Encoders

Encoders are rotary position sensors, used for measuring motor/actuator rotor angle with respect to the stator. Encoders can be either incremental or absolute - MD only work with selected absolute encoders.

MD motor controllers can work with up to 2 rotary position encoders at a time. There are two roles, the encoder can have:

• Main Encoder - used for motor commutation and motion control by default,

• Auxiliary (Aux) Encoder - used mostly getting absolute actuator position (after gearbox), optionally can be used for motion control, or not used by driver (can be read via register by host).

Main Encoder is required for operation. The MD does not currently support sensorless control mode.

Supported Encoders

Encoder type	Supported Role	Notes
ONBOARD	Main / Aux	Mounted on every MD
ME_AM On-axis	Main / Aux	Present in some of MA-x actuators
ME_AM Off-axis	Aux	Present in some of MA-x actuators
RLS_17B_RS422	Main / Aux	High precision position sensor. Present in some of MA-x actuators, mainly hollow shaft (HS) series.
CM_OFFAXIS	Aux	Present in some of the CubeMars actuators
RLS_17B_SPI	Main / Aux	High precision position sensor variant. Present in some MA-x actuators
RLS_ORBIS_RS422	Main / Aux	-
CE300	Aux	-

Note

Custom encoder support.

If you would like to use specific position sensor that is not currently supported, we are open to introducing it to MD Ecosystem. Please contact us at: contact@mabrobotics.pl.

Onboard Encoder

Each MD unit, features a 14-bit absolute encoder on the PCB. The Encoder is mounted on the bottom layer in the middle of the driver - in the center point of all mounting holes. This allows for mounting the driver right behind the motor, creating a compact and power-dense actuator with minimal moving parts.

Auxiliary (Aux) Encoders

If actuator has a gearbox, Main encoder (measuring motors’ rotor angle) cannot determine output shaft position on startup. This is where Aux Encoder can be used to supplement the driver with absolute angle measurement after gearbox. Thanks to this, the actuators can keep track of its position between startups, which is especially useful while starting up mobile robots.

ME-am

ME-am is a simple, reliable and cost-effective solution for adding additional position sensor to MD based actuators. ME-am can work in one of two placement configurations:

• ME-am placed axially on the output shaft with a regular diametrically magnetized magnet

• ME-am placed non-axially together with a diametrically magnetized ring magnet

Note

ME-am encoder can be connected to MD20 and MD80 using AUX1 (6-pin) connector.

CM encoder

MD drivers can also work with encoders supplied in many CubeMars actuators (such as AK10-9). These encoders are usually already mounted inside of the actuator. These encoders are perfectly fine to determine actuator on startup, but due to their noisy nature, are not recommended to be used for motion control.

Note

CM encoder can be connected to MD20 and MD80 using AUX1 (6-pin) connector.

RLS Precision Encoders

While Onboard and ME-am encoders provide sufficient precision for most general-purpose applications, there are situations where high-precision, through hole (hollow shaft) encoder is required. For those cases we worked together with RLS to provide easy way to connect MD to a few of RLS encoders We support RLS AksIM2 and RLS Orbis with RS-422 interface with the sizes ranging from 22 to 80mm.

Warning

RLS AksIM2 and Orbis with RS-422 interface can be connected to MD20 and MD80 using AUX2 (10-pin) connector.

RS-422 is on-demand feature of MD drivers, and by default it is not available on the board. Contact MAB Robotics support at support@mabrobotics.pl, for more information.

This connector and RS-422 interface are shared with the external torque sensor - only one RS-422 device can be connected at a time.

For mechanical assembly instructions, please refer to the RLS documentation. For connecting RLS RS422 based encoders to MD in custom application, please refer this instruction.

Aux Encoder Modes

With Aux encoder connected, MD can use its data in a few different ways. For most applications, where the encoder is used to determine absolute position of the shaft after a gearbox, STARTUP mode is the most suitable.

