46 / 84
disk, which is often needed when using
optical encoders. At the same time, the
capacitive encoder provides equivalent
or superior accuracy compared to other
encoder technologies, with a typical
accuracy value of 0.2 degrees.
Another advantage of the capacitive
encoder is that resolution can be
adjusted dynamically, whereas optical
encoders are fixed such that changing
the resolution requires fitting a different
encoder. Adjustable resolution delivers
advantages in development, by allowing
engineers to make any changes to
prototypes quickly and easily, and
also aids production supply-chain
management by allowing a single stock-
keeping unit (SKU) to be used in multiple
motor controls of different resolutions.
The encoder housing is designed for
easy assembly and supports multiple
mounting options. In addition, multiple
sleeve sizes are provided to suit
commonly used motor shaft diameters.
Energy Savings and
Efficiency Gains
The AMT encoders have very low current
requirements, with some series drawing
less than 10mA of current at the highest
resolution. This corresponds to 0.2W in
a four-motor system operating at 5V.
Recall that optical and magnetic
encoders can draw considerably higher
current, thereby significantly increasing
overall power consumption in a multi-
motor system. Table 1 shows the power
consumed purely by the encoders
of a four-motor system such as a
drone or mobile robot, comparing the
performance of CUI AMT capacitive
encoders with optical and magnetic
alternatives.
Capacitive encoders are shown to
offer a superior energy-efficient
solution, and can give designers
more freedom to manage the limited
power budget in mobile and battery-
operated applications. Moreover, the
capacitive encoder’s operating current
is independent of the resolution setting,
which allows the encoder settings to
be optimized without compromising
application power consumption.
When paired to a brushless dc motor,
capacitive encoders also allow faster
and easier digital “zeroing” to align
the encoder U, V and W signals with
the rotor windings. To align an optical
encoder, the rotor is usually locked in
a known position and the code wheel
is physically aligned. The motor is then
back-driven while using an oscilloscope
to verify correct alignment of the
back-EMF and encoder zero-crossing
points. This is an iterative process that
can take 15 to 20 minutes, although
small errors may remain. These errors
prevent the motor running at maximum
efficiency, thereby wasting precious
battery energy. It is even possible that
the inability to optimize alignment may
force the engineer to over-specify the
motor to ensure the desired torque.
In contrast, digital zeroing by
programming the encoder using a
software application ensures perfect
alignment every time. The process takes
seconds to complete and eliminates
unit-to-unit variability. The motor is
energized to lock the rotor in the desired
position, and the encoder is set to zero
at this position using a single command.
No additional instruments are required.
By allowing accurate, repeatable
alignment, this technique ensures that
the motor is able to run smoothly at
optimum efficiency thereby delivering
best performance and maximizing
battery life in mobile applications.
Benefits Beyond Battery-
Operated Applications
Market demands for better performance
may provide enough encouragement
for designers of battery-operated
appliances to take advantage of
capacitive encoders to boost the
efficiency of next-generation drive
systems. On the other hand, the
emergence of new regulations
governing efficiency of electric motors
show that regulators are taking
a progressively tougher attitude
toward the efficiency of electric-
motor systems. These include the
EU’s latest IEC 60034-30-1 standard,
which has introduced a new Super
Premium Efficiency level for three-
phase induction motors. In the US,
the Department of Energy (DoE) will
bring new legislation into force in mid
2016 that both increases minimum
efficiency standards and includes types
of motors not previously covered.
The DoE’s analysis estimates that
more than 70% of the total potential
energy savings achievable through
the new legislation can be realized
through system-level savings such as
improving component efficiency and
using smaller motors where possible.
Capacitive encoders can help towards
using smaller motors, and allow
designers to reduce the power draw
of their entire systematic government
agencies continue to focus more
sharply on this area.
Conclusion
Affordable automation in the form of
small mobile robots and civil drones
could revolutionize activities such as
manufacturing, distribution and asset
management.
Low-power,
precision
capacitive
encoders can deliver valuable savings
in overall energy consumption for
these motor-rich mobile applications,
while also streamlining development
and manufacturing as well as reducing
maintenance overheads so helping to
reduce ownership costs. As emerging
government regulations indicate an
increasingly strict attitude to the energy
efficiency of motor systems, designers
could find even more incentives to
adopt this technology.
Encoder model
Type
Operating voltage
Current at highest reso-
lution
Power in 4-motor
system
AMT10
Capacitive
5V
6mA
0.12W
Competitor 1
Optical
5V
85mA
1.7W
Competitor 2
Magnetic
5V
160mA
3.2W
Table 1. Encoder power consumption comparison
New-Tech Magazine Europe l 46