WOBBLarm

Can these hobby servos lift a 500 g payload through this desktop arm's pick and place?

Report
2026-10-15 19:09

FAILS

The shoulder servo stalls. Carrying 500 g through this move needs 1.35 N m (13.7 kg cm) almost at rest, where the TowerPro MG996R at 6 V gives 1.07 N m (11 kg cm). The hand hits the table at 0.241 s.

hardest joint

shoulder

needs

1.35 N m

servo gives

1.07 N m

heat

362% of rated

Confidence high

passed hand statics, forward run, time step convergence, energy account

Simulation

Torque and speed each servo needs against its torque speed line

Share of each servo's available torque through the move

The move at several instants, from the side and from above

Checks

status	check	result
pass	hand statics	at the pick pose the moments of the weights about each joint come to shoulder 1.29 N m, elbow 0.671 N m, wrist 0 N m, and MuJoCo's inverse dynamics needs 1.29, 0.671, 0 N m (1.7e-14% apart)
pass	forward run	the run with the servos' real limits falls 84.1 degrees behind the move, as the inverse dynamics says it must
pass	time step convergence	how far the shoulder falls behind 58.8 deg with the normal step and 58.7 deg with half the step (0.031% apart)
pass	energy account	the work of the forces (drives 0.521 J, constraints -0.826 J) matches the change in kinetic and potential energy, -0.305 J, to 0.13% of the largest energy flow

How to fix it

Drive the elbow through 2.5:1 and the shoulder through 5:1 with a belt or gear stage

Each servo then sees a fraction of the torque and turns that much faster, with 10% lost in the stage. Solving the move again, the busiest servo uses 44% of its torque at speed and the warmest 85% of its continuous rating.

Use a DSSERVO DS3218 at the elbow and a ROBOTIS Dynamixel XM430-W350 at the shoulder

Solving the move again with these, the busiest servo uses 39% of its torque at speed and the warmest 98% of its continuous rating.

Balance the shoulder with a 0.644 N m spring or 660 g on a 100 mm tail

Either carries about half the holding torque. Solving again with the counterweight, the shoulder uses 83% of its torque at speed and 233% of its continuous rating, and the base servo swings the extra mass, where a spring adds none.

Keep the payload under about 25.1 g

That is the heaviest load these servos carry through the same move over and over without running hot, found by solving the move at each weight. A single move without a stall works up to about 368 g.

Events

event	time	detail
shoulder servo at its limit	0.000 s	asks 1.29 N m (13.1 kg cm), gets 1.08 N m (11 kg cm)
pick	0.000 s
elbow servo at its limit	0.120 s	asks 0.654 N m (6.66 kg cm), gets 0.645 N m (6.58 kg cm)
wrist servo at its limit	0.190 s	asks 0.148 N m (1.51 kg cm), gets 0.141 N m (1.44 kg cm)
the hand hits the table	0.241 s	up to 97.7 N
base servo at its limit	0.420 s	asks 3.35 N m (34.1 kg cm), gets 1.07 N m (11 kg cm)
the base touches the upper arm	0.444 s	up to 141 N
lifted	1.200 s
over the place point	2.400 s
placed	3.600 s
end of the hold	4.100 s

Assumptions

Wobbl guessed: servo not named, so Wobbl assumed a TowerPro MG996R at every joint.
The arm has a TowerPro MG996R at 6 V at the base, shoulder and elbow and a TowerPro MG90S at 6 V at the wrist. Each can carry 33% of its stall torque continuously unless a rating is given.
The move picks at 200, 0, 40 mm, lifts 80 mm, swings over to 0, 200, 40 mm and lowers, each step in 1.2 s on a minimum jerk profile, then holds 0.5 s. The heating estimate repeats that loaded cycle.
The servos follow the move by computed torque, as a well tuned controller would, and a hobby servo's own loop lags a little more.
Each servo's motor, felt through its gearbox, adds the inertia of a 20 ms mechanical time constant, typical of small brushed motors, since servo datasheets do not give it.
The 500 g payload and the gripper are point masses at the tip, and the base is screwed to the table.
Bodies are rigid, so links do not bend. A long or thin printed link flexes and rings more than this shows.
Contacts are stiff springs that let parts sink into each other by the depth the contact check reports.
Joints and gears have no backlash, and friction acts only where the inputs give it.