Measuring RPM with S7-1200 HSC and Proximity Sensor

Overview

Counting rotational speed with a single inductive proximity switch is a classic S7-1200 application. The standard CPU 1212C includes onboard high-speed counter (HSC) inputs rated for signals up to 100 kHz, so a gear-tooth or slotted-disc target is a clean fit. This reference walks through the engineering workflow used on a 25-tooth gear driven by a 1000 RPM motor, where the resulting sensor frequency is 416.67 Hz, and shows how to convert that count into both RPM and linear web speed for an 800 mm drum.

Three implementation paths are documented here, ordered from simplest to most flexible:

1. Edge counting in ID1000 with a cyclic-interrupt time base (works on any S7-1200 firmware).
2. Frequency measurement with the extended HSC instruction CTRL_HSC_EXT (firmware 4.0 and later on CPU 1211C/1212C/1214C).
3. Period measurement using CTRL_HSC_EXT in period mode for very slow shafts.

Prerequisites

Reference the S7-1200 Programmable Controller System Manual (entry ID 109478121) for the exact HSC input pin map of the CPU 1212C. On the 1212C, the onboard inputs Ia.0 through Ia.5 are HSC-capable, with the maximum input frequency of 100 kHz on Ia.0 to Ia.3 (single-phase) and 30 kHz on Ia.4 to Ia.5.

Step 1: Signal Frequency Analysis

Before any code, calculate the maximum pulse rate the sensor will deliver. The 25-tooth gear at 1000 RPM produces:

f = (RPM × teeth) / 60 = (1000 × 25) / 60 = 416.67 Hz

This is well below the 100 kHz HSC ceiling, so the application is comfortable on a stock 1212C with no signal board or fast counting module. The constraint that previously blocked the user was a sensor with a switching frequency under 1 kHz, which would alias or stall at 416 Hz. A standard 1 kHz inductive proximity (for example, a Sick IME12 or ifm IFS204) is sufficient. For higher tooth counts or shaft speeds, recompute f first and confirm headroom:

Step 2: HSC Hardware Configuration in TIA Portal

1. Open the device view of the CPU 1212C and select Properties → Pulse Generators (PTO/PWM) initially to confirm the CPU is not consuming the input for PTO.
2. Open Properties → High Speed Counters (HSC) and add a new HSC instance (HSC_1 by default).
3. Set:
   • Type of counting: Count continuously (no preset, no gate control unless used).
   • Counting direction: Single-phase with internal direction control.
   • Input: Ia.0 (or the physically wired pin).
   • Initial count value: 0.
4. Set the input filter to a value appropriate for 416.67 Hz. The default filter of 0.8 µs (100 kHz) is fine. If noise is present, raise the filter to 6.4 µs (still 78 kHz).
5. Assign the process image for the HSC count to ID1000 (the default for HSC_1). ID1000 is a 32-bit DINT accessible from any OB, FB, or DB.

Background on HSC configuration, ID1000 mapping, and the maximum input frequency table is documented in the S7-1200 System Manual, section on High-Speed Counters.

Step 3: CTRL_HSC_EXT Frequency Measurement

The extended HSC instruction provides built-in frequency and period measurement, which is the cleanest path on firmware V4.0 and later. From the TIA Portal instruction tree, navigate to Technology → Counting → CTRL_HSC_EXT and drag it into a function block. The F1 help for the instruction lists every parameter, including the new PERIOD and FUNCTION selectors.

Sample call inside OB1:

// CTRL_HSC_EXT instance DB "DB_HSC_EXT"
"DB_HSC_EXT"(HSC    := 1,            // HSC_1
             PERIOD := 1000,         // 1000 ms measurement window
             FUNCTION := 1,          // 1 = frequency measurement
             MEASURED_FREQ => #MeasFreqHz,
             EDGE_COUNT   => #EdgeCount,
             ERROR        => #Err,
             STATUS       => #Status);

Parameter notes:

• PERIOD sets the integration window in milliseconds. 1000 ms gives a direct Hz reading at 1-second resolution; 100 ms gives 10 Hz steps but updates 10× faster. Trade off noise versus latency.
• MEASURED_FREQ returns Hz as DINT (real * 1000 in some firmwares, raw Hz in V4.2+, confirm via F1 status word).
• EDGE_COUNT is the number of edges captured inside PERIOD, useful as a cross-check against ID1000.

Apply the standard speed equation:

RPM = (EdgeCount × MeasurementFactor) / PeriodTime

where MeasurementFactor = 60 / teeth. With 25 teeth and a 1000 ms window, the formula collapses to RPM = EdgeCount × 2.4. For 416.67 Hz, EdgeCount = 416 or 417 and the displayed RPM = 999.4 to 1000.8, within one pulse of resolution.

Step 4: Cyclic Interrupt OB for Time-Based Counting

When CTRL_HSC_EXT is unavailable (firmware < 4.0) or when a second, independent channel is needed, an OB200 (Cyclic Interrupt) is the standard approach. The pattern is identical to legacy S7-300 work.

1. Add a new Cyclic Interrupt OB (default OB200). Set its phase offset to 0 and execution time to 1000 ms for a one-second RPM update.
2. Inside OB200, snapshot ID1000 and compute the delta against the previous snapshot stored in a static tag of an FB or in a global DB.

