S7-1200 Motor Synchronization with HSC Encoders and MC_GearIn

1. System Overview

Two-motor speed synchronization on a SIMATIC S7-1200 requires three coordinated functions: encoder pulse acquisition through high-speed counters (HSC), velocity measurement in user program, and a motion command that drives the follower axis. When the requirement is "speed_1 == speed_2", the simplest implementation is two independent TO_PositioningAxis objects commanded by MC_MoveVelocity. When the requirement is high-accuracy phase-locked speed tracking at varying setpoints, the implementation must use position synchronization via MC_GearIn (electronic gearing), where the master axis position increments drive the follower axis.

This reference covers the closed-loop architecture where Motor 1 (master) is governed by a potentiometer and Motor 2 (follower) is governed by an analog voltage reference to its drive. Both motors have incremental rotary encoders wired into the S7-1200 HSC inputs. The PLC reads the actual velocity of Motor 1 from HSC1 feedback and either mirrors it as a velocity setpoint or uses it as the master position stream for the follower axis.

2. Hardware Prerequisites

Verify that both drives accept an analog reference and that the drive parameter for analog input scaling matches the PLC engineering units (e.g., 0-10 V = 0-1500 rpm).

3. Encoder Wiring and HSC Channel Assignment

The S7-1200 onboard HSC supports up to six high-speed counters (HSC1-HSC6) depending on CPU model. Each HSC accepts an incremental encoder on a dedicated pair of digital inputs and supports quadrature counting modes (1x, 2x, 4x).

Wire Motor 1 encoder to HSC1 inputs (I0.0 = Phase A, I0.1 = Phase B) and Motor 2 encoder to HSC2 inputs (I0.2 = Phase A, I0.3 = Phase B). For long cable runs in electrically noisy cabinets, use shielded twisted-pair cable with shield grounded at the cabinet entry panel only.

Enable the HSC channels in TIA Portal under Device Configuration > CPU > Properties > High-Speed Counters (HSC):

• Enable HSC1: Type = Counting, Counting mode = Quadrature 4x, Initial count value = 0
• Enable HSC2: Type = Counting, Counting mode = Quadrature 4x, Initial count value = 0

4. Encoder-Based Velocity Calculation

Raw HSC count values are integer positions, not velocities. Velocity must be calculated in the user program by sampling the counter value over a fixed window. The standard Siemens idiom is to read the HSC count at a cyclic interrupt OB (e.g., OB30 at 50 ms or 100 ms), compute the difference, and divide by the sample time and pulses-per-revolution.

Formulas for converting HSC counts to engineering units:

n_rpm[rev/min] = (DeltaCount * 60000) / (PPR * SampleTime_ms * QuadratureFactor)

v_mm_per_s    = (DeltaCount * (DistancePerRev_mm * 60)) / (PPR * SampleTime_ms * QuadratureFactor)

omega_rad_per_s = (DeltaCount * 2 * PI) / (PPR * SampleTime_ms * 0.001 * QuadratureFactor)

Where QuadratureFactor = 4 for quadrature-4x mode and SampleTime_ms is the cyclic OB period. Example: PPR = 1024, sample time = 100 ms, 4x mode, observed DeltaCount = 1024:

n_rpm = (1024 * 60000) / (1024 * 100 * 4) = 60000 / 400 = 150 rpm

The HSC IDB exposes Count (DINT, signed 32-bit) and NewCount flag in the instance data block. Always use a local L-tag to capture Count at the start and end of each OB30 cycle to avoid mid-scan race conditions.

5. Motion Control Architecture Selection

Three architectures are viable. The selection depends on the required tracking accuracy and whether the master speed is constant or varies.

For two motors running with a constant speed set by the operator, Architecture B is sufficient and easiest to commission. For any application where the master ramps up/down or where angular phase must be preserved (printing, cutting, conveyor-to-conveyor transfer), Architecture C is required.

6. Technology Object Configuration

Per the S7-1200 Motion Control Function Manual, create two TO_PositioningAxis objects in the project tree:

Because Axis_1 has no drive and no PTO output, define it as a "virtual axis" by linking it to an external encoder only. The TO_ExternalEncoder technology object must be added first under Technology > External Encoder; bind it to HSC1 with the same PPR. Then in Axis_1 configuration, set Encoder connection to this external encoder.

7. Speed-Mode Implementation with MC_MoveVelocity

Architecture B uses two independent motion commands, each driven by the analog pot:

1. Read pot from AI0 (0-27648 raw = 0-10 V) and scale to Velocity_setpoint in user units/s using NORM_X and SCALE_X.
2. In OB1 (or OB30 for deterministic rate), call MC_MoveVelocity on Axis_1 with Velocity = Velocity_setpoint. Execute = Run_enable AND MasterEnable.
3. Mirror Velocity_setpoint to MC_MoveVelocity on Axis_2 with the same velocity value. Output to drive 2 via analog AQ.
4. Read actual velocities from each axis' ActualVelocity tag in the Axis DB.

SCL example for Axis_2 command:

IF "Run_Enable" AND "Master_Enable" THEN
    IF "MC_MoveVelocity_2_Done" OR "MC_MoveVelocity_2_Error" THEN
        "MC_MoveVelocity_2"(
            Axis := "Axis_2",
            Execute := TRUE,
            Velocity := "Velocity_Setpoint_RPM",
            Direction := 2,           // 2 = current direction
            CurrentVelocity := 0,
            Position := 0,
            Aborting := TRUE
        );
    END_IF;
ELSE
    "MC_Halt_2"(
        Axis := "Axis_2",
        Execute := TRUE,
        Deceleration := 5000
    );
END_IF;

This open-loop velocity mirroring is accurate to about ±1% under steady state. The lag during ramp is one PLC scan. For higher precision, layer a closed-loop trim:

// Sample every 100 ms in OB30
"v1_actual" := "MC_Axis_1".ActualVelocity;
"v2_actual" := "MC_Axis_2".ActualVelocity;
"v_error"   := "v1_actual" - "v2_actual";

// Integral trim, apply to AQ2 offset
"v_integral" := "v_integral" + ("v_error" * "Ki" * 0.1);
"v_integral" := LIMIT(-500.0, "v_integral", 500.0);
"AQ2_output" := "V_setpoint_scaled" + "v_integral";

Tune Ki starting at 0.05 and increase by 50% until the velocity error settles within ±2 rpm under step load changes.

