Siemens 840D sl U/W Axis Setpoint Exchange for a Shared Servo Motor

On a horizontal borer retrofit, seven NC axes (X, Y, Z, V, B, U, W) plus a spindle (SP) are driven from a single Sinumerik 840D sl controller. U and W are mechanically independent slides (each with its own external feedback encoder) but they share one servo motor through a clutch mechanism: when the clutch is engaged to the U side, the motor drives U; when engaged to the W side, the motor drives W. Only one of the two axes can be torque-producing at any instant, yet both must be visible on the HMI and both must be programmable.

The Siemens mechanism that satisfies this requirement is the NCK Setpoint Exchange function, documented in the Special Functions (S9) section of the DOCON CD / Programming and Operating Manual for 840D sl. The same end-result can be obtained with the newer Container Axes feature on SW 4.7 SP2 and later, but on a typical retrofit with an existing basic PLC program the classic S9 setpoint exchange remains the most common and most portable solution.

This reference covers the NC configuration, PLC interface, clutch sequencing, HMI display of both axes, and the verification steps required to bring the U/W pair up on a 840D sl retrofit.

1. Application Topology

The hardware topology for the U/W pair is shown below. Both axes feed back to the NCK through separate encoder modules; only one of the two axes is logically "active" on the drive side at a time.

The dashed arrows from the W axis and from the W slide represent the inactive state: the setpoint exchange logic in the NCK only routes the velocity command and controller enable to the drive for the active axis, while the inactive axis remains in follow-up mode and continues to display its position from its own external encoder.

2. Functional Concept of Setpoint Exchange

Setpoint exchange does not multiplex the drive at hardware level. Inside the NCK, two distinct axis objects exist (AX5 = U, AX6 = W). Only one of them is bound to the drive's setpoint slot at any moment; the other is parked. The PLC triggers the switchover through the axis-VDI interface, and the NCK then performs an orderly hand-off:

1. Controller enable of the outgoing axis is revoked (DB31..DBX2.1 falls).
2. Pulse enable of the outgoing axis is revoked (DB31..DBX21.7 falls).
3. Position feedback of the outgoing axis is frozen in follow-up mode.
4. The drive's setpoint and enable lines are reassigned to the incoming axis.
5. The incoming axis is brought to its current actual position, controller enable is granted, and pulses are enabled.
6. The exchange-complete status bit is set in the VDI of the incoming axis.

The clutch itself is a mechanical device outside the NCK. The PLC must engage the clutch to the new side before requesting the setpoint exchange, and confirm engagement through a position-proximity or pressure-switch feedback before releasing the old axis. Allowing the NCK to drive through an unselected gear would produce a position lag and a 25050 (axis contour monitoring) alarm within a few milliseconds.

3. Prerequisites

• Option bit: Setpoint exchange requires the Special Functions option to be enabled on the CF card. Verify with MD11500 $MN_PREP_TASK_CODE and the option screen under Commissioning > Options in Sinumerik Operate. The option string usually reads Setpoint exchange / additional axis or appears under the S9 family.
• Drive slot: One drive on the DRIVE-CLiQ bus (e.g. SINAMICS S120 with one Motor Module and one SMC/SME for the motor encoder, plus two additional encoder modules for the U and W external encoders).
• Topology: The motor must remain visible to the drive, while the two encoders appear as separate encoder objects so each NC axis can bind to its own actual value.
• PLC program: The basic PLC program (840D sl base PLC, FB1/FB2/FC100 family) must include the setpoint exchange handling block. On SW 4.5 and later, the block is part of the standard template and only the application-specific state machine needs to be added.
• Safety: If the machine has a Safety Integrated (SPL) configuration, the U/W pair must be modelled as a single drive for the safe stop path (SS1/STO). The two mechanical axes cannot have independent safe stop signals; the shared drive dictates a shared STO.

4. NC Configuration: Machine Data

The U/W pair is set up as two fully-fledged NC axes. The single drive is referenced by both axes' setpoint machine data. The actual-value machine data is unique per axis because each axis has its own external encoder.

4.1 Channel and axis assignment

4.2 Axis-specific setpoint and actual-value data

4.3 Setpoint-exchange enabling bits

5. PLC Configuration: VDI Interface and Setpoint Exchange

Each NC axis has a 32-byte VDI image in DB31.. (DB31 = axis 1, DB32 = axis 2, ... DB36 = axis 6 = W, DB37 = axis 7 = spindle, etc. The exact DB number is set by MD13050 $MN_AXIS_EXTENSION_VDI or by the basic PLC program's axis mapping).

5.1 VDI bits used by the setpoint exchange

On the basic PLC template (FB2/FC100 family), the setpoint exchange is implemented as a small ladder block that coordinates the clutch outputs and the VDI bits. The application only needs to call the FB and pass the desired axis identity.

