Keywords: CMOS Battery, Rethink Sawyer, Intera OS, Embedded Linux, Robot Hardware, Joint Actuation, Solenoid, Encoder Diagnostics

System Overview and Initial Problem Description

This document details the diagnostic and repair procedures undertaken for a Rethink Sawyer robotic arm, integrated into a Sawback mobile manipulator platform (utilizing a Clearpath Ridgeback base). Significant challenges were encountered during the initial setup and commissioning of the arm.

Initial Boot Sequence Failure and Subsequent Control Issues

The primary boot-up sequence of the robot failed due to a depleted CMOS battery, which resulted in system clock synchronization errors. Subsequent to CMOS battery replacement, a secondary issue manifested: the inability to transmit motion commands effectively to the arm.

Joint Enablement Anomaly: right_j1

The Sawyer arm supports two primary operational modes: an SDK mode, facilitating control via the Intera SDK over ROS (Robot Operating System), and an Intera mode, which permits direct control using the integrated human-machine interface (HMI) on the arm. Standard procedure requires enabling the arm via a control knob post-boot. During this process, the system reported the following error:

[ERROR] Failed to enable robot, press the knob to try again.

Concurrently, a red status LED illuminated, indicating a fault associated with the right_j1 joint—specifically, an inability for the joint brake to disengage. Preliminary diagnosis indicated the fault originated within the unlatching mechanism of the right_j1 joint.

Manual articulation attempts of the right_j1 joint revealed it was seized, accompanied by audible grinding sounds, strongly suggesting an internal mechanical or electro-mechanical fault.

Further investigation involved partial disassembly of the right_j1 joint for visual inspection of its internal components, with a focus on the brake/locking mechanism. The solenoid responsible for actuating this mechanism exhibited no movement during robot enable attempts. In contrast, observation of the equivalent solenoid in the adjacent, functional right_j2 joint showed correct actuation. This localized the anomaly to the right_j1 solenoid assembly or its immediate control circuitry.

Corrective Actions and Component-Level Repair

The identified problem with the right_j1 unlatching mechanism necessitated disassembly of the joint to diagnose the specific mechanical or electrical fault.

Encoder System Verification

Initial diagnostic efforts focused on the right_j1 joint encoder. The joint housing was opened to access the encoder wiring. A digital multimeter (DMM) was utilized to perform continuity tests across all relevant conductors and connections. These tests confirmed the integrity of the encoder wiring; no open circuits or faulty connections were detected.

Brake Mechanism Analysis

Subsequent to verifying the encoder system, attention shifted to the joint’s solenoid-actuated brake mechanism. This mechanism, by design, engages a mechanical lock on the joint’s gearing when the solenoid is de-energized.

Solenoid Electrical and Functional Tests

A systematic electrical evaluation of the right_j1 solenoid was performed prior to full disassembly.

1. A DMM was used to measure the continuity of the solenoid coil. The coil exhibited good continuity, indicating its electrical integrity.
2. The DMM was also employed to verify the presence of the energizing voltage signal from the robot’s controller during attempts to enable the joint. This voltage was confirmed to be present.

Following these in-situ DMM tests, the solenoid was electrically isolated from the robot and connected to an external DC power supply, with the appropriate voltage applied to directly command actuation. Despite direct energization, and with prior DMM tests confirming coil continuity and the presence of a control signal from the robot, the solenoid demonstrated no mechanical movement.

This comprehensive series of tests conclusively isolated the fault to an internal mechanical issue within the solenoid assembly itself, rather than an electrical fault in its coil, the control signals, or associated wiring. Consequently, the solenoid unit was completely disassembled for detailed inspection of its internal components.

Identification and Rectification of Internal Solenoid Fault: Deformed Spring

Upon disassembly of the solenoid, the root cause was identified: the internal actuating spring, while not fractured, had become detached from its retaining cylinder and was severely deformed (flattened), losing its required coiled geometry. This spring is a critical component, as it acts upon an internal plate to retract the brake mechanism when the solenoid is energized. In its deformed and detached state, the spring was incapable of providing the necessary actuation force.

The repair procedure involved:

1. Careful manual reformation (rewinding) of the flattened spring to restore its original helical shape.
2. Secure re-attachment of the reformed spring to its designated mounting point on the internal cylinder.

This was a precise operation, requiring careful alignment to ensure the spring was correctly seated against its actuating plate and that the entire mechanism could move without impedance once the solenoid was reassembled. After successful reformation and re-installation of the spring and confirming the plate’s freedom of movement, the solenoid and subsequently the entire right_j1 joint were reassembled.

Post-reassembly, the right_j1 joint and the Sawyer arm responded correctly to enable commands and subsequent motion control tests.