Overview: P2506 LU per Load Revolution
P2506 is the SINAMICS S120 / SIMOVERT MASTERDRIVES MC parameter that defines how many internal Length Units (LU) the position controller counts for one full mechanical revolution of the load-side element. In the Basic Positioner (EPOS) firmware of SINAMICS S120, P2506 is the single scaling parameter that ties the drive's internal 32-bit position counter to real machine geometry. A wrong P2506 value causes systematic following errors, wrong MDI setpoints, position-controller drift on every traversing block, and occasional EPOS faults F07449 (following error), F07450 (position actual value failed) or F07451 (position controller output limited).
This reference covers the underlying LU mathematics, the AVWF (Actual Value Weighting Factor) carried over from MASTERDRIVES MC parameters P169 and P170, the resolver pole-pair dependency, the 1FK7080-2AF71-1SH0 worked example with a 1400/104 gearbox and a 25-tooth belt wheel, and the equivalent SINAMICS S120 / TIA Portal parameter set used during MC→S120 migration. Engineers migrating glass-handling belt drives from SIMOVERT MASTERDRIVES MC to SINAMICS S120 with CU320-2 DP will find a drop-in parameter mapping plus a field commissioning procedure here.
leadingValueFactor on the TO_SynchronousAxis).Prerequisites
- SINAMICS S120 with CU320-2 DP or CU320-2 PN control unit, firmware V4.4 or later, with Basic Positioner licence enabled (order code F01).
- 1FK7 synchronous servomotor with integrated multipole resolver; the 1FK7080 frame is p = 4 pole pairs.
- MASTERDRIVES MC backup (DriveMonitor .dnc or .bck) carrying P169, P170, P171 and P2506 reference values.
- Mechanical reference: gearbox ratio (1400/104), load wheel tooth count (25) and either pitch diameter or measured linear travel per revolution.
- Starter V5.x, Startdrive V16 / V17 / V18 or TIA Portal V16 / V17 / V18 / V20 with S7-1200 / S7-1500 Motion Control toolbox.
- PROFIBUS-DP or PROFINET connection to the drive for online commissioning.
Underlying Length Unit (LU) Math
Siemens drives do not use engineering units (mm, deg) directly inside the position controller. Each position increment is an integer Length Unit (LU). The drive converts LU to mechanical position through three nested scaling factors:
- Resolver resolution per motor revolution (encoder / pole-pair dependent, set in p0420 or legacy P171).
- Actual Value Weighting Factor (AVWF) - sub-revolution interpolation, legacy MC parameters P169 and P170.
- Load-revolution scaling (P2506) - mechanical distance per load revolution.
For MASTERDRIVES MC, the LU-per-motor-revolution formula is:
LU_motor_rev = 2^P171 × p × AVWF
where P171 is the actual-value resolution in bits, p is the number of resolver pole pairs, and AVWF = P170 × 10^(P169-8) (legacy MC encoding).
Resolver Pole Pairs on 1FK7080
The 1FK7080-2AF71-1SH0 is an 80-frame 1FK7 high-inertia servomotor with integrated multipole resolver. The 1FK7 family uses p = 4 pole pairs on the resolver track. The resolver emits one sine / cosine period per pole pair per mechanical revolution, so p = 4 yields 4 sinusoidal periods per motor revolution. This value is read from the electronic nameplate (p0307, p0314) and from the resolver rating (p0400.0 = 1001 for resolver 2-pole, p0400.0 = 1003 for multipole). On 1FK7 80-frame, the resolver type code is 1003 (multipole, p = 4). The parameter p0425 sets the resolver pole-pair number for the encoder evaluation; the correct value is 4.
Worked Example: 4096 × 4 × 1.82690264
With P171 = 12 (factory setting for MASTERDRIVES MC actual value resolution, equivalent to 12-bit = 4096 increments per electrical revolution at the resolver output), p = 4, and AVWF = 1.82690264 (P169 = 1, P170 = 82690264 as loaded from the MC backup), one motor revolution equals:
4096 × 4 × 1.82690264 = 29 931.987285 LU
This is the position increment count the MASTERDRIVES MC accumulates for one full mechanical turn of the motor shaft. The same calculation applies on SINAMICS S120 with p0420 = 12 bits and p2506 / AVWF combined. The result 29 931.987 LU per motor revolution is the resolver-internal count after AVWF interpolation.