8. Closed-Loop Synchronization with MC_GearIn (Electronic Gearing)

When the operator turns the pot, the master velocity ramps. To maintain phase lock, the follower must execute MC_GearIn against the master:

1. Home both axes (or use passive homing on power-up).
2. Start the master with MC_MoveVelocity.
3. Call MC_GearIn on Axis_2 with Axis_1 as the master, RatioNumerator = 1, RatioDenominator = 1, Acceleration = 10000, Deceleration = 10000.
4. The follower position will increment by 1 user unit per master unit. Use RatioNumerator/RatioDenominator for ratio scaling.

SCL snippet:

IF "Gear_Enable" AND NOT "MC_GearIn_Busy" THEN
    "MC_GearIn_DB"(
        Master := "Axis_1",
        Slave  := "Axis_2",
        Execute := TRUE,
        RatioNumerator := 1,
        RatioDenominator := 1,
        Acceleration := 10000.0,
        Deceleration := 10000.0,
        Aborting := TRUE
    );
END_IF;

MC_GearIn is available on S7-1200 firmware V4.2 and higher; refer to the S7-1200 Motion Control V5 manual for full parameter description and graph behaviour.

9. Drive Configuration for Analog Setpoint

Configure the drive accepting the AQ signal to operate in speed-control mode with analog input as primary setpoint:

For SINAMICS G120 with CU240E-2, the equivalent parameters are in the SINAMICS G120 Parameter List (P15 free function blocks, P2250 technology controller selector). Set P2250 = 0 (analog setpoint), P1500 = 0 (analog main setpoint) to bypass any internal ramp generator that could fight the PLC.

10. S7-1200 CPU Scan Budget

Motion Control on S7-1200 uses a fixed interpolator (IPO) cycle of 4 ms. The user program must complete within the cyclic OB1 budget; for two axes plus HSC reads plus analog processing, expect:

Keep OB1 cycle time below 8 ms. Use OB30 at 50 ms for the velocity-trim PID loop rather than OB1, so jitter from MC blocks does not perturb the integral term.

11. Commissioning Procedure

1. Wire both encoders; verify pulse integrity with a hand-crank and the HSC IDB online Count value.
2. Download the hardware configuration. Go online and confirm both HSC channels show "OK" in Online > Diagnostics.
3. Configure both axes; commission Axis_2 with drive in jog mode to verify direction and scaling.
4. Perform passive homing of both axes using the encoder Z-pulse as the homing marker. Verify Axis.XIST1 returns to 0 at the marker.
5. Enable the master with MC_Power and MC_MoveVelocity at 100 rpm. Confirm Motor 1 spins.
6. Enable MC_GearIn on Axis_2. Confirm Motor 2 accelerates to match. Monitor Axis_2.ActualPosition and Axis_1.ActualPosition; the ratio should hold exactly.
7. Apply pot step changes (0%, 50%, 100%) and verify synchronous ramp.
8. Run a 30-minute soak test; log both ActualVelocity values to a data block for trending.

12. Verification and Diagnostics

Add a watch table with the following tags to verify synchronization during commissioning:

Use the TIA Portal Traces function with a 100 ms recording rate on both ActualVelocity tags. Plot the difference to visualize drift. Drift above 0.5% indicates drive-side ramp or scaling error; drift above 1% indicates an encoder pulse-loss or HSC wiring issue.

13. Troubleshooting Matrix

14. Edge Cases and Field-Proven Caveats

• Encoder Z-pulse for homing: Without homing, the gear ratio holds but the absolute phase relationship is unknown. If the application requires a known phase (e.g., blades aligned), wire the encoder Z signal to an HSC marker input or a fast digital input and trigger MC_Home with Mode = 3 (passive homing on Z-pulse).
• Loss of encoder feedback: HSC retains the last count but velocity calculation becomes 0. Add an underflow/overflow check using CTRL_HSC_EXT with upper/lower limits; raise a drive fault on violation.
• Drive stop category: Ensure the drive is configured for STOP category 1 (controlled ramp-down) not category 0 (coast) when MC_Halt is issued. Category 0 on the master with the follower still geared will saturate the follower drive.
• Power-up sequencing: Always power the master first, home it, then engage the follower with MC_GearIn. Reversing the order causes an immediate large following error on engage.
• Firmware versions: MC_GearIn behaviour changed slightly between FW V4.2 and V4.4 (Abort handling on master stop). Review the firmware release notes on the Siemens Industry Online Support portal before commissioning.

15. Alternate Controller Notes

For applications where the S7-1200 scan budget is insufficient (e.g., above 4 axes or 1 ms IPO requirement), migrate to a SIMATIC S7-1500 with a 1 ms IPO cycle or to a SINAMICS S120 drive pair with DCC (Drive Control Chart) doing the gearing at the drive level. The latter eliminates the PLC entirely and reduces velocity tracking error to below 0.01%.

For cost-sensitive single-axis duplication where only speed (not phase) must match, a simple analog passthrough from drive 1 output to drive 2 input is acceptable and removes the PLC from the control loop entirely. The PLC then serves only for start/stop and monitoring.