Upgrading MM420 to SINAMICS S120 for 1mm Motor Sync Precision

1. Application Overview: Steel Forming Line with Four Synchronized Motors

A typical steel forming line requires precise mechanical coordination between four asynchronous induction motors driving sequentially-coupled processes. In the reference configuration discussed in field practice, the line architecture is:

The line is treated as a rigid product path: the steel piece must traverse all four stations without slipping relative to any driven roller, and the cut/stamp position on Axis 4 must land within ±1 mm of the geometric reference set by Axis 1. Because position is defined by the steel itself (not the motor shaft), the load-side encoder must be mechanically coupled to the strip — typically via a spring-loaded measuring wheel riding on the strip surface.

2. Problem Statement: Why MM420 Cannot Deliver 1 mm Synchronization

The MicroMaster 420 (MM420) is a general-purpose V/f and Flux Current Control (FCC) drive. It is not designed for position-synchronized multi-axis coordination. The following constraints are inherent to the MM420 platform and were specifically called out by experienced practitioners in the field discussion:

1. No closed-loop torque or speed control with encoder feedback. MM420 does not accept an encoder option module. Speed estimation is derived from the V/f curve and motor model, leaving slip uncompensated.
2. No vector control. Without field-oriented control, transient torque response and steady-state speed accuracy under load steps are too poor for synchronized registration of a moving strip.
3. Cascaded communication delay. The position signal path FM350 → CPU backplane → PROFIBUS → MM420 → inverter → motor introduces 2–4 communication cycles of delay. At a 4 ms PROFIBUS cycle, that is 8–16 ms of lag, which at 1 m/s line speed equals 8–16 mm of position error — three orders of magnitude beyond the 1 mm target.
4. No gearing/synchronization primitives. MM420 has no electronic cam, no virtual master, and no ratio block. Synchronization must be performed entirely by the PLC at the application layer, multiplying the closed-loop delay.

Conclusion: A 1 mm synchronized registration cannot be achieved on a four-motor line using MM420, regardless of PLC scan time or encoder choice. The drive platform itself is the bottleneck.

3. Encoder Resolution and Mechanical Calculations

Before migrating, validate that the proposed encoder placement provides sufficient native resolution. The source describes the following scenario:

• Measuring wheel diameter: 80 mm (D = 80 mm, radius r = 40 mm)
• Encoder A: 1000 p/r on Axis 1 (roll forming)
• Encoder B: 2000 p/r on Axis 4 (press)

Circumference:

Single-channel pulse equivalent (linear travel per pulse):

Δx_A = 251.327 / 1000 = 0.2513 mm/pulse (Axis 1)
Δx_B = 251.327 / 2000 = 0.1257 mm/pulse (Axis 4)

With quadrature decoding (×4 evaluation of A and B channels in FM350 or SMC30):

Δx_A_quad = 0.2513 / 4 = 0.0628 mm/count (Axis 1)
Δx_B_quad = 0.1257 / 4 = 0.0314 mm/count (Axis 4)

The 4 pulses/mm observation from a single channel matches the 0.2513 mm/pulse calculation for the 1000 p/r encoder. This is correct and indicates the encoder resolution is not the limitation — the drive control loop is.

4. Recommended Architecture: SINAMICS S120 with Technology CPU

The field consensus for a four-axis, 1 mm synchronized steel line is the SINAMICS S120 drive system paired with a SIMATIC Technology CPU. The recommended topology is:

• SIMATIC S7-300T CPU 315T-3 PN/DP or 317T-3 PN/DP as the master controller. The "T" Technology CPUs include integrated motion control functions (positioning, gearing, camming) and synchronized PROFIBUS/PROFINET interfaces for isochronous drive communication.
• SINAMICS S120 drive system with a CU320-2 Control Unit. CU320 manages all drive objects (one per motor module) inside its own servo cycle, eliminating per-axis PROFIBUS round trips for the inter-axis synchronization.
• Line Modules (Smart Line Module or Basic Line Module) sized to the total line kVA.
• Motor Modules (Booksize or Blocksize), one per axis, in servo or vector control mode.
• SMC30 Sensor Module Cabinet-Mounted for each axis requiring encoder feedback. SMC30 accepts HTL/TTL incremental encoders and is parameterizable in STARTER/SIMOTION Scout.

The Technology CPU performs the application-level synchronization (master position → slave ratio → cam profile for the press) and exchanges setpoints with the drive over PROFIBUS DP with isochronous mode (isochronous PROFIBUS, equidistant bus cycle). The drive-to-drive synchronization for slave following is handled inside CU320, which drastically reduces the closed-loop delay relative to the MM420/standard CPU approach.

