Robot Arm Kit for Romi

This Robot Arm Kit is an accessory for the Romi Chassis that allows it to manipulate small objects around it. The kit includes a micro gripper with paddles that can slide up to 32 mm apart, arm linkages for supporting the gripper, and a platform to mount everything to. The assembly is actuated by three included servos that are specially modified to provide access to their feedback potentiometers through a fourth (green) wire, which lets you detect if something is interfering with their ability to get to their commanded positions. This feedback also enables more complex closed-loop control of the arm from your main controller.

The Romi arm mounts on the rear half of the Romi chassis, just above the motors, using the included aluminum standoffs. Two standard-size servos sit at the base of the arm and control the height and tilt angle of the micro gripper while a micro servo is housed inside the gripper to actuate the paddles themselves. The larger lift and tilt servos can briefly draw up to around 1.8 A each when commanded to move abruptly, but the typical current draw should be under an amp each. The stall current of the micro gripper servo is approximately 0.8 A at 6 V. All three servos are intended to be powered from 4.8 V to 6 V (nominal) battery packs or a suitable power supply in that general range. For each servo, there is an approximately 1:1 correspondence between the servo position in milliseconds and the feedback voltage in millivolts, so for example, the feedback voltage will be around 1.5 V at the position corresponding to 1.5 ms servo pulses.

Range of motion of the height of the Robot Arm for Romi.

Range of motion of the wrist tilt on the Robot Arm for Romi.

Range of motion of the gripper on the Robot Arm for Romi.

The gripper, which can be mounted to the end of the arm in either a horizontal (scoop) or vertical (crane) orientation, uses a rack and pinion design with the pinion gear mounted to the servo spline and opposing racks to keep the gripper paddles synchronized and parallel to each other throughout the range of motion.

Romi arm with horizontal gripper orientation installed on a Romi.

Romi arm with vertical gripper orientation installed on a Romi.

The maximum recommended payload of the arm is 3.5 oz (100 g), and the gripper can open to a maximum width of 32 mm. Please note that the full range of motion of the gripper servo requires pulses beween 0.5 ms (fully open) to 2.4 ms (fully closed), which well beyond the standard hobby servo range of 1 ms to 2 ms, so you should make sure your control electronics is capable of generating such signals if you care about getting the full range.

Kit contents

• One Micro Gripper with Position Feedback Servo
  • One micro servo with position feedback
  • One top micro gripper case
  • One bottom micro gripper case
  • One micro gripper plate
  • One micro gripper pinion gear
  • Two micro gripper paddles with rack
• One Romi expansion platform
• One main arm
• One rear tilt arm
• One front tilt arm
• One pivot stand
• Two servo mounting brackets
• One short linkage
• Two pivot transfer linkages
• One tilt servo horn
• One lift servo horn
• One alternate tilt servo horn attachment
• One alternate life servo horn attachment
• Two standard-sized servos with position feedback
• Six 20 mm nylon spacers, 4 mm OD, 2.7 mm ID
• Three 6 mm nylon spacers, 4 mm OD, 2.7 mm ID
• Two 1.5″ aluminum standoffs, #2-56 thread, M-F
• Two 1″ aluminum standoffs, #2-56 thread, M-F
• Six 1″ #2-56 Machine screws
• Three 3/8″ #2-56 machine screws
• Four 1/4″ #2-56 machine screws
• Thirteen #2-56 hex nuts
• Nine #2 washers
• Six 8 mm M3 machine screws
• Six M3 hex nuts