The
heart of today's high performance servo-positioning systems is the position
feedback encoder used to control the motion loop in closed loop or servo
systems. Whether rotary or linear motion, it provides the electronic signals
needed to accurately position the motion system's mechanism or the servo
motor (rotary) or stage (linear). As engineers are trying to improve system
performance and reduce system costs, encoder selection is becoming increasingly
important to the success of their design.
Three encoder sensor
technologies, optical, magnetic and electric, have dominated the market
for the past 50 years, with optical encoders preeminent in the medium
to high-resolution servo-motion market. The optical encoder is outdistancing
its magnetic sensor counterparts in the high-performance motion control
arena as defined by position resolution from 5 microns to 5 nanometers.
Click here to see the entire Mercury™ encoder family, twenty one
models in all.
Market
Intelligence Identifies Encoder Design Opportunities
The most meaningful product improvements are often a response to the customer's
needs and such was the case with the new Mercury™ Encoder Family
from MicroE Systems. To gain a better understanding of key problems encountered
by users of high-performance optical encoders, MicroE Systems undertook
a marketing study to determine the design objectives for their new family
of precision optical encoders. Here is what the company discovered:
Engineers
Want It Smaller, Faster and Cheaper
1. Reduce the Size of the Optical Sensor Head - While many
applications are being downsized to reduce cost and improve performance,
the high-performance optical encoder had not enjoyed similar advancements.
This prevented its use in many new applications with restricted space.
2. Make Sensor
Head Alignment Easy - Many optical encoders were difficult to
align or failed to remain aligned over the length of travel. Trained technicians
with oscilloscopes or other gear were required to install and align these
encoders. Often, final tightening of the sensor mounting screws threw
the encoder out of alignment…
3. Make Sensor Head Less Prone to Collision - Many high-performance
optical encoders required clearances between the scale and the sensor
of less than a half a millimeter. This condition led to potentially dangerous
collisions and sensor head failures if surface flatness was not held to
tight tolerances or if debris/hardware was encountered along the motion
path.
4. Add Programmable Interpolation Settings - Most encoders were supplied with an inflexible fixed interpolation multiplier. If higher resolution was needed, the encoder had to be returned for an upgrade or replacement. This led to extra cost, design delays and equipment down time.
5. Reduce the Initial Cost & Cost of Ownership - In many high-performance positioning systems, the optical encoder was the most expensive element. For example, a high resolution linear or rotary encoder would have cost up to $1800 in single quantity. Both the initial purchase price and cost over the life of the system had to be reduced.
The
Solution: Mercury™ Next Generation
Encoder Technology
How does Mercury™ achieve an effective solution? First, we simplified
the optical design. This process reduced the sensor height dramatically,
minimized optical component count and cost, and enhanced signal fidelity.
PurePrecision ™ Optics enables the smallest sensor height and the
widest alignment tolerances.
The optical system used in the Mercury™ encoder design detects the interference between the diffraction orders of reflected laser light to produce nearly perfect sinusoidal signals. A photodetector array is placed within the interference pattern for signal detection. This construction results in an extremely compact sensor. Named PurePrecision™ optics, this patented design shrinks the Mercury™ sensor to 8.4 mm high or about one half the height of the smallest high-performance encoder sensor on the market.
Because the optical interference pattern is large (the reflected light pattern), it produces a nearly sinusoidal signal within this region. Alignment tolerances are 300% more relaxed than other designs. Additionally, the sensor can be used in either linear or rotary applications. All in all, PurePrecision™ optics yields a compact, highly accurate encoder that can be easily integrated into many applications.
Electronic Design
Enhancements
The second area of improvement was component integration. A micro-photonic
system was developed that combined the laser source, the detector and
the signal processing electronics. This component integration follows
the trend in the semiconductor industry. So, if one can produce a functional
optical encoder with a lower parts count, it has to be "good news"
because unit cost is reduced, reliability is enhanced and sensor size
is cut in half compared to other design alternatives.
A Mercury™ encoder
is half the height of
competitive high-performance encoders.
Innovative
Optical and Electronic Design Facilitates
High Performance and Ease of Use
As stated earlier, the technology platform of the Mercury™ encoder
family delivers a 300% advantage in alignment tolerance or a ±
2 degree (mechanical) "sweet spot" in the Theta Z axis. The
problem of shifting alignment after sensor mounting has been eliminated.
In addition, systems with mounting pins referenced to a precision datum
of tight mechanical tolerances lets users place the sensor against the
pins for self-alignment. This process eliminates all alignment procedures
and is only possible with the Mercury™ encoder.
Mercury™
encoder sensors have a ± 2
degree sweet spot that makes alignment
easy and superior to all other encoders.
In designs where
the sensor is not positioned against a reference datum and alignment procedures
must be performed, our SmartPrecision™ electronics allow you to complete
the alignment procedure in under 30 seconds.
When aligning the encoder sensor over the length of travel, the SmartPrecision™
LED signal strength indicators display signal strength for visual confirmation.
No scopes or test meters are needed to insure that optimal signal strength
is being produced from the encoder sensor.
