LOGO! 8 Up/Down Counter: Analog Direction Detection from AI

Problem Statement: Counting Potentiometer Revolutions on a Single Analog Input

A single-turn potentiometer driven by a lifting mechanism rotates more than 360° during normal operation. The analog voltage at the wiper only describes the position within one revolution, so a measurement system that records only the analog value loses the absolute height whenever the wiper leaves its 0–10 V window. The engineer needs an incremental revolution counter that uses the same analog input, because the LOGO! 8 base module (BM) does not have free digital inputs for a quadrature encoder or a limit switch.

The implementation strategy in this article derives the direction of motion (clockwise / counter-clockwise) directly from the slope of the analog value, then feeds that direction signal into the LOGO! 8 Up/Down Counter (Vor-/Rückwärtszähler) function block. Every crossing of the analog signal through a configured threshold generates a count pulse; the direction input decides whether the internal counter increments or decrements. The result is a 16-bit revolution counter that can be scaled, exported to the LOGO! web server, and logged to the SD card as a CSV column.

Hardware Requirements

Reference the LOGO! 8 System Manual (entry ID 109751612) for terminal assignments, the LOGO! Soft Comfort V8 Operating Manual (entry ID 109751609) for software block references, and the Siemens LOGO! product page for current catalog numbers and firmware releases.

LOGO! 8 Software Environment

LOGO! Soft Comfort V8.4 is the engineering tool for LOGO! 8 base modules running firmware FS:04 or higher. The function block library used in this article resides in the SF (Special Function) palette:

• SF – Counter: Up/Down Counter (B005 family) with Cnt, Dir, R inputs and a 16-bit internal count value.
• SF – Analog: Analog Threshold Trigger, Analog Comparator, Analog Multiplexer, Math (Add/Sub/Mul/Div), PI Controller, and Analog Filter.
• SF – Edge / Pulse: Edge-triggered flag (Flanke) for one-shot pulse generation on signal transitions.
• SF – Latching: RS / SR latching relays for sticky direction state.
• SF – Timers: Pulse generator (Taktgeber) used to pace the analog sample-and-compare cycle.

The full block reference and parameter dialog structure is described in the LOGO! Soft Comfort V8 manual, chapter 4 (function blocks) and chapter 6 (analog value processing). The diagrams in the remainder of this article follow the LOGO! 8 ladder logic (FBD) representation.

Analog Input Configuration

The potentiometer is wired as a voltage divider between U+ and M, with the wiper connected to AI1. The LOGO! 8 BM models 6ED1052-1MD08-0BA1 and 6ED1052-1HB08-0BA1 accept 0–10 V on AI1–AI4 with reference to M. The internal ADC is 10-bit, so the value at the input of any analog block is a normalized integer 0–1000.

To insert the analog input in LOGO! Soft Comfort:

1. Open the programming canvas. From the toolbar, drag the Analog Input block (icon: blue triangle) onto the sheet.
2. Double-click the block. In the Block Properties dialog, set Input to AI1, Type to 0–10 V, and assign the block number (default AI1).
3. Check Enable smoothing if available in the firmware version. The smoothing factor (1–255) controls the depth of an internal moving average; a value of 8–16 is appropriate for a 1 Hz mechanical movement.

The AI1 block output (a virtual analog net) is the source for all subsequent threshold and comparator blocks. Do not assign it to a digital coil – the signal is analog, not Boolean.

Direction Detection Theory

A single analog sample cannot indicate direction; direction is a property of two samples. Specifically, if sample_n > sample_n-1 the value is rising, if sample_n < sample_n-1 it is falling, and if equal the value is stationary. The classic textbook solution stores sample_n-1 in a memory location and subtracts it from sample_n; the sign of the difference gives the direction. PLC-class controllers, including LOGO! 8, do not provide explicit analog memory, so the equivalent effect is built from three primitives:

1. A fixed-frequency sampling clock that paces when direction is evaluated.
2. A pair of analog threshold triggers with hysteresis that act as 1-bit memory cells encoding the “I was above” / “I was below” history of the most recent threshold crossing.
3. An edge evaluator on the threshold outputs that produces a single-shot pulse each time the value crosses a threshold in a particular direction.

For a single-turn 0–10 V potentiometer, configure two overlapping thresholds. Use a 250-unit-wide hysteresis band (default LOGO! setting is 250 / Gain 1), which is large enough to suppress contact noise and small enough to resolve a 1° rotation at the steepest part of the pot taper.

Program Structure

The complete FBD program consists of the following net groups, each labelled with a free marker (M1, M2, …) for diagnostics.

The two thresholds are deliberately asymmetric: the rising-edge detector watches the LOW threshold (200/100) because the first event in a CW rotation is the analog value crossing 200 going up. The falling-edge detector watches the HIGH threshold (800/700) because the first event in a CCW rotation is the analog value crossing 800 going down. By separating the two regions, the direction is unambiguous even if the user wiggles the mechanism back and forth through a single threshold.