Encoder Mode	Description
STARTUP	Initial position from aux encoder, report main encoder values, motion based on main encoder.
MOTION	Initial position from aux encoder, report aux encoder values, motion based on aux encoder.
REPORT	Initial position from aux encoder, report aux encoder values, motion based on main encoder. Calibration of the aux encoder is impossible.
MAIN (legacy)	Position from aux encoder, is mapped as main encoder, and used for both control and commutation. Note: This is only applicable for drivers with firmware 2.x.x. For v3.0.0+ firmware, the main/aux encoder is set though mainEncoder and auxEncoder registers.
DUAL	Initial position is computed from phase difference between aux encoder and main encoder. This is the mechanical configuration shipped in some actuators, notably MA-P-110. Refer to dual encoders section.

Warning

The non-axial configuration outputs a nonlinear position values. This means it requires a full calibration (your setup should be able to rotate by at least one full rotation), and in case of the report mode it will output nonlinear position and velocity readings that will have to be compensated in the host’s software

Not all modes are recommended for every encoder. The off-axis placed CM or ME-am encoder is inherently more noisy and less accurate and thus we recommend using it only in STARTUP mode. Please refer to the table below:

Encoder type	Supported modes	Recommended
ME_AM_CENTER	STARTUP / MOTION / REPORT	STARTUP / MOTION / REPORT
ME_AM_OFFAXIS	STARTUP / MOTION / REPORT	STARTUP / REPORT
RLS_17B_RS422	ALL	ALL
CM_OFFAXIS	STARTUP	Only off-axis configuration is supported.
RLS_17B_SPI	ALL	ALL
RLS_ORBIS_RS422	ALL	ALL
CE300	STARTUP	STARTUP

ME-am - Mechanical mounting

How to add an encoder to the driver setup:

• make sure the encoder sensor is placed correctly:

• • in case of axially placed sensors make sure they are placed in center at correct height above the magnet (1 mm is usually optimal),

  • in case of non-axial configuration make sure the magnet is close to the ring magnet (<0.5mm) and the sensor IC is at least 2mm above or below the ring magnet horizontal plane.

• Connect the MD with the encoder using a picoblade series cable assembly and connect power to the MD,

• Modify the motor config file, or directly access registers, according to your setup and save it to the MD,

• Calibrate the MD using candletool md calibration command,

• (if applicable) Calibrate the output encoder using candletool md calibration -e aux command

• Test the encoders using candletool md test encoder main (and candletool md test encoder aux) command.

• Use the candletool md info command to make sure there are no errors and the test results (min, max and stddev errors) are within your expectations.

• The external encoder is ready to use!

Warning

Most of the auxialary encoder require sum-millimeter mounting for conitnous and reliable operation. It is recommended to use CNC-machined mounting brackets for attaching the encoder. 3D printed enclosures often do not provide sufficient precision and stiffnes (especially after heating up), and can lead to more noisy and reliable readings.

Drives with DUAL encoders

Some actuators, like the MA-P-110, use two encoders coupled by gears with a 35:36 ratio. Because the two gears turn at slightly different speeds, the phase difference between their readings changes as the output rotates. The drive uses this difference (the Vernier principle) to compute the absolute output shaft position, without an encoder mounted directly on the output.

Compared to a ring encoder, this gives high precision with a simpler mechanical design. The trade-off is a limited absolute measurement range of 0 - 35 rad. Outside this range the absolute position wraps around.

Actuator setup

Keep the measurement range in mind when setting up the actuator. If your application depends on absolute position, set the zero position near the middle of the range (17.5 rad), so the joint can move the same distance in both directions without leaving it.

The absolute position is available in the dualEncoderAbsPosition register (0x027). It is not affected by offsets or zeroing, so you can always use it to check where the joint is within the range:

candletool md register read dualEncoderAbsPosition --id 100

candletool md --id 100 info also shows it in the aux encoder section when the aux encoder mode is DUAL.

Note

MA-P-110 actuators ship calibrated and zeroed at 17.5 rad absolute position. This gives ±17.5 rad (about ±2.78 output revolutions) of absolute feedback around zero.