using encoders. A user may say that
encoders feel more precise, more responsive, and more accurate over analog
solutions; I would think that comment is logical as the technology is more
precise, more responsive, and more accurate.
Does that added control precision make you a better driver or give you a
competitive edge? Maybe, or not, it’s
not a question I can answer as it depends on the drivers ability to utilize the
added precision.
Question
#4 – Are encoders worth the extra cost?
Answer
#4 - The encoder solution will always be more beneficial to the professional
racing simulation market like www.Pabst-Racing.com
where top drivers train and utilizing professional built racing simulator
cockpits. The reliability and precision
response of encoders are a key objective in the professional racing arena. For some simulation enthusiast the extra cost
will be justifiable for a more advanced interface system with higher
resolutions, enhanced sensor reliability and more diverse controller
programmable features. However for other
wheel / pedal purchasers today’s analog potentiometer interfaces may serve them
well and work adequately for the average racing simulation hobbyist.
Ending Comments:
For the future I personally would like to see input
interfaces continue to develop and become more cost affective for the
simulation community. As a next step I
think mixing the two sensor technologies for a steering wheel encoder solution
and potentiometers pedal solution would provide a good mid range cost affective
wheel / pedal product that would provide the advantages of both
technologies. Using potentiometers for
the three pedals (gas, clutch & brake) would save cost and a single encoder
for steering would provide superior steering response and greater resolution.
Author:
Kevin L. Martin.