mjbots

mjbots product lines

Who mjbots is

mjbots (mjbots Robotic Systems) makes motor controllers and accessories for robots whose joints are torque-controlled, such as legged robots and backdrivable arms. Its moteus controllers are small circuit boards that mount on the back of a brushless motor, read its position with an on-axis magnetic encoder and run field-oriented control. Firmware is open source under the Apache 2.0 licence.

For OEM engineers, mjbots is a fast route from a hobby or drone-class brushless motor to a working servo joint, especially for prototypes and research robots.

History and ownership

The About page says mjbots has provided components and assemblies for dynamic torque-controlled robots since 2019. The website does not publish a street address, ownership details or headcount. The current store lists controllers, a motor and accessories rather than complete actuators.

Product portfolio

All moteus boards share the same 5 Mbps CAN-FD data link, integrated absolute magnetic encoder, 15 to 30 kHz control rate and -40 to +85 °C rating. They differ in voltage, current and size:

• The moteus-c1 is the smallest, at 38 × 38 × 9 mm and 8.9 g. It runs on 10 to 51 V with 20 A peak and 5 A continuous phase current (14 A with cooling) and 250 W peak.
• The moteus-r4 (r4.11) measures 46 × 53 mm and runs on 10 to 44 V with 100 A peak and 12 A continuous phase current (32 A with cooling) and 900 W peak.
• The moteus-n1 measures 46 × 46 × 8 mm, runs on 10 to 54 V, and gives 100 A peak, 9 A continuous (26 A with cooling) and 2 kW peak. It adds an RS422 connector and auxiliary SPI, UART, I2C, quadrature and Hall inputs.
• The moteus-x1 is the largest, at 56 × 56 mm and 23.7 g, with 120 A peak, 25 A continuous (62 A with cooling) and 1.3 kW peak on 10 to 54 V.

Developer kits bundle a controller with the mj5208 motor. Accessories include a CAN-FD to USB adapter, a Raspberry Pi expansion board with several CAN-FD ports, and power distribution boards. A bare controller needs a CAN-FD or UART adapter, which is not included.

Technologies

• Field-oriented control. FOC with an on-axis encoder gives smooth torque at low speed, which matters for backdrivable joints.
• CAN-FD. At 5 Mbps, one bus can carry fast torque commands to several joints. Check update rates with the number of joints you plan.
• Open source. Open firmware lets a team inspect and modify control loops. In production, it also means you own configuration control.

The large gap between “without thermal management” and “with thermal management” continuous currents is the key spec to read. For example, the moteus-x1 is 25 A uncooled and 62 A cooled. As a rule of thumb, design to the uncooled figure unless the board is bolted to a heat sink you have tested.

See how to select a PCB-mount servo drive, how servo drives work and frameless motor sizing for robot joints.

Industries served

mjbots aims at legged robots, robot arms and research robotics, where low cost and fast iteration matter more than industrial certification. The boards also suit test rigs and small automation projects that need a compact torque-controlled axis.

Worked example: matching a controller to a motor

A knee joint uses a brushless motor with a torque constant of 0.1 Nm/A behind a 6:1 reduction and needs 24 Nm peak at the output. The motor must give 4 Nm, which is 40 A of phase current. The moteus-c1 (20 A peak) is too small, while the r4, n1 or x1 all cover 40 A peak. If the joint must hold 8 Nm continuously, that is 13.3 A continuous, above the r4’s 12 A uncooled rating but within the x1’s 25 A, so the x1 is the safer choice without a tested heat sink.

Things that catch engineers out

• Adapters. A bare controller cannot be configured without a CAN-FD or UART adapter, so order one with the first boards.
• Voltage headroom. The r4 tops out at 44 V; a 12-cell battery at full charge exceeds that, so use the n1 or x1 for 12S packs.
• Encoder alignment. The on-axis encoder needs a diametric magnet centred on the motor shaft; poor alignment degrades torque ripple.

Global and UK presence

mjbots sells direct online with international shipping and lists no UK office or distributors. If you are moving from prototype to production, check the lead time, firmware version control, EMC testing and whether you need CE or UKCA paperwork for the finished product.

What to send when specifying a joint drive

Send the motor’s torque constant, phase resistance and pole pairs, the supply voltage, the continuous and peak phase currents, the bus and update rate, the encoder arrangement and the cooling available.

Specify Motion is independent of mjbots and is not a distributor for the company. Our engineers can help source equivalents from our supply network on published figures; see find a replacement or the servo drives hub.

mjbots history: key dates

1. 2019mjbots begins supplying components for dynamic torque-controlled robots

mjbots servo drives

PCB-mount drive

moteus-r4 (r4.11)

46 × 53 mm brushless FOC controller with an integrated absolute magnetic encoder for 10 to 44 V: 100 A peak phase current, 12 A continuous (32 A with cooling), 900 W peak and 5 Mbps CAN-FD.

PCB-mount drive

moteus-n1

46 × 46 × 8 mm brushless controller for 10 to 54 V with 100 A peak and up to 26 A continuous phase current (with cooling), 2 kW peak electrical power, CAN-FD and RS422 plus SPI, UART and quadrature auxiliary ports.

PCB-mount drive

moteus-x1

56 × 56 mm brushless controller for 10 to 54 V with 120 A peak and up to 62 A continuous phase current (with cooling), 1.3 kW peak electrical power, CAN-FD and RS422.

PCB-mount drive

moteus-c1

Small 38 × 38 mm brushless controller for 10 to 51 V with 20 A peak and up to 14 A continuous phase current (with cooling), 250 W peak, an integrated absolute encoder and CAN-FD.