Bosch Rexroth 21/1F-D Servo Drive – DIAX Series

Bosch Rexroth 21/1F-D: Single-Axis Digital Servo Amplifier in Closed-Loop Motion Control Architecture

The Bosch Rexroth 21/1F-D is a single-axis digital servo amplifier belonging to the Rexroth DIAX / Indramat legacy drive platform. Within a servo control loop, this unit occupies the power-stage and current-regulation layer between the motion controller and the servo motor. It receives torque or velocity setpoints via a digital fieldbus (SERCOS fiber-optic ring) or an analog ±10 V reference, executes real-time current-vector control at the PWM switching layer, and returns encoder or resolver feedback to the upstream controller at cycle times consistent with the DIAX04 firmware architecture. The result is deterministic torque delivery with position error typically held below ±1 encoder count under steady-state load.

In multi-axis machine tool installations — milling centers, grinding machines, and servo-press lines — the 21/1F-D operates as a subordinate node on a SERCOS ring, receiving interpolated position commands from an IndraControl or compatible CNC at 1 ms or 2 ms cycle intervals. Its internal current controller runs at a significantly faster rate, decoupling mechanical resonance from the position loop and allowing the machine builder to tune axis stiffness without destabilizing the outer position loop.

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Technical Parameters

Hardware Logical Analysis

The 21/1F-D implements a three-tier cascade control structure in firmware. The outermost position loop closes at the SERCOS cycle boundary (1 ms or 2 ms), the velocity loop executes at a sub-millisecond rate derived from the encoder/resolver interpolation engine, and the innermost current (torque) loop operates at the PWM carrier frequency — typically 4 kHz to 8 kHz. This separation of bandwidths is the architectural reason the drive can maintain stiff axis response without requiring the CNC to handle current regulation directly.

The resolver interface uses a dedicated ASIC-based R/D (resolver-to-digital) converter that tracks rotor angle continuously, independent of the main DSP cycle. This means commutation angle is always current even during DSP interrupt latency, preventing torque ripple at low speeds — a critical requirement in grinding and honing applications where surface finish is directly coupled to velocity uniformity.

EMC design on the DIAX platform uses a combination of common-mode chokes on the DC bus rails, shielded gate-drive transformers to isolate the IGBT gate signals from the control board ground plane, and a dedicated PE (protective earth) bonding point on the heatsink. The result is that conducted emissions on the AC supply side remain within EN 61800-3 Category C2 limits without an external line filter in most cabinet configurations — reducing BOM cost and panel space.

The DC bus architecture supports shared-bus operation: multiple 21/1F-D units can connect their DC bus rails together, allowing regenerative energy from a decelerating axis to be consumed by an accelerating axis on the same bus segment. This reduces peak demand on the mains supply and lowers heat dissipation in the cabinet, which is measurable in high-duty-cycle press and stamping applications where axes are continuously accelerating and decelerating in coordinated sequences.

Fault detection logic monitors IGBT junction temperature via NTC sensors embedded in the power module, DC bus voltage within ±5% of nominal, phase current symmetry, and resolver signal amplitude. Any parameter exceedance triggers a latched fault with a coded error number stored in non-volatile memory, retrievable via the IndraWorks commissioning tool or the front-panel H1 display. This on-board fault history reduces mean time to diagnose (MTTD) during field service calls.

System Integration Benefits

• Deterministic SERCOS cycle synchronization: The drive phase-locks its internal clock to the SERCOS ring master, ensuring all axes in a multi-drive system execute their current-loop updates within nanoseconds of each other — a prerequisite for coordinated contouring in 5-axis machining.
• Plug-compatible with Rexroth DIAX04 topology: No re-engineering of the control cabinet wiring harness, motor feedback cables, or CNC parameter sets is required when replacing a failed unit with another 21/1F-D of the same sub-variant.
• Shared DC bus energy recovery: Regenerative braking energy is redistributed across the bus segment, reducing peak mains current draw by up to 30% in coordinated multi-axis cycles.
• On-board diagnostic transparency: Coded fault history in non-volatile memory allows field engineers to reconstruct the sequence of events leading to a fault without oscilloscope instrumentation.
• Resolver-based absolute commutation at power-on: The drive establishes correct commutation angle within one resolver electrical cycle after power-up, eliminating the need for a homing move before the first motion command in many machine configurations.
• Firmware-selectable operating modes: Torque control, velocity control, and position control modes are selectable via SERCOS IDN parameters, allowing the same hardware to serve as a spindle drive, feed axis, or force-controlled press axis depending on the application.
• Thermal derating management: The drive automatically reduces peak current limits as heatsink temperature approaches the thermal threshold, preventing hard shutdowns during sustained high-load cycles and giving the machine controller time to complete the current machining pass before requesting a pause.
• IndraWorks software integration: Full parameter read/write, oscilloscope trace capture, and firmware update are supported via the RS232/RS485 service interface and the Rexroth IndraWorks DS engineering environment, reducing commissioning time compared to drives requiring proprietary handheld programmers.

Quality Assurance & Global Logistics

Every Bosch Rexroth 21/1F-D unit supplied by siemensplc.com is sourced through verified industrial channels with full traceability documentation. Prior to dispatch, each unit undergoes a functional verification sequence: DC bus charge/discharge cycle, IGBT gate-drive integrity check, resolver signal amplitude and phase verification, and a no-load motor spin test where applicable. Units supplied as new carry the original Bosch Rexroth factory warranty documentation. Refurbished units carry a 12-month warranty from the dispatch date, backed by our in-house technical team in Xiamen, China.

Logistics operations are managed from our Xiamen warehouse, with direct access to Xiamen Gaoqi International Airport and Xiamen Port — two of China’s primary export hubs for industrial components. Standard international shipments are dispatched via DHL Express, FedEx International Priority, or UPS Worldwide Expedited, with typical transit times of 3–7 business days to Europe, North America, Southeast Asia, and the Middle East. Export documentation including commercial invoice, packing list, and certificate of origin is prepared in compliance with destination country customs requirements. For time-critical plant shutdowns, same-day dispatch is available for in-stock units when orders are confirmed before 14:00 CST.

Contact Information

📧 Email: [email protected]
💬 WhatsApp: +86 18359268345
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📍 Location: Xiamen, China
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