Troubleshooting S7-1500 Positioning Axis Faults on Hydraulic TOs

Overview

The Siemens S7-1500 and S7-1500T CPU families expose the TO_PositioningAxis technology object (TO) as a unified motion primitive for both electric and hydraulic axes. When the same TO is bound to a hydraulic proportional valve through an analog output (AQ) module and a contactless magnetostrictive position sensor is used for feedback, the commissioning engineer must reconcile three independent parameter sets before MC_MoveJog can be executed without a positioning alarm: encoder configuration, mechanical lead/pitch, and the valve characteristic curve managed by the SIMA Hydraulics library.

This reference consolidates the field-proven procedure for clearing the positioning fault, reading the <TO>.ErrorWord tag to identify the alarm group, and configuring the SIMA Hydraulics function blocks so the controller can drive the proportional valve with a compensated setpoint. The same procedure applies to S7-1500T axes with an additional load-side encoder, with the proviso that the T-CPU exposes an extra ActualPosition2 tag for the second feedback channel.

Architecture: TO_PositioningAxis Driving a Hydraulic Proportional Valve

The TO_PositioningAxis is normally configured for electric drives that use PROFIdrive over PROFINET or an analog setpoint interface. The TO is also suitable for hydraulic proportional valves that accept a voltage or current command. The PLCopen motion-control instructions (MC_Power, MC_MoveJog, MC_MoveAbsolute, MC_MoveRelative, MC_Stop, MC_Halt, MC_Reset) operate identically; the differences are in the data block parameters of the TO and the addition of the SIMA Hydraulics function blocks in the user program.

When the axis is bound to a hydraulic proportional valve, the closed-loop controller inside the TO drives the AQ output, and the actual position is fed back by an SSI or analog magnetostrictive position sensor mounted on the cylinder. Because the hydraulic valve has a dead zone, hysteresis, and a nonlinear flow-vs.-command characteristic, the SIMA Hydraulics library wraps the inner loop with a characteristic-curve compensation block (LSimaHydTO_MC_SetCharacteristic). The outer LSimaHydTO_HydAxisCall runs in the cyclic task and feeds the corrected setpoint into the TO each cycle.

Wiring topology

The CPU publishes the setpoint on the configured AQ channel, the proportional valve driver conditions the current to the valve solenoid, the cylinder moves, and the magnetostrictive sensor returns the actual position on the SSI or AI channel. The TO closes the loop on the actual position value.

Mechanics and Encoder Parameter Configuration

Open Technology objects > TO_PositioningAxis > Configuration > Mechanics in TIA Portal and set the mechanical parameters to match the physical system exactly. The mechanical parameters define how load revolutions are translated into position units and which side of the gearbox the encoder sits on.

The most common source of a positioning fault on a hydraulic cylinder is an incorrect Distance per revolution or a wrong Encoder type setting. For a Temposonics RH or Balluff BTL7 magnetostrictive sensor, set the TO to Absolute encoder (SSI) with a data length of 24 or 25 bits and the code type configured per the sensor datasheet (typically gray for SSI). For a sensor with analog output, set the TO to Analog measuring encoder and bind the sensor to an AI module input, then scale the AI range to the cylinder stroke length in the TO's analog scaling parameters.

Verification: open-loop motion test

Disable the position controller (Configuration > Control loop > Position control > Disabled) and run a MC_MoveJog command. If the cylinder extends and retracts proportionally to the jog velocity setpoint, the mechanics and encoder parameters are correct. If the cylinder moves the wrong direction, invert the encoder polarity in the TO configuration. If the cylinder hunts, overshoots, or stops short, re-check the distance-per-revolution value and the load-gear ratio.

Magnetostrictive Position Sensor Setup

Contactless magnetostrictive position sensors (Temposonics RH/RP/EP from MTS Sensors, Balluff BTL7) deliver either an SSI frame, an analog 0-10 V or 4-20 mA signal, or a start/stop pulse. The selection on the TO must match the sensor variant exactly. Resolution is typically 1 to 5 micrometres with a linearity of 0.01% to 0.05% of full stroke, well within the closed-loop accuracy of a hydraulic axis.