Applying the 1400/104 Gearbox
The gearbox ratio 1400/104 means that the load-side output rotates 1400 revolutions per 104 motor revolutions - this is a geared-up planetary stage where the load wheel turns faster than the motor. The relationship between motor revolutions and load-side revolutions is therefore:
motor_revs_per_load_rev = 104 / 1400 = 0.074286
load_revs_per_motor_rev = 1400 / 104 = 13.461538
One gear-shaft revolution (load side) corresponds to:
LU_load_rev = LU_motor_rev × 1400 / 104
= 29 931.987285 × 13.461538
= 402 930.4038 LU per load revolutionVerify the direction sign in the drive commissioning tool: a positive MDI setpoint must produce a positive position increment after the gearbox. If reversed, toggle p0410 bit 0 (signal direction reversal) or swap the resolver A+/A- and B+/B- leads on the Sub-D connector at the SMC10 / SMC20 Sensor Module.
Calculating P2506 from Mechanical Geometry
P2506 must equal the number of LU the load moves per single revolution of the load-side element. For a 25-tooth belt wheel driving a belt on a glass-handling conveyor, the formula is:
P2506 = (teeth × pitch_length) × (LU_per_length_unit)
The pitch length is the centre-to-centre distance from one tooth to the next along the belt. The LU_per_length_unit factor lets the engineer trade off resolution against numeric range. For the SINAMICS S120 linear configuration in TIA Portal, see Linear configuration in TIA Portal V20 for the canonical parameter list. The default values for "Reference speed", "Number of motor revolutions", "Number of load revolutions" and "Length units per load revolution" are reset by the TIA Portal configuration wizard; the engineer must re-enter the calculated value when migrating from a MASTERDRIVES MC backup.
Procedure: Measure Belt Travel per Wheel Revolution
- Mark one tooth of the 25-tooth wheel with a paint pen or scribe. Mark the wheel hub with a fixed reference line on the gearbox housing.
- Mark the belt with a reference line (use the frame edge or a paint dot).
- Disable the position controller (p2580 = 0) and jog the drive in velocity mode (p1151) for exactly one wheel revolution. Stop when the marked tooth returns to the reference point on the wheel hub.
- Measure the linear distance the belt has travelled, e.g. with a steel tape or digital caliper. Record the value in mm with 0.1 mm resolution.
- Repeat the measurement five times and average. If the spread between minimum and maximum is greater than 2 percent, the gearbox has measurable backlash; reduce the MDI setpoint range, apply backlash compensation (p2516 / p2517) or replace the gearbox.
Choosing the LU Length Unit
SINAMICS S120 lets the engineer pick the engineering length unit on the linear axis. The recommended settings for a belt-driven glass-handling axis are:
| Length Unit | LU equivalent | Typical P2506 range | Use case |
|---|---|---|---|
| 1 LU = 1 µm | 1000 LU = 1 mm | 10 000 - 1 000 000 | Glass-handling belts, precision conveyors |
| 1 LU = 0.1 µm | 10 000 LU = 1 mm | 100 000 - 10 000 000 | Sub-µm positioning, optical alignment |
| 1 LU = 1 mm | 1 LU = 1 mm | 10 - 1 000 | Coarse long-travel axes, table feeders |
| 1 LU = 10 µm | 100 LU = 1 mm | 1 000 - 100 000 | Standard conveyors, packaging lines |
For a glass-belt application with 25 teeth and an assumed 20 mm pitch, circumference = 25 × 20 = 500 mm. With 1 LU = 1 µm, P2506 = 500 000 LU per load revolution. Confirm with the measured travel value before saving to ROM.
Migration from MASTERDRIVES MC to SINAMICS S120
The MC parameter P2506 is reused almost identically in SINAMICS S120, but the AVWF concept is split across two parameter groups: encoder scaling (p0418, p0420) and load scaling (p2506). The mapping below is valid for firmware V4.4 to V5.2 on CU320-2 DP / CU320-2 PN with a Sensor Module SMC10 (resolver) or SMC20 (incremental encoder).