In addition,
the electronics establish a 1 LSB (least significant bit) index/home position
with the push of a button. The process is very precise and very important;
if you can't locate "home" position with great accuracy, then
what's the point of a high-resolution encoder? LEDs indicate successful
acquisition of the index/home position location by the SmartPrecision ™
electronics. Finally, SmartPrecision™ electronics automates gain, offset
and phase adjustments. No equipment is needed, and there are no potentiometers
to adjust so manufacturing and field service time/costs are greatly reduced.
Mercury™ encoders are the fastest high-performance encoders available.
Highest Operating Speeds
Mercury encoders can operate at higher speeds and resolutions than other
encoders. All traditional
A quad B output encoders reduce resolution as operating speed increases.
MicroE Systems' optics and electronics allow speeds of 7.2 meters/second
at higher resolutions than any comparable encoder. Standard SPI signal
output, based on high speed serial word, enables operation at 7.2 meters
per second with up to 5 nanometer resolution. The Mercury™ Si models
are the fastest encoder systems in the industry.
The First Programmable Encoder System
Common complaints from encoder users center around the inflexibility of
encoder operating parameters and the inability to easily monitor or diagnose
encoder operation. SmartPrecision™ software solves these problems. It
is the first software system developed for encoder setup, monitoring and
diagnostics.
. 
SmartPrecision™
software lets you program resolution, output frequency and alarms, as
well as viewing data in a
variety of plots. Nothing else comes close to this level of performance
and flexibility.
Programmable
Resolution and Output Frequency
Obtaining the best performance from a motion control platform requires
the correct system resolution at the desired speed. SmartPrecision™
solves the system optimization problem, namely, selecting the optimal
resolution (e.g. - number of pulses or steps). SmartPrecision™ software
allows the engineer to program the encoder's resolution in integer steps
from X4 to X4096, directly from the main user interface. Linear resolutions
range from 5 micrometers to 5 nanometers; Rotary resolutions range from
6600 counts-per-revolution (CPRs) to 67.2 million CPRs.
Enhanced
Diagnostics
SmartPrecision™ software allows the engineer to display various performance
parameters to ensure optimal operation. All that is required is a pc or
lap top computer. The system displays the signal as a Lissajous data plot,
showing the raw and corrected output from the encoder sensor. This feature
is particularly interesting in the manufacturing environment, as engineers
can capture data to document proper encoder setup. Used this way, SmartPrecision
software can be a very effective manufacturing quality assurance tool.
Use
SmartPrecision™
software to view a Lissajous plot of the
raw and corrected encoder signal. Ideal for
documenting proper setup during manufacturing operations.
Other data plots can be used to locate the index and see when the sensor is positioned over the scale's index mark. Sensor output can be verified over length of scale using the Signal Strength plot. The position readout, displayed in the engineering units of the designer's choice, also provides confirmation of the system's actual position as calculated by the motion controller.
Use SmartPrecision™
software to view encoder
position and velocity or signal strength over the length of travel.
Diagnose Mercury Encoder Performance
SmartPrecision™ provides the user with diagnostic tools to monitor motion
system performance. The software can capture alarms while the system continues
to operate. It can also read the encoder's hour meter to monitor system
usage. Any deviations in system position or speed are recorded and time
stamped. Signal data can be electronically recorded, emailed and analyzed
by our application engineering team to determine if there are any encoder
problems.
Application Success Story
Any new high performance motion control device is only as successful if
OEMs buy it. Today, hundreds of OEMs around the world have specified Mercury™
encoders in their designs. Here is just one example.
MI Technologies is a world leader in control measurement systems and services for the antenna, radome, RCS and component characterization markets. Precision measurements and unequaled accuracy are critical to their success when profiling the performance of new customer designs and equipment.
Mark Hudgens, senior mechanical engineer at MI Technologies, was tasked with designing a two-axis gimbal for the company's latest antenna measurements system. Small system size and ultra-high performance were major design objectives. The motion platform was required to track velocity with ultrahigh resolution and accuracy. And it had to maintain exceptional system repeatability.
"After considering
alternative technologies and other encoder systems, we were running out
of options," confided Hudgens. He had heard about the tiny high performance
encoders from MicroE Systems and decided to evaluate them. "The small
sensor size was impressive. 
In
fact, the Mercury 3000 had the only sensor that could easily fit into
the available space. And the Mercury 3000 was a fraction of the cost of
our next best option," Hudgens added. He continued, "Finally,
only MicroE Systems SmartPrecision™ software provided the flexibility
to make this design easy to build and set up in the development environment."
Hudgens' primary complaint with other encoders was that he was required
to purchase an interpolator with a fixed multiplier. 
The programmable M3000 Mercury™ encoder and SmartPrecision™
software solved this problem by giving him the ability to program the
exact resolution his system required. "It's very helpful," stated
Hudgens.
Final
Comments
The combination of Mercury programmable encoders and SmartPrecision™
software represents a new level of performance in the world of motion
control. Mark Hudgens of MI Technologies said, "Our gimbal now performs
like a champ and we saved thousands of dollars by using MicroE Systems'
programmable encoder and SmartPrecision™ software. It's an unbeatable
combination."
The
Mercury™
3000 programmable encoder and
SmartPrecision™
software were the enabling technologies
that allowed MI Technologies to meet their motion
platform performance goals.