SIMA Hydraulics Library and Call Order

The SIMA Hydraulics library is published on the Siemens support site as the LSimaHydTO package. It contains the FBs needed to operate a hydraulic axis on top of a standard TO_PositioningAxis. The library documentation entry Library LSimaHydTO lists every function block in the package and their call order. The application example SIMA Hydraulics application example (MC_PreServo) provides a complete TIA Portal project that can be opened, compiled, and adapted.

The LSimaHydTO_MC_SetCharacteristic FB uploads a measured valve characteristic curve into the TO. The FB must be called once at startup, before the cyclic LSimaHydTO_HydAxisCall is called. If the FB is not called, the controller assumes a linear valve curve, and any nonlinearity in the real valve causes a steady-state position error and a positioning alarm during dynamic moves.

Call sequence

1. Insert the LSimaHydTO_MC_SetCharacteristic FB once in OB100 (startup) or a one-shot routine.
2. Pass the data block or dataset ID that contains the cam table (typically generated by LSimaHydTO_MC_LearnCharacteristic or imported from a CSV file exported by the SIMA commissioning tool).
3. Assign the output of the FB to the corresponding hydraulic axis Axis input.
4. Call LSimaHydTO_HydAxisCall in the cyclic task (OB1 or a servo-equivalent task) so the inner loop is updated each cycle.
5. Issue MC_Power.Enable := TRUE and wait for StatusWord.X3 (OperationEnable).
6. Run MC_MoveJog with a low velocity to verify the closed loop.

Library versioning and availability

The library is delivered as a TIA Portal library package (.zip) and can be imported via Options > Global libraries > Open library. Check the release notes in the library package for the minimum TIA Portal version and the minimum CPU firmware required. Library version 2.x targets TIA Portal V17 and later, version 3.x targets TIA Portal V20 and later. Mixing a V20 library with a V17 project is not supported.

ErrorWord Tag and Alarm Groups

The <TO>.ErrorWord tag on a positioning axis aggregates all active technology alarms into a 16-bit WORD. Each bit identifies a specific error group. When the ErrorWord is non-zero, the axis is in an error state and most motion commands are rejected until the cause is cleared and MC_Reset is issued.

For the official description of the ErrorWord tag on the S7-1500/S7-1500T positioning axis, see the Siemens TIA Portal Help: ErrorWord tag (positioning axis) - TIA Portal V20 documentation.

Bit interpretation

While the exact bit mapping is version-specific, the canonical groups on a positioning axis ErrorWord are:

Companion tags

When the ErrorWord indicates a non-zero group, use the following tags to drill down:

• <TO>.ErrorDetail - DWORD with the detailed error code
• <TO>.ErrorAdditionalInfo - additional context such as the failing parameter ID
• <TO>.ErrorReaction - the reaction the controller took (e.g. stop with ramp, immediate stop)
• <TO>.WarningWord - WORD with non-fatal warnings
• <TO>.StatusWord - WORD with axis state bits

Position Controller Tuning (Kv Gain)

The TO_PositioningAxis closes the position loop with a P-action controller. The proportional gain is expressed as the velocity gain Kv in units of (1/s) or as a direct loop gain. The closed-loop bandwidth and the steady-state position error are linked by:

v_setpoint(t) = Kv * e(t)

where e(t) = position_setpoint(t) - position_actual(t). A higher Kv reduces the steady-state error but increases the closed-loop bandwidth, which can excite hydraulic resonances around 30-80 Hz that are common in long-bore cylinders. A lower Kv damps the loop but leaves a steady-state offset that the SIMA cam characteristic must correct.

