Coaxial Virtual 4-bar Differential Arm
Two motors on a shared axis driving a virtual four-bar that keeps the end effector level while a differential controls wrist roll. The most compact arm-and-wrist on the field — and the hardest to get right.
Before you start
Read up on differential drivesA differential mixes two inputs into two outputs. If both motors turn the same direction, the arm pivots and the wrist stays level. If they turn opposite, the wrist rolls and the arm stays put. Sum and difference of motor angles control sum and difference of arm/wrist positions. before starting — this design depends on understanding the kinematics.
1Draw the kinematics in 2D
On the front plane, sketch the arm length, end effector position, and the four-bar links that keep the effector level as the arm swings. The kinematics have to work on paper first — there's no fixing this in CAD.
Verify by hand: as the arm pivots through its range, the effector mounting face should stay horizontal.
2Lay out the two coaxial motors
Both motors mount concentric to the arm's pivot axis, one on each side of the side plate. Through-shaftFor a clean coaxial design, both motors' output shafts share the same rotation axis as the arm's pivot. One motor drives the arm; the other drives a coaxial sleeve that runs the differential belt out to the wrist. is what enables the differential.
Account for motor body width — your arm has to clear both motors' bodies through its full swing.
3Build the virtual four-bar
The four-bar uses one rigid arm link plus three belt-driven pulleys. As the arm rotates, the belt forces the end-effector pulley to counter-rotate, holding the effector level.
Belt tension is critical here — a slack belt lets the effector droop, a tight one increases joint friction. Add an idler with adjustment for tensioning.
4Wire the differential to the wrist
The second motor drives a pulley coaxial to the first. A second belt runs from that pulley out to the wrist. The math: wrist angle = (motor1 + motor2)/2 + arm angle correction; effector roll = (motor1 - motor2)/2.
Test the math in Onshape: drive both motor angles and verify the arm and wrist move as the kinematics predict. Build a state tableTest extreme states: both motors at 0°, both at 90°, one at 0° and the other at 90°, etc. If any of these produce nonsense kinematics, your geometry or belt ratios are wrong. first.
5Stiffen everything
The arm wants to whip — a 2 kg end-effector on a 0.5 m arm at full extension is a huge moment. Cross-brace the arm itself, support the pivot with bearings on both ends, and use the largest pulleys your packaging allows so belt tension translates to less arm deflection.
Drive the arm to full extension under simulated load and check for visible flex. Anywhere it flexes, you'll lose end-effector position accuracy.
This is one of the most capable manipulators in FTC, and one of the most temperamental. Test the math, test the math, and then build the parts as stiff as you can.