| MASTERDRIVES MC | Function | SINAMICS S120 equivalent | Notes |
|---|---|---|---|
| P169 | AVWF exponent | p0418.0 / p0420 (encoder bits) | Combined with p2506; not a standalone S120 parameter |
| P170 | AVWF mantissa | Contributes to p2506 (load) | Multiply with 2^P171 × p to get LU per motor rev |
| P171 | Actual value resolution (bits) | p0420 (resolution bits) / p0418 (fine resolution) | Same value carried over |
| P2502 | Encoder configuration | p2502 | Identical parameter name, identical value range |
| P2506 | LU per load revolution | p2506 | Direct copy if LU/load is identical |
| P2533 | Position controller Kp | p2533 | Identical; may need re-tuning with S120 firmware |
| P2534 | Position controller Tn | p2534 | Identical |
Re-deriving P2506 from the MC Backup
If the MC backup gives P169 = 1 and P170 = 82690264, the AVWF is 1.82690264. For a 1FK7 motor at p = 4 and P171 = 12, the load-side P2506 must be recomputed as:
P2506 = (2^P171 × p × AVWF) × (motor_revs_per_load_rev)
P2506 = (4096 × 4 × 1.82690264) × (104 / 1400)
= 29 931.987285 × 0.074286
= 2 223.45 LU per motor rev
OR
P2506 = 29 931.987285 × 13.461538 = 402 930.4 LU per gear-shaft revolutionThe choice depends on what the drive considers the "load side". On SINAMICS S120, the load side is defined by p2503 (Load gear unit, numerator) and p2504 (Load gear unit, denominator). Configure:
p2503 = 104 (motor revolutions)
p2504 = 1400 (load revolutions)Then enter P2506 = 402 930 LU per load revolution with 1 LU = 1 µm scaling. This is the value the S120 expects when the load-side feedback is the resolver through the gearbox and the load wheel is the driven element.
Drive Parameters Required by EPOS
| Parameter | Value | Meaning |
|---|---|---|
| p2502 | 1 | Encoder assignment for position actual value |
| p2503 | 104 | Load gear, motor revolutions |
| p2504 | 1400 | Load gear, load revolutions |
| p2505 | 1 | Resolution per motor revolution (resets with p2506) |
| p2506 | 402 930 | LU per load revolution |
| p2507 | 0 | Modulo correction not active |
| p2508 | 1 | Reference signal active |
| p2580 | 1 | EPOS mode (positioning) |
Commissioning Steps on CU320-2 DP
- Connect the drive via PROFIBUS-DP (CU320-2 DP) or PROFINET (CU320-2 PN) to Starter / Startdrive / TIA Portal.
- Run the drive wizard (automatic drive configuration) so the 1FK7080 motor is identified from the electronic nameplate. Verify p0300 = 202 (1FK7), p0301 = 0 (1FK7080-2AF71-1SH0 from the part number mapping).
- Configure the Sensor Module: p0400.0 = 1003 (multipole resolver, p = 4) and p0425 = 4 (resolver pole pairs). Verify the resolver cable shielding is bonded at the SMC10/SMC20 end only.
- Configure the load gear (p2503 = 104, p2504 = 1400) and enter the calculated p2506 = 402 930. Save with "Copy RAM to ROM" (p0977 = 1).
- Configure EPOS operating mode (p2580 = 1 for basic positioning) and set the position controller Kp (p2533) and integral time (p2534). Initial Kp = 1.0 (1/s) and Tn = 0 ms for first commissioning; tune to a critically-damped response.
- Run the reference-point approach (p2596) and verify the home position (r2521) is stable over five power cycles.
- Execute a relative MDI setpoint of 10 000 000 LU (10 m with 1 LU = 1 µm) and measure the actual belt travel. Recompute P2506 if the error is greater than 0.1 percent.
