LegBoard

Stomp Leg Board

Each leg of the robot needs a control board to interface to the servos valves and position sensors. The servos communicate with proprietary serial protocol and the position sensors measure joint angle which needs to be converted to cylinder length to use the built-in feedback loop.

Board Layout

I added a representative connector for each connection this board will need and picked a pinout. The layout of the connectors is being designed in parallel with the rest of the leg, hopefully that will be settled soon. The pinout can be freely changed, please update the Block Diagram schematic if it does.

Electrical Specifications

Power

Power is distrubeted from a central power distribution block on the robot directly from a battery pack. The supplied voltage will be in the 18-28V range. Each leg board will be supplied through a fuse at the distribution block.

Enfield S2 servo valve output

Signal	Description
Output Voltage	24V nominal, the data sheet specs 10-28v
Output Current	1A per valve
RXD	5V TTL asynchronous serial data from servo valve
TXD	5V TTL asynchronous serial data to servo valve
Feedback	0-10V cylinder position measurement

The feedback signal should have an analog bandwidth of at least 1kHz.

Angular Position Sensor

Signal	Description
+5V	Power to angle sensor, 10mA
Vsense	Measured angle, 0.25 to 4.75 over specified angle range

The measurement circuit for Vsense should have at least 1kHz bandwidth.

Communications Bus

The communication bus connects all the Leg Boards to the platform microcontroller. The two connectors pass through A and B lines to make wiring simple.

Signal	Description
A	RS-485 A line
B	RS-485 B line
GND	Reference voltage for RS485 transcievers
Upstream	Connected to the Leg Board closer to the microcontroller or the
	microcontroller
Downstream	Connected to the Leg Board farter from the microcontroller

I'm not sure how the grounding should work, it may be that ground should not be wired in the harness on these connetors to avoid a ground loop, but I would like to provide it in case we have signal integrity problems.

The Upstream and Downstream pins should each be connected to a processor GPIO pin. I would like to use them to avoid having to hard code addresses on each board.

Signal Processing

Since the position sensors we are installing measure angle and the servo valves need to know cylinder length, each channela $i$ needs to calculate

$$\theta_i = as_i * (V_{sense} + ao_i) l_{cylinder} = a_i + b_i \cos(\theta_i + \phi_i ) V_{feedback} = fo_i + fs_i * l_{cylinder}$$

Where $as_i$ and $ao_i$ define the conversion from sensor voltage to angle in radians, $a_i$, $b_i$, and $phi_i$ define the geometry of the link, and $fo_i$ and $fs_i$ define the conversion from cylinder length back to voltage.

This calculation should take at most 20us per channel to ensure low latency and CPU usage for the full calculation. This should not be hard for a microcontroller with a clock rate of at least 50MHz.

Communications Protocols

Enfield S2

The processor will need to implement a serial port interface to the Enfield S2 for delivering command positions and monitoring status. The reverse-engineered protocol is described in the README.

The pressure and feedback registers should be continously monitored while sending the current position command.

Leg Bus (RS-485)

The commnuication between the Platform microcontroller and the Leg Controllers over the RS-485 bus should implement the MODBUS protocol.

TODO: MODBUS Register Map.