// OB200 - Cyclic Interrupt, 1000 ms
// Input: ID1000 (DINT) - HSC count from CPU
// Tags in global DB "DB_Speed":
//   "DB_Speed".PrevCount   : DINT
//   "DB_Speed".EdgeCount   : DINT
//   "DB_Speed".RPM         : REAL

#EdgeCount := "ID1000" - "DB_Speed".PrevCount;
"DB_Speed".PrevCount := "ID1000";

// EdgeCount pulses per 1000 ms = Hz
// RPM = Hz × 60 / 25 teeth
"DB_Speed".RPM := INT_TO_REAL(#EdgeCount) * 60.0 / 25.0;

Catch for 32-bit overflow: ID1000 is a DINT. At 416.67 Hz it overflows 2,147,483,647 in about 57 days. For an unattended line, reset the count or use a modulo counter in the DB.

Step 5: Linear Speed Calculation

Once RPM is known, linear web speed on the drum is:

V (m/min) = π × D (m) × n (RPM)

For the 800 mm drum at 1000 RPM:

V = π × 0.8 × 1000 = 2513.3 m/min

This is the value at the drum surface, not at the gear. The user case studies a 15 m/min web, which corresponds to roughly 6 RPM at the drum, not 1000 RPM. Confirm which shaft the proximity sensor actually monitors before applying the formula. If the sensor is on a low-speed take-up shaft, the divider changes; if it is on the motor, multiply by the gear ratio between motor and drum.

Reciprocal form (used by the user to back-compute n from a measured V):

n (RPM) = (V (m/min) × 1000) / (π × D (mm))

Substituting V = 15 m/min and D = 800 mm:

n = (15 × 1000) / (π × 800) = 5.97 RPM

Step 6: Memory Allocation Discipline

A common fault on first-time S7-1200 work is overlapping memory tags. The original program read MD201 while MD210 held the snapshot, and MD212 was a DINT that overlapped both. Memory bits and bytes in the M area (and in unoptimized DBs) lay out byte-by-byte, and a DINT occupies 4 consecutive bytes. Use the rules below:

Step the start addresses of any multi-byte M tag by a multiple of 4 to avoid silent overlap. The clean fix is to declare every speed, count, and status value inside a global DB with structured names, e.g. DB_Speed.RPM_Sec, DB_Speed.RPM_Min, DB_Speed.WebSpeed_mpm. The compiler then manages byte offsets and the cross-reference shows all reads/writes. Search the cross-reference for Ms (or any temporary test tag) and delete them before sign-off.

Step 7: Sensor Wiring and Filtering

Wire the proximity switch to a 24 V DC source from the CPU's sensor power output (terminal 24 V on the front connector). Use a shielded cable with the shield bonded at the cabinet entry, and keep the run away from VFD output cables to avoid capacitive coupling. Confirm the sensor is PNP (sourcing) so that the active-high edge matches HSC expectation on the S7-1200. If the sensor is NPN, add a 4.7 kΩ pull-up to 24 V at the input terminal and verify the resulting logic level.

Set the input filter in the CPU device configuration. The 1212C filter choices are:

• 0.8 µs → 100 kHz
• 1.6 µs → 100 kHz
• 3.2 µs → 100 kHz
• 6.4 µs → 78 kHz
• 12.8 µs → 39 kHz

For 416 Hz, any filter setting works. Use 6.4 µs (78 kHz) when the cable run exceeds 10 m or the installation shares the tray with motor power.

Step 8: Verification and Commissioning

1. Force a known frequency from a signal generator or function-generator app on the input. Inject 416.7 Hz at 50% duty, 24 V swing. The online watch table for ID1000 must increment by roughly 416 each second.
2. Spin the shaft by hand at a measured RPM. Use a tachometer on the same shaft as a cross-check. Expected reading at 1000 RPM with 25 teeth = 999.6 to 1000.8 RPM, single-pulse resolution.
3. Validate CTRL_HSC_EXT output MEASURED_FREQ matches the ID1000 delta divided by the PERIOD.
4. Monitor the HSC STATUS word for non-zero values. STATUS = 0 means healthy. See the S7-1200 system manual HSC error codes for the full list; common values 0x0001 to 0x000F indicate illegal parameter combinations at configuration download.
5. Run a 10-minute soak. Watch for dropped counts (HSC_VALUE stops incrementing while the shaft turns), which usually point to filter settings, sensor supply sag, or a wiring shield not terminated.

Troubleshooting Matrix

Sensor Selection Quick Reference

Generic application guidance on proximity-based RPM sensing (resolution versus pulses per revolution) is summarized in the Opto 22 RPM Measurement Techniques technical note (form 1784), which is independent of the PLC choice and is useful background when justifying the tooth count.

Final Notes on Field Practice

• Capture the calculation basis as a comment block in the FB so the next maintainer sees which shaft and which diameter the constants refer to.
• Save the HSC DB as a know-how-protected block (right-click → Know-how protection) only after the program is signed off, because protected blocks cannot be re-edited without the password.
• If the drum reverses, change the HSC to count continuously with direction control and track direction as a sign on the delta. CTRL_HSC_EXT frequency measurement always reports the absolute value, so it cannot directly indicate direction.
• When a single CPU must measure more than 6 HSC channels, switch to an SM 1231 high-speed counter signal module. The 1212C only has 6 HSC-capable inputs on the CPU itself.