Verification Checklist
| Test | Acceptance | Tool |
|---|---|---|
| Referencing (homing) | Reference point stable over 5 power cycles | Starter trace on r2521 |
| Direct MDI 1 m | Actual travel = 1.000 m ± 0.001 m | Steel tape |
| Direct MDI 10 m (back and forth) | Following error < 5 000 LU | r2563 trace |
| Continuous belt run 1 h | No EPOS fault F07449, F07450, F07451 | Fault buffer p0945 |
| Reversing test 100× | Position offset < 1 LU at end | r2521 readback |
| Backlash check (reversal) | Position error < 50 LU at direction change | Trace r2563 |
Troubleshooting Matrix
| Symptom | Probable Cause | Corrective Action |
|---|---|---|
| Position always too small (load undershoots) | P2506 too high | Reduce P2506 by ratio of commanded / actual distance |
| Position always too large (load overshoots) | P2506 too small | Increase P2506 by ratio of commanded / actual distance |
| Position reverses sign when direction toggled | Encoder direction inverted | Toggle p0410.0 or swap resolver A+/A- and B+/B- leads |
| F07449 following error during traverse | Position controller Kp too low or P2506 wrong | Increase p2533, recheck P2506 and load gear ratio |
| F07450 position actual value failure | Encoder error or p2502 wrong | Verify p2502 = 1, check SMC10/SMC20 LEDs |
| Resolver loss F31110 / F31111 | p0425 wrong, cable fault | Verify p0425 = 4, check resolver shield and connector pinout |
| Gearbox backlash visible at reversal | Backlash comp not enabled | Configure p2516 (backlash) and p2517 (compensation factor) |
| Drift over long traverses (10+ m) | Tooth pitch uncertainty, P2506 rounding | Re-measure travel, recompute P2506 with more decimal places |
| F07451 position controller output limited | Mechanical jam, controller saturated | Inspect belt tension, check r2526 (controller output) |
| Homing offset stable but jumps after power cycle | Save-to-ROM not performed, p0977 = 0 | Execute p0977 = 1 to save parameters persistently |
Notes on TIA Portal Linear Configuration
When configuring the SINAMICS S120 in a TIA Portal project for an S7-1200 or S7-1500 controller, the Mechanics tab in the drive configuration window exposes the load gear, feed constant, and the number of length units per load revolution. The wizard default is 10 000 LU per motor revolution with a 1:1 load gear. When migrating from a MASTERDRIVES MC application, override these defaults with:
- Load gear motor revolutions (p2503) = 104
- Load gear load revolutions (p2504) = 1400
- Number of length units per load revolution (p2506) = 402 930
The wizard's auto-generated drive data block (DB) in the PLC project automatically picks up the LU-to-engineering-unit conversion. Confirm the same factors are visible in the technology object (TO) configuration if you use TO_SynchronousAxis / TO_PositioningAxis. See the TIA Portal V20 documentation at Linear configuration - SINAMICS Motion Control for the canonical list of parameters exposed by the configuration wizard.
Field-Proven Caveats
- The 1FK7080-2AF71-1SH0 motor frame is a high-inertia design; expect a mechanical time constant 30-50 percent higher than a 1FK7 compact frame of the same rated torque. Start EPOS tuning with a softer Kp.
- The 1400/104 gearbox is unusual - typical planetary reducers are 5:1, 10:1, 20:1, 50:1. Verify the gearbox nameplate matches 1400/104 before commissioning. A common error is misreading the ratio, which manifests as P2506 errors of 13× in the wrong direction.
- The 25-tooth belt wheel pitch may not equal the belt pitch if the wheel is overdriven or underdriven. For round-tooth belts, the pitch line is the centre of the tooth, not the belt surface. Use the manufacturer's pitch diameter, not the outer diameter.
- Glass-handling belts require extremely low following error to avoid cracking the glass. Tune p2533 (Kp) to a critically-damped response with following error under 100 LU during normal traverse.
- EPOS position controller Kp has a maximum value of 9999.99 (1/s). Higher gains are not effective on a 1FK7 with this load inertia.
FAQ
What is the exact formula for P2506 on SINAMICS S120?
P2506 = 2^P171 × p × AVWF × (motor revolutions per load revolution). For 1FK7 motors with p = 4, P171 = 12, AVWF = 1.82690264 and a 1400/104 gearbox where the load side turns 1400 times per 104 motor revolutions, P2506 = 29 931.987 × 13.4615 = 402 930 LU per load revolution with 1 LU = 1 µm scaling.
How do I migrate MASTERDRIVES P169 / P170 to SINAMICS S120?
Combine the AVWF mantissa P170 and exponent P169 into a single AVWF factor, multiply with 2^P171 and the pole-pair count, then re-derive P2506 via the load gear ratio. The S120 p2506 absorbs both the AVWF and the load-revolution scaling.
Can I change the length unit from mm to µm after commissioning?
Yes, by changing p2506 and any HMI / PLC user-side conversion constants. Reset p2506, save to ROM with p0977 = 1, and run the verification procedure (1 m, 10 m, reversing test) again before resuming production.
What is the maximum P2506 value on S120 EPOS?
1 000 000 000 LU per load revolution on firmware V4.4 and later. If the calculated value exceeds this, switch to a coarser length unit (e.g. 10 µm per LU) and update the user-side scaling factor in the PLC user program.
Why does the load position drift over long traverses?
Drift over long distances is normally caused by an incorrect P2506 value or by gearbox backlash. Re-measure the belt travel per wheel revolution and adjust P2506 by the ratio of commanded to actual distance; if drift persists, enable backlash compensation (p2516) and recheck the load gear ratio p2503 / p2504.