FM 354 Servo Drive: Current Limiting Limitations for Soft-Landing

Problem Overview

Engineers attempting to implement a "soft-land" sequence — driving a moving element into a workpiece at a controlled, very low contact force (in the order of 0.25 N for a SMAC LAL30-025-55 linear actuator) — frequently select the Siemens FM 354 positioning module because it slots directly into the S7-300 rack and advertises closed-loop servo control. The expectation is that the module can clamp motor current to limit contact force, monitor position error, and capture the position at which the rod stalls on the workpiece surface.

That expectation does not match the FM 354's actual capability set. The module is a positioning controller, not a torque/force controller. It contains no programmable current limit, no torque-feedback loop, and no force-window output. The only contact-related functions the firmware exposes are digital switching-point detection and set reference point on event. If the application requires controlled current/torque to achieve a non-damaging soft-land, the FM 354 will, in practice, either:

1. Hard-drive the actuator into the surface because no current clamp is applied, or
2. Trip on following-error / overcurrent protection when the actuator mechanically stalls, with no record of the touchdown position.

FM 354 Architecture and Capability Set

The FM 354 is documented in the SIMATIC S7-300 FM 354 Servo Drive Positioning Module Manual. From the manual and from field experience the relevant capabilities and limits are:

Key manual passage (paraphrased): If no DB-MD is present on the FM 354 when the control is switched on, the module can only communicate by way of the MPI interface. In practical terms this means commissioning must complete the parameter download before the module becomes a usable positioning controller.

Root Cause: Why FM 354 Cannot Perform Soft-Landing Force Control

A soft-landing sequence, as described in the original application, requires three real-time control primitives that the FM 354 does not provide:

1. Adjustable motor current ceiling. The drive stage contains fixed hardware I²t protection set by the servo amplifier's rating, but the FM 354 user program cannot command "use only 5 % of rated current during this move." The MLFB of the connected amplifier, not the FM, sets the ceiling.
2. Torque/force feedback. A force-controlled soft-land needs a real-time measurement of rod force, typically from a load cell or from the actuator's built-in force sensor. The FM 354 front connector carries incremental encoder inputs, enable, ready, and a small number of digital I/O. There is no analog torque input and the module does not interpret the SMAC LAL-series "force" output as a torque reference.
3. Position capture at the instant of mechanical stall. The natural FM 354 reaction to a stalled actuator is to declare a following-error fault and abort the move. The position register is updated continuously, but the firmware does not latch the position at the moment following error begins and then release the axis back to a controlled state. By the time the PLC reads the position, the actuator has typically bounced off or been driven into a hard stop.

What the FM 354 can do is treat the actuator as a positioning axis and use its on-board switching inputs as contact sensors. That is the basis of the only workable soft-land scheme on this module: a switching-point approach.

Workaround: Switching-Point Approach on FM 354

The switching-point approach assumes a digital contact signal (a microswitch, a force-sensor comparator output, or the SMAC LAL's internal position-monitoring flag) is wired to one of the FM 354's digital inputs. The procedure is:

1. Issue a slow velocity move toward the workpiece in the FM 354's velocity mode.
2. The PLC monitors the assigned switching input (or the SMAC LAA-5 "in-position/at-target" flag).
3. On the rising edge of the contact input, the FM 354 latches the current actual position and the PLC reads it from the axis feedback interface.
4. The PLC issues a stop command and (optionally) commands a reverse jog to "back off" until the contact input clears, which gives a second position reading and a contact-bandwidth number.
5. The latched position is written to the machine data as the new reference / zero point for subsequent switch-test moves.

Limitations of this approach:

• The "soft" in soft-land comes only from a low approach velocity. The actual contact force is determined by the actuator's stall force at that velocity, not by a programmed current limit. With a SMAC LAL30-025-55 the stall force is several newtons — easily enough to overshoot a 0.25 N target.
• Contact must produce a clean digital edge. If the part deflects elastically, the contact input may chatter, producing multiple position captures.
• There is no in-process force readout. The application cannot distinguish "touching with 0.2 N" from "touching with 1.5 N."

Alternative Controllers That Support Current/Torque Limiting

For a true soft-landing, the controller must be able to clamp current (and ideally read force) at the contact event. The table below lists controllers that can perform this function natively, in roughly increasing order of integration with an S7-300/S7-400 PLC.

For applications that must stay on the S7-300 footprint, the typical migration is from FM 354 to a SINAMICS V90 + S7-1500/TIA Portal combination with PROFINET telegram 750 carrying the torque limit. The PLC writes the torque limit word (in 0.1 % units of rated torque) into the telegram during the approach move and reads the actual torque word back during contact.

SMAC LAL30-025-55 Linear Actuator Integration

The LAL30-025-55 is a moving-coil linear actuator with 30 mm body, 25 mm stroke and a 55 N peak force rating. The "soft-land" use case is one of the actuator's stated strong points, but it requires a controller that can command peak current and that can read the actuator's on-board force sensor output (0–10 V or via the LAA-5 communications connector).