5. Component Selection Matrix

6. Encoder Migration: FM350 to SMC30

The existing FM350-1 counter modules can be removed once the SINAMICS S120 system is in place. The SMC30 module takes over encoder evaluation directly inside the SINAMICS drive. The migration touches the following points:

1. Encoder wiring is moved from the FM350 front connector to the SMC30 sub-D connector. Pinout of the SMC30 is documented in the SINAMICS S120 Control Units and Supplementary System Components manual.
2. The 24 V encoder supply is provided by the SMC30 (X520 pins 7, 8, 15, 16 for +24 V; 6, 14 for ground). The existing FM350 wiring scheme is not reused.
3. Encoder signal level (TTL 5 V differential, HTL 24 V single-ended) must be configured in the SMC30 DIP switches and matched in the STARTER/SIMOTION Scout commissioning tool under Drive → Commissioning → Encoder.
4. The position value is exposed by SINAMICS as process data word r0481 (encoder 1 actual position value Gn_XIST1) and is mapped to the PROFIBUS telegram for the Technology CPU to read.

7. Topology and Communication Architecture

The following SVG depicts the recommended topology. The PLC-to-drive bus is isochronous PROFIBUS, and the drive-internal axis-to-axis sync is handled in the CU320 cycle, which is typically 1 ms.

8. Commissioning Procedure

Commissioning follows the standard Siemens motion sequence. The order of operations matters because the drive tuning must be complete before the synchronization master is enabled.

1. Mechanical pre-check. Verify the measuring wheel pressure on the strip (typically 5–15 N depending on strip gauge), verify the wheel is free of contamination, and check that the wheel rotation axis is perpendicular to the strip travel direction.
2. CU320 and Motor Module bring-up. Use STARTER to load the SINAMICS S120 project, perform automatic drive identification, and run the motor data identification routine at standstill and at partial speed.
3. Encoder calibration. With the SMC30 connected, run the encoder adjustment routine in STARTER (Commissioning → Encoder → Commissioning interface). Confirm the direction sense (positive count when the strip moves downstream) and that the displayed linear position matches a physical reference distance.
4. Speed controller tuning. Run the speed controller autotune for each axis. Default load inertia can be estimated from roller diameter and conveyed mass; refine if the line is mechanically coupled.
5. Position controller tuning. Set the position loop gain Kp in the Technology Object (T-CPU → TO Axis → Dynamic response) starting at a low value (e.g., 1.0 1/min) and increase until position following error is stable.
6. Synchronous operation commissioning. Configure the gearing Technology Object between the master axis (Axis 1, the line reference) and each slave axis. Set the ratio such that strip travel, not motor rotation, is the unit of sync. Activate the cam Technology Object for the press axis to define the die engagement window.
7. End-to-end verification. Run the strip through the line and verify that the press die lands on the scribed reference mark on the strip within ±1 mm.

9. Programming: Synchronous Operation on the T-CPU

The Technology CPU exposes the synchronization as a Technology Object (TO) of type SynchronousAxis or Cam. Configuration is done in STEP 7 with the optional Technology package. The following example is illustrative of the structure used; it is not a complete working program and should be adapted to the actual TO names in the project.

// In OB1 or the motion OB
// Enable master axis (Axis 1 - roll forming)
IF "StartLine" THEN
    "TO_Axis_1".ControlWord.%X0 := TRUE;   // Enable
    "TO_Axis_1".ControlWord.%X1 := TRUE;   // Enable2 / NoStop
    "TO_Axis_1".SpeedSetpoint := "LineSpeed_mm_s";
ELSE
    "TO_Axis_1".ControlWord.%X0 := FALSE;
END_IF;

// Gear slave axes 2 and 3 to master
"TO_Axis_2".GearRatio.Numerator   := 1;
"TO_Axis_2".GearRatio.Denominator := 1;
"TO_Axis_2".MasterAxis := "TO_Axis_1";
"TO_Axis_2".ControlWord.%X0 := "StartLine";

"TO_Axis_3".GearRatio.Numerator   := 1;
"TO_Axis_3".GearRatio.Denominator := 1;
"TO_Axis_3".MasterAxis := "TO_Axis_1";
"TO_Axis_3".ControlWord.%X0 := "StartLine";

// Press axis (Axis 4) follows via cam
"TO_Axis_4".MasterAxis := "TO_Axis_1";
"TO_Axis_4".Cam := "CAM_PressStroke";   // defined in cam editor
"TO_Axis_4".ControlWord.%X0 := "StartLine";
"TO_Axis_4".ControlWord.%X4 := TRUE;     // Enable cam synchronization

The cam profile CAM_PressStroke is designed in the cam editor such that the press die descends at a defined strip position and retracts before that position is reached by the next strip. The synchronization ensures that the die engagement event is bound to strip position, not to motor shaft position.