5.2 Structured-text state machine for the clutch + exchange

The pattern below is typical for an 840D sl retrofit using SCL in the basic PLC. It assumes a single output ClutchCmdU (true = engage clutch to U) and a single feedback ClutchAtU from a pressure switch or proximity sensor.

// U/W setpoint exchange state machine
CASE iState OF
  0:  // Idle - no axis active
      ClutchCmdU := FALSE; ClutchCmdW := FALSE;
      AxisU_Enable := FALSE; AxisW_Enable := FALSE;
      IF bSelectU OR bSelectW THEN
          IF bSelectU THEN iState := 10; ELSE iState := 20; END_IF;
      END_IF;

 10:  // Engage clutch to U
      ClutchCmdU := TRUE; ClutchCmdW := FALSE;
      AxisU_Enable := FALSE; AxisW_Enable := FALSE;
      IF ClutchAtU AND NOT ClutchAtW THEN iState := 11; END_IF;

 11:  // Grant controller + pulse enable to U
      AxisU_Enable := TRUE;  // DB31 / axis U / DBB2 = controller enable
      AxisU_Pulse  := TRUE;  // DB31 / axis U / DBB21 = pulse enable
      AxisW_Enable := FALSE; AxisW_Pulse := FALSE;
      IF bSelectW AND NOT bSelectU THEN iState := 12; END_IF;

 12:  // Switch from U to W
      AxisU_Enable := FALSE; AxisU_Pulse := FALSE;
      // wait for NCK to release the active axis; status bit DB31.DBB69.0
      IF NOT AxisU_Active THEN iState := 20; END_IF;

 20:  // Engage clutch to W
      ClutchCmdU := FALSE; ClutchCmdW := TRUE;
      AxisU_Enable := FALSE; AxisW_Enable := FALSE;
      IF ClutchAtW AND NOT ClutchAtU THEN iState := 21; END_IF;

 21:  // Grant enable to W
      AxisU_Enable := FALSE; AxisU_Pulse := FALSE;
      AxisW_Enable := TRUE; AxisW_Pulse := TRUE;
      IF bSelectU AND NOT bSelectW THEN iState := 22; END_IF;

 22:  // Switch from W to U
      AxisW_Enable := FALSE; AxisW_Pulse := FALSE;
      IF NOT AxisW_Active THEN iState := 10; END_IF;

END_CASE;

6. HMI Display of Both U and W Axes

The Sinumerik Operate HMI displays all NC axes defined in the channel. The parked axis shows its actual position (from its own external encoder) but its setpoint is frozen; the controller enable and pulse enable indicators are off. This satisfies the requirement to "display U and W separately on the monitor".

6.1 What the operator sees

6.2 HMI configuration

• Axis name in the channel axis list is taken from MD20080 $MC_AXCONF_CHANAX_NAME_TAB; both U and W appear in the axis list because both are referenced in MD20070 $MC_AXCONF_MACHAX_USED.
• Operator can press the axis-select key on the HMI to switch the axis-view focus between U and W. The setpoint exchange is then driven by the PLC state machine, not by the HMI key.
• To add an additional user-defined axis indicator (e.g. "U active" lamp), bind an HMI variable to a flag set inside the state machine above.
• If a custom soft-key or extended display is required, use the standard Operate configuration in sl > config > operator panel and add the U/W indicator under Diagnostics > Axis.

7. NC Part-Program Behaviour

From the NC program perspective, both U and W are normal channel axes. The setpoint exchange is invisible to the part program as long as the PLC has the correct axis active. The NCK automatically stops in the same block if the desired axis is not the active one and the setpoint exchange is part of the channel reset sequence.

Typical pattern (DIN/ISO):

; Bore with U axis (cross-slide in)
G1 G94 F200 U50.0
G0 U0

; ... wait for PLC to switch to W axis ...
M61          ; user-defined M-code: "select W axis, engage clutch, wait for ready"

; Bore with W axis (outboard slide)
G1 G94 F200 W40.0
G0 W0
M62          ; user-defined M-code: "switch back to U axis"

The M-codes M61 and M62 in this example are user-defined; they call the PLC state machine above via the basic PLC M-code decoder (default mapping in DB250 / DB251). On completion, the NC program continues only after the M-code acknowledge is returned, which the state machine should hold off until the new axis is fully released and the new axis is active.