Key parameters to verify against the SMAC datasheet (in the source and on the manufacturer's product page) before sizing the controller:

A linear scaling of force to current: at 0.25 N commanded, the controller must deliver approximately I = 0.25 N / (55 N / I_peak). If the actuator's peak current is, e.g., 3 A, the soft-land setpoint is ~13.6 mA. Most digital servo drives cannot resolve this in 16-bit current commands, so the controller must be one whose current loop is calibrated for low-end linearity (SMAC's own amplifiers and SIMOTION with the forceControl TO both qualify).

LAA-5 Connector Reference

The actuator's LAA-5 male input carries power, encoder, and serial control lines per the SMAC LAA-5 pinout page. Pin assignments relevant to a soft-landing controller swap are summarised below; verify against the live SMAC datasheet before commissioning because pin numbering and signal names occasionally differ between actuator revisions.

Implementing a Soft-Landing Sequence With FM 354 (Switching-Point Method)

Where the FM 354 must be used because the rest of the machine is already wired for it, the following ladder-style outline captures the only available soft-landing logic. The output "contactPos" is the captured position that the application uses as the new zero.

1. Configure the FM 354 axis machine data:
   • MD 30: switching-point assignment (which digital input is the contact flag).
   • MD 21: maximum approach velocity (low — e.g., 1 mm/s).
   • MD 12: following-error window (set wide enough to not trip on normal contact; e.g., 0.5 mm for a 25 mm stroke).
   • MD 14: jerk limit / acceleration ramp to minimise impact when the contact flag eventually fires.
2. From the S7-300 user program, write a velocity command to the FM 354 user DB (e.g., DB 1000, DW 12) with the low approach speed.
3. Enable the move and poll the switching-input bit in the FM 354 status word (e.g., DB 1000, DW 0, bit 5).
4. On the rising edge, latch the current actual position from the FM 354 feedback interface (DB 1000, DW 14) into a marker word (MW 200 — "contactPos").
5. Write zero velocity to the FM 354 command word to stop the move.
6. Optional reverse jog at 0.5 mm/s until the switching input clears; record the open position as "releasePos."
7. Subsequent switch-test moves are commanded relative to contactPos (or contactPos + offset to provide a known preload).

Sample ST pseudocode for the latch step:

IF "FM354_status".switchInput AND NOT "latchArmed" THEN
    "contactPos" := "FM354_actualPos";
    "latchArmed" := FALSE;
    "FM354_cmdVel" := 0.0;
END_IF;

Recommended Migration Path

If the application genuinely requires a controlled 0.25 N contact, the engineering-correct step is to retire the FM 354 from this function. Two paths preserve the rest of the S7-300 build:

1. Stay on Siemens: Add a SIMOTION D4x5 module to the rack, configure a "forceControl" technology object, drive the LAL30-025-55 directly through a SINAMICS S120 double-axis module. The PLC and HMI layers stay STEP 7 / TIA Portal; only the motion kernel changes.
2. Standalone motion controller: Replace the FM 354 + S7-300 positioning task with a SMAC LCC-10 (or equivalent) plus a small S7-1200 used only for sequencing and HMI gateway. The SMAC controller provides peak/continuous current limits and reads the actuator's force sensor natively.

In both cases, retain the FM 354 only for any remaining axes on the machine that are pure positioning (no soft-land) and that cannot economically be re-wired.

Verification Procedure

After commissioning, verify both the soft-land repeatability and the position-capture integrity:

1. Static current-limit test (new controller only). Command a slow approach into a calibrated load cell, read actual force at the moment of contact, and confirm the commanded current produces the expected force within ±10 % of 0.25 N.
2. Position repeatability test (FM 354 switching-point method). Run 50 approach cycles into the same fixture, log contactPos each time, and compute 3σ. The FM 354 encoder readback at 1 µm resolution should yield ≤ 5 µm 3σ for the LAL30-025-55.
3. Following-error sanity check. With a low approach velocity, the FM 354 should not trip a following-error fault at contact; if it does, the MD 12 window is too tight. Re-tune MD 12 and re-run the test.
4. Overcurrent fault test. Disable current limiting (where the controller allows it) and command a hard approach; confirm the drive stage trips on I²t protection rather than the PLC application code, so the fault is recoverable.

Field-Proven Caveats

• DB-MD download on the FM 354 is required at every cold start if the project uses non-volatile machine data. A missing DB-MD leaves the module responding only on MPI, which looks like a "dead" module to the rest of the S7-300 — confirm the configuration has been downloaded and the FM 354 is in RUN before debugging motion faults.
• On the SMAC LAA-5 connector, the shield/chassis pin must be bonded to the cabinet ground at the entry plate; otherwise the RS-485 force-data stream will be corrupted by drive PWM noise during the approach move.
• If a switch under test must not be mechanically actuated by the approach, the switching-point workaround is unsafe — substitute a non-contact sensor (optical, capacitive, or eddy-current) at the same geometry as the contact point.
• The 0.25 N target is 0.45 % of the LAL30-025-55's 55 N peak force. With the FM 354 there is no command resolution for that — accept that you are doing a "slow approach with switching-point capture," not a "0.25 N soft-land."

Troubleshooting Matrix