Field-proven starting point for a hydraulic cylinder:

• Kv = 1 to 5 (1/s) for a soft, well-damped axis
• Kv = 5 to 15 (1/s) for a stiff, high-response axis
• Acceleration limit = 0.5 to 2 m/s^2 for the first commissioning run
• Velocity limit = 50% of the valve rated flow for the first commissioning run

Reduce Kv in 10% steps if the axis oscillates at the target position. Increase Kv in 10% steps if the axis approaches the target slowly or leaves a steady-state offset after the cam characteristic has been applied.

MC_MoveJog Commissioning Procedure

Once the mechanics, encoder, and SIMA Hydraulics parameters are loaded, commission MC_MoveJog in the following order. Each step is a gating condition for the next.

1. Verify MC_Power.Enable is TRUE and the TO has reached the Standstill or Discrete Motion state. Read StatusWord.X3 (OperationEnable) and StatusWord.X7 (HasError).
2. Set Velocity and JogDuration in the MC_MoveJog instance. Start with a low velocity (5% of the configured maximum) and JogDuration = 0 (continuous jog while input is TRUE).
3. Set Direction to 1 (positive) and apply the jog input. Watch the AQ output value in the online watch table; the voltage should ramp up smoothly, not jump.
4. Watch the actual position in the online watch table. The position should change proportionally to the commanded velocity and reach the software limit switches without overshoot.
5. Reverse polarity test by setting Direction = 0. If the cylinder extends in both directions or retracts when commanded to extend, invert the encoder polarity in the TO configuration.
6. Enable position control in the TO and repeat the jog. If a positioning alarm appears, read the ErrorWord, decode the active bit, and address the underlying cause.
7. Issue MC_Reset to clear the alarm, then re-arm the axis and resume jogging.

Cam Characteristic Learning

After open-loop motion is confirmed, the SIMA Hydraulics library expects a learned or imported cam table that maps the desired setpoint to the actual valve command at 5 to 11 points along the stroke. The learning procedure is:

1. Call LSimaHydTO_MC_LearnCharacteristic in manual mode. The FB will drive the valve through the configured setpoint points and record the actual cylinder response.
2. Store the resulting cam table in the data block assigned to the axis.
3. Call LSimaHydTO_MC_SetCharacteristic at the next startup so the controller uses the learned curve.
4. Repeat the MC_MoveJog test and verify the position error is within the target window.

If the learning run is skipped and the controller is left to assume a linear curve, the axis will exhibit a steady-state offset on one side of the cylinder and overshoot on the other. The closed-loop integral action eventually saturates the integrator, and the axis trips a Following error alarm that surfaces in the ErrorWord as a runtime or positioning error.

Open-Loop Verification and Troubleshooting Matrix

To confirm whether the fault is in the closed-loop controller or in the mechanics/sensor chain, disable the position controller and re-run MC_MoveJog. The cylinder should move at a constant velocity proportional to the jog setpoint, with no positioning alarm.

Open-loop success indicates:

• Mechanics and encoder are correct.
• Sensor is delivering valid position values.
• AQ output is wired correctly and the valve is responding.

Closed-loop success with the same setpoint then indicates the controller is now active. If the closed loop oscillates or alarms, the gain settings, dynamic limits, or the cam characteristic are wrong.

Troubleshooting matrix

Verification, Acceptance, and Safety

Once the positioning alarm is cleared, perform the following acceptance test before releasing the axis to production:

1. Cycle the axis 50 times in jog mode without an alarm. The ErrorWord must remain 0 throughout.
2. Command 10 absolute moves of varying length and verify the position is reached within the configured positioning window (typically 0.1 mm for hydraulic cylinders).
3. Command a step response and verify the settling time is within the process requirement.
4. Capture the AQ output and the actual position on a trend; the AQ should not oscillate around the steady state, and the position should converge smoothly.
5. Power-cycle the CPU and confirm the cam characteristic and axis parameters are restored from the remanent data blocks.
6. Document the final parameters in the project documentation, including the cam table, the Kv gain, the velocity and acceleration limits, and the encoder configuration.

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