8. Safety Considerations

• Both U and W are tied to the same Motor Module from a Safety Integrated (SI) perspective. The STO/SS1 path is shared; the safe brake management must be modelled accordingly.
• The clutch feedback must be hard-wired to a safety-relevant input if a category-1 stop is required. A drifting clutch under STO is unacceptable: the parked axis could become mechanically loaded while the drive is de-energised.
• The PLC must interlock the M-codes that drive the U/W swap with the active safety stop. If SS1 is active, the swap must be blocked.
• External encoders on both axes should be monitored for plausibility against the motor encoder. A divergence beyond a configurable threshold indicates a clutch that failed to engage fully.

9. Commissioning Procedure

1. Drive and encoder topology. In Sinumerik Commissioning Tool / Startdrive, confirm that one drive and two encoder objects are present, and that the U and W encoders are physically distinct modules.
2. NC machine data. Set MD10000, MD20070, MD20080 to include U and W; set MD30110 to the same drive number for both; set MD30220 to the two distinct encoder module numbers.
3. Option enable. Verify the setpoint exchange option is enabled under Commissioning > Options. Without the option, the NCK ignores the duplicated MD30110 and the second axis becomes a simulation axis.
4. PLC state machine. Implement the state machine above; load and download the PLC project; bring the controller up in PLC stop first and observe the state transitions in the watch table.
5. Dry test (no NC motion). Manually request a U-to-W exchange from the watch table; verify the VDI status bits flip, the clutch feedback matches, and both axes display their actual positions correctly.
6. Motion test. Run a small part program that moves U, exchanges, moves W, and exchanges back. Monitor the contour deviation in the axis service screen; a healthy exchange shows < 2 mm transient on the parked axis (no real motion, but the setpoint freeze transition is visible).
7. Full axis service screen. Open Diagnostics > Axis on the HMI for both U and W; confirm K_v, follow-up mode, controller enable, and pulse enable all read as expected.
8. Safety test. Trigger SS1 from the safety test selector; confirm the drive de-energises and the clutch is held in its current position (typically a spring-applied brake on the gear side).

10. Diagnostics and Alarms

The most common alarms when the setpoint exchange is misconfigured are listed below. These are taken from the Sinumerik 840D sl Diagnostics Manual.

10.1 Live diagnostics checklist

• Commissioning > Axis > Axis U: controller enable on, pulse enable on, follow-up off, K_v matches commissioning sheet.
• Commissioning > Axis > Axis W (parked): controller enable off, pulse enable off, follow-up on, position actual updates as the slide is moved by hand.
• PLC watch table: ClutchAtU XOR ClutchAtW is always true; both never true, both never false (clutch must not be in a mid-position state).
• Drive trace: trace the velocity setpoint during an exchange; the setpoint for U should ramp to 0 inside 200 ms, then a brief flat, then the W setpoint ramps in.

11. Troubleshooting Matrix

12. Alternative: Container Axes (SW 4.7 SP2 and later)

On 840D sl SW 4.7 SP2 and later, the same problem can be solved more declaratively using Container Axes. A container is a logical group of axes of which only one is active. The exchange is configured by NCK commands and does not require a manual PLC state machine for the VDI bits. Container axes are the recommended long-term approach for new retrofits, but the classic setpoint exchange remains the supported, documented mechanism for the existing installed base.

Key MDs for container axes:

• MD12750 $MN_AXCT_NAME_TAB - container names
• MD12751 $MN_AXCT_AXCONF_ASSIGN_TAB - slot assignments
• MD12752 $MN_AXCT_DEFAULT_AXCONF - default assignment after reset
• NC commands AXCTSW (axis container switch) and AXCTSWC (axis container switch with check)

13. Verification Checklist

Before signing off the U/W axis pair, confirm every item below.

• Both U and W visible in the channel axis list on the HMI.
• Position actual on both axes updates when the corresponding slide is hand-cranked (clutch disengaged, drive de-energised).
• Referencing of U works; referencing of W works; referencing of W after a U-to-W exchange returns to the same machine zero within the encoder resolution.
• Clutch engage time measured at < 200 ms; PLC dwell after feedback set to at least 1.5× the worst-case engage time.
• No 25050 contour-monitoring alarm during 100 consecutive exchanges under load.
• No 20140 alarm latches in the alarm log after a power cycle.
• SS1 from the safety test selector drops the drive and the clutch is held (no drift on the parked axis).
• The NC program that uses both U and W runs the test part to print with no deviation on the diameter or surface.

14. Related Siemens Documentation

Refer to the official Siemens documentation for the latest MDs and PLC blocks:

• Sinumerik 840D sl Diagnostics Manual
• Sinumerik 840D sl Programming Manual (basics)
• Sinumerik 840D sl Programming Manual (advanced)
• Sinumerik 840D sl Special Functions (S9) - DOCON CD
• SINAMICS S120 Commissioning Manual
• Sinumerik 840D sl Lists Manual (machine data)

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