Resolving MC_MoveJog Acceleration Override on S7-1500 Motion

Resolving MC_MoveJog Acceleration Override on S7-1500 Motion Control

When commissioning an S7-1500 or S7-1500T motion control axis with the MC_MoveJog technology instruction, a common field problem is that the operator-supplied acceleration and deceleration values from the HMI do not produce the expected ramp behavior on the drive. The instruction reads the HMI tag without raising a fault, but the actual acceleration profile on the motor does not match the value written. This article identifies the root cause, walks through the unit conversion between HMI ramp-time seconds and the technology object's acceleration units, and provides a verified configuration sequence for the S7-1517F-3PN controller and TP1200 Comfort HMI on TIA Portal V15.1 with Motion Control V4.0.

1. Problem Statement

The reported symptom occurs under the following conditions:

• Controller: SIMATIC S7-1517F-3PN (any S7-15xF or S7-15xT with Motion Control V4.0 or later is affected).
• HMI: SIMATIC TP1200 Comfort.
• Engineering: TIA Portal V15.1, Motion Control V4.0.
• Axis type: Speed axis (rotary, unit 1/min).
• Instruction: MC_MoveJog with Acceleration and Deceleration inputs bound to HMI tags.
• Velocity range: 0–1380 1/min written from HMI.
• Operator input: Ramp-up / ramp-down time in seconds (example: 1.0 s for full speed).

Computed acceleration in the program: 1380 1/min ÷ 1 s = 23.0 1/s². The user expects this value to be passed to the drive. Instead, the drive's ramp-up time remains pinned at the default 2 s and ignores the user value. The instruction itself returns no error code; Busy, CommandAborted, and Error are not asserted.

2. Root Cause Analysis

Three independent issues converge in the reported configuration. Each must be resolved for the jog motion to follow the HMI-defined ramp.

2.1 Acceleration is not a ramp-up time

The inputs Acceleration and Deceleration on MC_MoveJog are LREAL values expressed in the unit 1/s² (or the configured acceleration unit of the axis), not in seconds. Writing a ramp-up time of 1.0 from the HMI passes 1.0 1/s² to the instruction, not a 1-second ramp. With a velocity of 1380 1/min = 23 1/s and an acceleration of 1 1/s², the axis would theoretically take 23 s to reach full speed — which silently violates the technology object's limits and clamps to the configured maximum.

Per the official Siemens documentation, when Acceleration or Deceleration is set to a value greater than 0, the instruction uses that user-defined value. If the value is below the configured minimum or above the configured maximum, the instruction uses the corresponding limit value of the technology object. This clamping is silent — no diagnostic is raised unless the technology object's configuration window explicitly monitors the override. See the MC_MoveJog V4 manual for the complete behavior matrix.

2.2 Drive-side ramp generator is still active

When a Siemens drive (SINAMICS V90, G120, S120, etc.) is controlled via the technology object through PROFIdrive, the technology object manages acceleration and deceleration entirely on the PLC side. The drive-internal ramp-function generator (RFG) must be disabled — otherwise the final ramp observed at the motor is the maximum of the two ramp limits.

For PROFIdrive-compliant drives, the technology object automatically sends a control word (STW) bit that disables the drive-side RFG when the telegram contains the appropriate bits. If a non-PROFIdrive telegram is in use, or if the drive's p1120 (ramp-up) and p1121 (ramp-down) parameters are non-zero, the drive imposes its own ramp on top of the PLC command.

2.3 Unit mismatch between HMI and axis

The HMI displays ramp-up time in seconds (operator-friendly). The technology object expects acceleration in 1/s². The conversion logic in the PLC is missing or applied to the wrong tag. In the reported case, the user correctly computed 23.0 1/s² for 1380 1/min over 1 s, but this computed value is being written to the wrong input or the value is being clamped at the axis's default acceleration rather than accepted.

3. Affected Versions and Telegrams

4. Step-by-Step Resolution

4.1 Verify the technology object configuration

1. Open the technology object (TO) configuration for the speed axis in TIA Portal.
2. Navigate to Basic parameters > Dynamics.
3. Record the values for:
   • Default acceleration (LR_DynamicsDefault
   • Default deceleration
   • Maximum acceleration (MaxAcceleration)
   • Maximum deceleration
   • Maximum velocity
4. Set the maximum acceleration to a value higher than the maximum HMI ramp-derived acceleration. Example: for 1380 1/min in 0.1 s the acceleration is 230 1/s². Set maximum to 500 1/s² to allow headroom.
5. Compile and download the TO configuration.

4.2 Implement the unit conversion in the PLC

The PLC must convert the HMI ramp-time input (seconds) into the technology object's acceleration unit (1/s²) before writing to MC_MoveJog.Acceleration and MC_MoveJog.Deceleration.

