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mjbots power_dist and mjpower-ss reference

A. Operation

The power_dist and mjpower-ss boards provides pre-charging, an external power switch, and a power connector fanout for use in robotic applications. Additionally, the power_dist provides energy monitoring and over-current protection, and a soft switch capability.

The input can be directly connected to a battery or power supply. The output can be connected to high capacitance loads.

For the power_dist, a CAN-FD port allows host software to monitor the state of the switch, input voltage, power, and energy, and request that shutdown be delayed to accomplish a "soft power down". This documentation refers to the most recent (r4.5b) version of the power_dist. r4.3 versions are largely compatible with this as well.

A. Register command set (power_dist only)

The power_dist uses the same register framing and CAN-FD as the mjbots moteus controllers, with alternate definitions for the registers. moteus reference

The default ID for the power_dist is '32', (which notably differs from the default id of 1 for the moteus controller).

A.2

The following additional mappings are provided.

A.2.a Energy (measured in W*hr)

  • int8 => 1 LSB => 1 W*hr
  • int16 => 1 LSB => 0.01 W*hr
  • int32 => 1 LSB => 0.000001 W*hr

Registers

0x000 - State

Mode: Read/write

The current operational state of the power_dist. Only some values may be written.

  • 0 => power off
  • 1 => precharging (read only)
  • 2 => power on
  • 3 => fault (read only)

If power off is commanded over CAN, the device will not enter sleep, which differs from the behavior when power off is achived through the physical switch.

0x001 - Fault Code

Mode: Read only

A 7 bit fault code.

0x002 - Switch status

Mode: Read only

Zero if the switch is current turned off, non-zero otherwise.

0x003 - Lock time

Mode: Read/write

Power will be maintained for this long after the switch has been turned off. The maximum value is 3276.7s, all register scalings report in units of 100ms (thus int8 will not be able to command or monitor the full possible scale).

0x004 - Boot time

Mode: Read

Time since the processor was powered.

0x010 - Output Voltage

Mode: Read only

The current output voltage

0x011 - Output Current

Mode: Read only

The current output current.

0x012 - Temperature

Mode: Read only

The current output FET temperature.

0x013 - Energy

Mode: Read only

Total energy provided to the downstream port since power was enabled.

B. diagnostic command set (power_dist only)

All tel and conf class commands from moteus reference are supported. In addition, the following diagnostic mode commands are available.

p lock

Request that the lock time be set.

p lock <time_in_100ms>

C. Mechanical / Electrical

Mechanical

power_dist: 4x M2.5 mounting holes are provided in a 74mm x 44mm rectangular pattern.

mjpower-ss: 4x M2.5 mounting holes are provided in a 64mm x 44mm rectangular pattern.

Pinout

XT-90 Input

The XT-90 connector has a - and a + imprint on the housing.

XT-30 Output

The XT-30 connectors have a - and a + imprint on the housing.

JST PH-4 Switch

A small white dot is present on the silk screen near pin 1.

  • 1 - LED+ - LED positive
  • 2 - LED- - LED ground
  • 3 - SWG - Switch ground
  • 4 - SWP - Switch positive

Onboard Switch (mjpower-ss only)

The onboard switch is connected in parallel to the external switch. If either the onboard switch is set to the ON position, or the external switch pins are tied together, the board will pre-charge and provide power to the output when the input voltage reaches a small threshold below the minimum input voltage.

JST PH-3 CAN (power_dist only)

Looking at the pins of the connector with the mjbots logo right side up, the pins are numbered from the bottom to the top.

  • 1 - CAN_H
  • 2 - CAN_L
  • 3 - GND

NOTE 1: CAN connections should be terminated by a 120 ohm resistor at both ends of a bus. Some mjbots products have built in termination resistors, such as the pi3hat. The fdcanusb has a software configurable termination resistor that is by default on. The power_dist board has no termination resistors. For very short runs, the system will work terminated only on one side. However, when runs become longer than 0.5m, you will likely need to terminate both ends. This can be done by crimping a 120 ohm resistor into a JST PH3 connector and connecting it to the open data connector.

NOTE 2: Ground may not be necessary, only one path through ground in a system should exist to avoid ground loops. In a typical robot application with a common ground, that role is filled by the power ground. However, in desktop applications, it may be appropriate to connect the CAN ground if the device power supply is otherwise isolated.

JST ZH-6 SWD (power_dist only)

Looking at the pins of the connector with the mjbots logo right side up the pins are numbered 1 to 6 from top to bottom.

  • 1 - NC
  • 2 - NRST
  • 3 - SWDIO
  • 4 - GND
  • 5 - SWCLK
  • 6 - 3.3V

D. Maintenance (power_dist only)

Building firmware

Building is supported on Ubuntu 20.04 with the following command:

tools/bazel test //:target

Flashing firmware

A firmware image (.elf file), can be flashed from a linux PC using the 6-pin SWD debug connector.

One time:

sudo apt install binutils-arm-none-eabi

Ensure that power is applied, the switch is turned on, and no loads are connected, then:

fw/flash.py optional/path/to/file.elf

openocd

As with moteus, openocd 0.11.0 or newer is required, see:

https://github.com/mjbots/moteus/blob/main/docs/reference.md#openocd