Bringing a Year-2000 Fanuc LR Mate Back to Life — Dead Batteries, a Fried Battery Holder, and a Disintegrated Rotor
The recommissioning started with a software-level fault (MCTL-013 enable input off) and quickly surfaced the most common issue on any robot that has sat unused: dead encoder backup batteries. The two 3V cells in the upper arm, wired in series for 6 V, were replaced with four AA cells — but a connector mix-up briefly shorted the supply, melting a battery holder contact. The fix was to abandon the battery holder entirely and run a bench power supply to the encoder backup circuit, with the caveat that switching it off erases all position memory.
After mastering (zeroing each joint against its witness marks via the teach pendant), the robot jogged and ran a test program successfully. XYZ-mode jogging moved the wrist endpoint in straight Cartesian lines by coordinating all joints simultaneously — a basic sanity check that calibration was correct. A disturbance-excess error on joint 5 flagged possible harmonic-drive wear, though that remains unconfirmed.
A second, crash-damaged robot provided a hardware teardown opportunity. The J2 harmonic drive is completely destroyed and the joint spins freely; J3's flex spline has begun to twist and buckle. One servo motor's rotor disintegrated — magnetic dust packed against the stator, locking the shaft — a condition the host calls unrepairable because the rotor's magnetic properties are permanently altered. The brake still clicks and draws current, making diagnosis without disassembly non-obvious.
The host closed with a comparison of the two main industrial reducer types: - Harmonic drives (strain wave): flex spline, rigid outer ring with two extra teeth, elliptical bearing — each input revolution shifts output by two teeth, giving 50:1–200:1 reduction in a compact package but with a fragile flex element. - RV reducers (cycloidal): two counter-phased cycloidal discs, rotating outer pins, and an upstream spur stage — the rolling pins replace sliding contact with a full cylindrical bearing surface, greatly extending service life. The UP6 uses RV reducers on joints 1–3 and harmonics on the wrist joints.
Bottom line: dead encoder batteries are a routine, low-stakes fix on aged Fanuc robots; the real risks are connector errors during battery swaps and blindly replacing fuses before understanding the root cause. Mechanical damage to gear reducers and rotors, by contrast, is expensive and largely non-repairable in the field.
The host walks through a full recommissioning of an early Fanuc robot, hitting the classic dead-encoder-battery problem, a self-inflicted wiring mishap that melted a battery contact, and a failed servo motor whose rotor literally came apart. The session ends with a deep-dive on harmonic drives and RV reducers.
Dead batteries are basically the standard issue with FANUC robots after they sit for a while. Not at all a big issue to fix.
MCTL-013 enable input is off is related to some sort of software configuration in the controller itself
The encoder batteries are located up in the top part of the arm, not in the base
The two encoder batteries are wired in series to produce 6V
The original batteries were 3V each; four 1.5V replacement cells wired in series substitute for them
If the power supply is shut off, the robot will forget where it is and lose all encoder position information
In jog mode, joint speed and linear move speed are limited well below the robot's actual maximum speed; full program speed requires predefined points in a written program
Linear moves in the test program are set to 500 mm per second, which is below the robot's full speed of approximately 1 meter per second
This robot is one of Fanuc's very first LR Mates and is from the year 2000
Basic stepper motors are uncommon in industrial grade machines since they have much lower power density than AC or DC servo motors
Most robot servos have spring-loaded brakes that require power to unlock and hold the motor stationary by default to prevent the arm from falling when unpowered
The brakes are sized to provide a bit above the continuous torque output of the motor, so stopping torque from brakes engaging at full speed is still within acceptable range
Shorting motor phases together when releasing a brake makes the motor stiff to turn, preventing the joint from moving fast
On the bad motor, the brake still draws power and makes a click sound like it is releasing, but the shaft stays locked in place with no backlash
The rotor on the failed motor came apart and magnetic dust covering it jammed against the inside of the stator preventing rotation; this is unrepairable because the magnetic properties of the rotor have changed
The J2 harmonic drive reducer on the crashed robot is completely destroyed and the joint is free to move; the J3 reducer is stiff and the flex spline has started to twist and buckle
The high-pitched squeaky sound from the joints at full speed is mainly from the servo drives running the motors at a lower switching frequency, not from failed gear reducers or motors
The UP6 robot uses small RV reducers on joints 1, 2, and 3 with small harmonic drives on the remaining joints
The rotating outer pins in RV reducers change the wear surface from a small contact point to a large cylindrical bearing surface, reducing friction and greatly improving reducer life
Accidentally swapped the connectors when plugging in replacement batteries, putting 6V on the inner pins which the circuit board shorts together, causing a dead short that melted a battery holder contact
Disturbance excess error appeared on joint 5, which could indicate one or more harmonic drives are in not super great condition
Blindly replacing fuses without knowing the original cause could potentially cause even more damage
Mastering (Fanuc's term for calibration) requires lining up witness marks on each joint to the zero position, then saving calibration in the teach pendant
After calibration, XYZ jogging mode moves the wrist endpoint in straight lines relative to world coordinates by coordinating all joints simultaneously, rather than controlling each joint individually
When troubleshooting a robot controller, check error meanings in the controller manual first; blown fuses due to improper or damaged wiring are one of the first places to look if getting bad errors or no power-on
Harmonic drives (strain wave gear reducers) use a thin flexible inner spline, a rigid outer spline with two more teeth, and an elliptical bearing that bends the flex spline to mesh at two points; each full input rotation shifts the splines by two teeth, yielding 50:1 to 200:1 reduction
RV reducers use cycloidal discs surrounded by rotating outer pins with a count off by one, plus additional spur gear reduction via offset crankshafts; two counter-phased discs cancel vibration and balance loads on crankshaft bearings
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