Formula for rotary speed axis (unit 1/min):

acceleration_1_per_s2 = (velocity_RPM / 60) / ramp_time_seconds
acceleration_1_per_s2 = velocity_RPS / ramp_time_seconds

Example ladder (FBD style) in TIA Portal:

// Inputs
// "HMI_Velocity_RPM"   : REAL   = 1380.0
// "HMI_RampTime_s"     : REAL   = 1.0
// "HMI_RampDown_s"     : REAL   = 1.0
// "Min_RampTime_s"     : REAL   = 0.05  (safety clamp to avoid div-by-zero)

// Calculate ramp-up acceleration in 1/s^2
#velocity_RPS := #HMI_Velocity_RPM / 60.0;
#safe_ramp_up := MAX(#HMI_RampTime_s, #Min_RampTime_s);
#safe_ramp_dn := MAX(#HMI_RampDown_s, #Min_RampTime_s);

"TO_MoveJog".Acceleration := #velocity_RPS / #safe_ramp_up;   // unit 1/s^2
"TO_MoveJog".Deceleration := #velocity_RPS / #safe_ramp_dn;   // unit 1/s^2
"TO_MoveJog".Velocity     := #HMI_Velocity_RPM;                // unit 1/min (axis unit)

4.3 Disable the drive-side ramp generator

For SINAMICS V90 with PROFINET (PROFIdrive telegram 1 or 2):

1. Open the V90 web server or STARTER / Startdrive commissioning tool.
2. Set p1120 (ramp-up time) = 0 and p1121 (ramp-down time) = 0.
3. Alternatively, verify the telegram configuration on the drive matches what the TIA Portal device configuration expects. Telegram mismatches can leave the RFG enabled.

For SINAMICS G120 / S120 with PROFIdrive:

1. Verify STW1 bit 4 ("Enable ramp-function generator") is being set correctly by the TO. The TO sends this bit as part of the cyclic control word.
2. Set p1120 = 0 and p1121 = 0 in the drive parameter list.
3. If using a free telegram 999, manually map the STW1 bit in the telegram configuration.

For third-party drives:

1. Disable the drive's internal ramp generator via the drive's parameter list.
2. Set the drive's acceleration/deceleration ramps to 0 or the minimum allowed value.

4.4 Verify the override is active

The technology object has a status word that indicates whether the user-defined dynamic values are active. In the online view of the TO:

1. Go online with the PLC.
2. Open the technology object monitoring panel.
3. Force MC_MoveJog.Velocity to 1380.0 and MC_MoveJog.Acceleration to 23.0.
4. Enable the jog command (set JogForward = TRUE).
5. Read the actual ramp time on the drive's trace or via r0020 / r0021 on SINAMICS drives.

5. Verification Checklist

6. Troubleshooting Matrix

7. Edge Cases and Field Notes

7.1 Acceleration = 0 behavior

If Acceleration is written with 0, the technology object uses the configured default acceleration from the TO dynamics. This is documented behavior across MC_MoveJog versions V2 through V8. Some versions of the help text state "if the value is > 0" — strictly greater than zero triggers the user override. A value of exactly 0 falls back to the default.

7.2 MC_Power abort behavior

The MC_MoveJog job is aborted when MC_Power.Enable transitions to FALSE. The override response depends on the axis configuration ("Behavior when disabling" set to "Cancel motion" or "Continue motion with ramp"). For jogging applications, the recommended setting is "Cancel motion" to prevent the axis from coasting.

7.3 Jerk-limited vs jerk-free acceleration

MC_MoveJog accepts a Jerk input as well. If jerk is left at 0, the motion is jerk-free (trapezoidal velocity profile). A non-zero jerk value applies an S-curve profile. For most manual jog operations, leave Jerk at 0 for the most responsive feel. See the MC_MoveJog V8 documentation for the complete Jerk parameter semantics.

7.4 Override during jog

The Override input on MC_MoveJog scales the dynamic values by a percentage (0–200%). If an override is active in parallel with the HMI acceleration tag, the effective acceleration is user_acceleration × override / 100. This can produce seemingly nonsensical results if the override is left at a non-100% value.

7.5 Reading current acceleration

The actual active acceleration can be monitored via the technology object's status data: <TO>.StatusWord.UserDataActive and the actual values via the cyclic I/O. Place the TO in an online watch table to verify the value being passed during the jog.

8. References to Official Siemens Documentation

• MC_MoveJog V4 manual (S7-1500 / S7-1500T)
• MC_MoveJog V2 manual (S7-1500)
• MC_MoveJog V8 manual (S7-1500 / S7-1500T)
• SIMATIC S7-1500 S7-1500T Motion Control function manual

9. FAQ