The Annin Robotics robot line is AR2, AR3, and AR4. AR4 hardware revisions are MK1, MK2, MK3, MK4, and MK5. What follows is the official revision history, not a spec sheet. If you need numbers (reach, payload, motors, DIP settings, BOM), use the build manual that matches your mark on the downloads page.

AR2

First major release of the project.

• Arduino Mega controller
• Separate control enclosure — electronics were not in the robot base
• Open-loop motors, no encoders
• J3 drive was chain and sprocket

AR3

Controller and encoder hardware both changed.

• Teensy 3.5 control board
• External CUI encoders and dual-shaft motors
• J3 drive changed from chain/sprocket to an L-series belt and pulley

AR3 still used a separate enclosure. Encoder and limit-switch voltage are not the same as AR4. The AR3 notes described those CUI encoders as adding closed-loop operation; that is the hardware history. How the stock software actually uses encoders is covered in Are the AR4 joint encoders closed-loop?.

AR4 MK1

This is the first AR4. Older posts often just call it “the AR4.”

• Teensy 4.1 control board
• External CUI encoders dropped; motors come with integrated encoders
• Electronics still in a separate enclosure (that moves into the base on MK2)

AR4 MK2

• All electronics moved into the robot base — no external control box
• Drives changed from the larger DM542T to the smaller DM332T
• J1, J2, and J3 motors updated to tighter-tolerance EG-series gearmotors

AR4 MK3

• J4 and J6 motors updated to EG-series gearmotors
• J3 drive updated to HTD-series belt and pulleys
• First aluminum-part change on the AR4 line: J6 housing and J6 motor mount, to fit the new motor

AR4 MK4

• Larger base enclosure
• 2×12 terminal board in place of the earlier 3-post terminal block
• Servo gripper control board moved into the base, so you do not need a separate gripper enclosure
• ACS712 current sensor added to protect the gripper from overheating
• J5 limit switch updated to a KW12 switch

AR4 MK5 (current production kit)

New combo kits we sell now are AR4 MK5 (you may also see it written AR MK5). We are not going to invent a mechanical changelog here. For the current bill of materials, print files, motor-step sheet, and wiring, use the AR4-MK5 Robot Build Manual and the MK5 section on the downloads page. That page also has an MK4-to-MK5 hall-effect sensor upgrade document if you are coming from MK4.

Published “AR4” specs — reach 26 in, payload 4.15 lb, repeatability 0.3 mm — live on our specifications article, which still cites the MK3 manual. Treat those as a starting point and verify them in the manual for the mark you actually have.

Software and firmware

Each mark has its own downloads tab. Pair the HMI version with the matching Teensy sketch (and the Nano/Mega aux sketch if you use one). Do not mix an AR3 Teensy 3.5 / CUI setup with current AR4 MK5 files.

What you should buy or upgrade to

If you are buying a new kit, it is AR4 MK5. If you already own an older arm, follow the upgrade-path article. That article currently walks AR2 / AR3 / MK1 / MK2 / MK3 up to MK4, not MK5. Use it together with the MK5 downloads for anything newer.

See What are the available upgrade paths for the AR4 robot arm? and Downloads.