10. Performance Verification and Acceptance Test

The acceptance test must demonstrate 1 mm registration accuracy on the actual product. The following procedure is recommended:

1. Mark three reference lines on a test strip at known positions (e.g., 100 mm, 250 mm, 400 mm from the strip leading edge) using a scribing tool.
2. Run the strip through the line at nominal speed and at slow speed (e.g., 20% of nominal).
3. Measure the offset between the scribed reference line and the press die edge on the formed part. Use calipers with 0.05 mm resolution or a vision system.
4. Record the position following error of each axis as reported by the Technology Object (PositionError, axis status word).
5. Pass criteria: |offset| ≤ 1 mm at nominal and slow speed, |following error| ≤ 2 mm peak (the tighter the better, but 1 mm is the contractual target).

If the offset is outside ±1 mm, increase the position loop Kp of the slave axes (Axes 2, 3) to reduce following error, and verify that the master axis (Axis 1) speed is sufficiently smooth — acceleration transients are the most common cause of position error excursions.

11. Alternative Path: MM440 with Encoder Module

One field respondent suggested that an MM440 with the encoder option module running in full vector control could improve the situation. The MM440 supports encoder feedback via the MM440 encoder module and offers sensorless vector control (SLVC) and closed-loop vector control (VC) modes. This is a viable single-axis upgrade but is not a viable path for the full four-motor synchronized line because:

• MM440 has no native inter-drive synchronization, so all four drives must be coordinated through the PLC.
• The PROFIBUS cycle on MM440 (typically 4–8 ms) is too slow for a 1 mm target at line speeds above ~0.25 m/s.
• MM440 has no electronic cam, so the press profile would still have to be generated in the PLC at the scan rate.

Use MM440 with vector control only if the budget does not allow a full S120 migration and the line speed is low (≤0.1 m/s). For the steel forming application described, S120 is the correct answer.

12. Troubleshooting Matrix

13. Field Commissioning Notes and Edge Cases

• Strip stretch. Steel strip stretches under tension. If the take-up reel is at the end of the line and the press is just before it, the strip at the press is in tension. Mount the measuring wheel on the press axis with a small tension-decoupling distance, or compensate in software by adding a stretch factor to the master position.
• Strip temperature. Hot-rolled strip expands. A 10 °C change in a 10 m strip section is ~1 mm of linear expansion. For hot applications, use a temperature-compensated virtual master or a separate measuring wheel on the strip downstream of the heater.
• Encoder mounting on a vibrating surface. Use flexible couplings and avoid rigid mountings on the press frame; the press impact transmits vibration back to the encoder housing and can cause count errors at the moment of stamping.
• Loss of encoder feedback. The SMC30 and T-CPU will detect loss of the A/B signal and fault the drive. Add a mechanical interlock or a current-proving input in the press axis logic to prevent the die from descending if the encoder is not valid.
• Mechanical coupling between axes. If two driven rollers are mechanically connected through the strip (one upstream of a tension zone, one downstream), the synchronization is implicit through the strip itself, but the drive controllers will fight each other. The Technology Object synchronous axis with a soft ratio (with a small allowable slip) is the correct way to handle this.

14. Summary of Migration Path

The MM420-based architecture cannot deliver 1 mm synchronization on a four-motor steel forming line because the drive itself is open-loop and the inter-axis communication path introduces uncompensated delay. The migration path is:

1. Replace MM420 with SINAMICS S120 (CU320-2 + Motor Modules + Line Module).
2. Replace FM350-1 with SMC30, keeping the existing 1000/2000 p/r encoders (which are already adequate in resolution).
3. Replace the standard S7-300 CPU with a Technology CPU (315T-3 PN/DP or 317T-3 PN/DP), or use a SIMOTION D controller if the application grows beyond gear/cam.
4. Configure isochronous PROFIBUS between the T-CPU and CU320, with the drive-internal cycle at 1 ms for inter-axis sync.
5. Use Technology Object synchronous axis and cam objects for the roller slaves and the press respectively.
6. Verify with the scribed-strip acceptance test, target |offset| ≤ 1 mm at nominal and slow speed.