With the right packaging option for the electronics, position encoders can keep pace with ever shrinking motors

By Bonnie Baker, Maxim Integrated

Encoders
are integral parts of motor-control systems that sense mechanical motion, then
generate digital signals in response to that motion. The trend today is to create smaller
mechanical and electronic devices, and the motor as well as its encoder are
part of this trend in applications such as robotics, drones, medical equipment,
and handheld devices. The major challenge that designers now face is trying to
fit the necessary number of devices into the encoder’s shrinking space allocation.

Whether
the system is an automated pick-and-place (x-y positioning), an automobile
(assisted lane detection), or a robotic arm, the position feedback information that
an encoder provides is an integral part of the motor-control system (Fig. 1). Encoders can provide either incremental
or absolute positions in motion applications. When an application needs a
relative motor position, an incremental encoder is useful. One uses these types
of encoders with AC induction motors. In contrast, pairing the absolute encoder
with permanent-magnet brushless motors assists in servo applications. The
encoder feedback ensures synchronization of the motor stator by providing rotor
direction, speed, and position.

Fig. 1: Human vs. robotic arm, where encoders
abound.

Embedded
in an encoder is a motor position-sensing device and several analog-to-digital
converters (ADCs). These circuits translate the motor’s motions such as speed, direction,
and shaft angle into electrical signals that they provide to the system microcontroller
or processor. In the example of Fig. 2,
the encoder uses an optical wheel that connects to a motor stator. The encoder’s
output signals provide information that allows the system’s microcontroller to
determine the speed of the stator and thereby control the speed of the rotation
of the motor and optical wheel.

Encoder_2

Fig. 2: Basic sensing circuit for an optical
encoder.

The
optical encoder uses an LED and a photodiode positioned opposite one another
with the windowed optical wheel between them. The illumination from the LED is
constant as the optical wheel turns. The LED light shines through each wheel
window one at a time. As the LED light shines through a window, the photodiode at
the input of the transimpedance amplifier (TIA) converts the impinging light
into a current (IPD). The IPD current flows through the feedback resistor, RF,
to create a voltage greater than VREF at the output of the TIA.

The
signal travels through a gain block and then the ADC-driving amplifier — in
this case, the MAX44242 — which is a 180-MHz, low-noise, fully differential
successive-approximation-register (SAR) ADC driver. Next to it is the ADC,
which continually samples the driver output to construct a square wave over
time.

The
diagram in Fig. 2 illustrates only
one channel of an optical encoder, but encoders typically have at least two
identical signal channels, each focusing on different windows. The appropriate
ADC encoder topology thus utilizes a dual, simultaneous SAR ADC — in this
example, the MAX11192. Dual encoders are available with a 12-, 14-, or 16-bit
resolution, with higher-resolution ADCs providing extra sensitivity and
positioning accuracy. For this encoder, the dual, simultaneous sampling SAR ADC
takes consistent snapshots, providing an instantaneous picture of the encoder’s
motor direction, speed, and position. Fig.
3
shows an example of the output of a two-channel encoder.

Encoder_3

Fig. 3: The optic output of an encoder.

Encoders
produce pulses, which indicate movement over short distances that the system
must then interpret, such as by counting pulses to convert them into position
information. There are two counting methods applied to the encoder’s output:

  • A count of pulses indicates movement and
    speed over time.
  • The channel order of the ØA compared to ØB indicates
    the direction.

The shrinking encoder

Encoder_4

Fig. 4: The encoder continues to shrink in
size.

Ideally,
encoders will have all of their electronics inside the housing or donut because
having the PCB inside the encoder minimizes interconnections. But as the
encoder’s cubic area lessens to match shrinking motors, the PCB’s square area also
reduces in size. Furthermore, with the space inside optical encoders continuing
to tighten, it has become even more critical to verify device layout in the development
phase. Small motor encoders can have a diameter as small as 25 mm with a shaft
diameter of 4 mm, so the design challenge becomes the process of populating the
smaller PCB with smaller devices.

The
components depicted in Fig. 2, plus
various capacitors, must all fit on the PCB of an encoder (Fig. 4). In this case, the MAX11192 family helps meet tight space restrictions
by offering a TFDN plastic package option, which has  3 x 2-mm dimensions and a 0.75-mm clearance.
Such ultra-small package options are allowing the shrinking donut shape and
internal PCB size of motor encoders to continue housing all of the needed
electronics.

Bonnie Baker is a
seasoned analog, mixed-signal, and signal-chain professional. She is a
published author and the author of “A Baker’s Dozen: Real Analog Solutions for
Digital Designers.” Bonnie has a Masters of Electrical Engineering from
University of Arizona (Tucson, Arizona) and
a bachelor’s degree in music education from Northern Arizona University
(Flagstaff, Arizona).