Step-by-Step Implementation

Step 1 – Build the sampling clock

1. Insert a Pulse generator (Taktgeber, B002 family). Set the time base to seconds and the pulse width to 0.1 s. Set the period to 0.5 s (2 Hz sample rate) for a slow lifting application; increase to 0.05 s for fast motion.
2. Wire the output of the pulse generator to flag M1. This is your sample tick.

Step 2 – Configure the two analog threshold triggers

1. Insert two Analog Threshold Trigger blocks from the SF Analog palette.
2. Block A: input = AI1, ON threshold = 200, OFF threshold = 100, Gain = 1. Output → M2.
3. Block B: input = AI1, ON threshold = 800, OFF threshold = 700, Gain = 1. Output → M3.
4. Enable Retentivity only if the LOGO! is restarted mid-motion; otherwise leave disabled so the thresholds reset on power-up.

Step 3 – Generate the rising pulse (count up)

1. Insert an Edge-triggered flag (Flanke, B002 family in the SF – Special category). Input = M2, edge type = rising.
2. Wire its output to M4. Each time the analog value crosses 200 going up, M4 fires a single scan pulse.

Step 4 – Generate the falling pulse (count down)

1. Insert a second Edge-triggered flag. Input = M3, edge type = falling.
2. Wire its output to M5. Each time the analog value crosses 800 going down, M5 fires a single scan pulse.

Step 5 – Wire the Up/Down Counter

1. Insert the Up/Down Counter (Vor-/Rückwärtszähler) from SF – Counter.
2. Wire M4 to the Cnt input, M5 to the Dir input (or, if you want rising = up and falling = down, wire the inverted M5 to Dir via a NOT block), and I1 (or any free digital input) to R for the reset.
3. In the parameter dialog set the On threshold to the maximum revolution count (e.g., 9999 for a 0–9999 rev display) and the Off threshold to 0. Enable Retentivity so the count survives a power cycle.

Step 6 – Expose the count

1. Right-click the counter block and assign the internal count value to a Variable Memory address (e.g., VW0). This makes the count available to the web server and to any analog output scaling block.
2. To drive a 0–10 V analog output proportional to revolution count, add an Analog Multiplexer or Math block: AQ1 = count × 0.001 (with Gain = 0.001 to map 0–9999 → 0–10 V).

Hysteresis and Noise Rejection

Potentiometer wipers generate Johnson noise and contact resistance variations. Without hysteresis the threshold triggers chatter and the counter increments or decrements falsely at rest. The 100-unit hysteresis band chosen in Step 2 corresponds to roughly 100 mV of analog window, which is comfortably above the LOGO! AI noise floor of ±3 LSB (±3 mV) and well below the desired mechanical resolution of one revolution per 360°.

For high-noise environments (VFD cabinets, unshielded motor cables) add a hardware low-pass filter at the wiper: 10 kΩ in series with 100 nF to M creates a 1.6 ms time constant that attenuates 10 kHz noise by 40 dB without affecting the 1–2 Hz motion. The LOGO! 8 software filter (Enable smoothing = 8) provides a second stage of 8-sample moving average, equivalent to a further 18 dB attenuation at 2 Hz.

Up/Down Counter Parameter Reference

The Cnt output of the counter block goes HIGH when the count reaches the On threshold, providing a clean “max reached” signal that can drive an alarm coil or a web-server indicator. The full parameter dialog is documented in LOGO! 8 System Manual, section 4.6.5.

Web Server and CSV Logging Integration

LOGO! 8 BM variants with Ethernet (6ED1052-1HB08-0BA1 and the 6ED1052-2HB08-xBA2) expose a built-in web server on port 80. By assigning the counter’s count value to a Variable Memory location (VW0 in the example), the value can be displayed in a user-defined web page. Use the Tools → Web Page Editor in LOGO! Soft Comfort to drag a “Variable” element onto the page and bind it to VW0.

For CSV data logging, place a FAT32-formatted microSD card in the LOGO! BM slot. The web server then appends a row every second to LOGO_SD.csv in the SD root. The CSV columns include all variable memory locations; the revolution counter (VW0) appears as a numeric column. Refer to the LOGO! Soft Comfort manual, chapter 7.4 (data logging) for SD card sizing and column ordering.

Commissioning and Verification

1. Connect the potentiometer to AI1 and power the LOGO! 8. Open LOGO! Soft Comfort and go online (Tools → PC → LOGO! Connection or Ethernet).
2. Open the online monitor (View → Online Monitor). Observe M2 toggling when the pot passes the 200-unit threshold and M3 toggling at the 800-unit threshold.
3. Manually rotate the pot slowly CW. Verify M4 fires once per crossing of 200 going up and the counter increments by 1.
4. Manually rotate the pot slowly CCW. Verify M5 fires once per crossing of 800 going down and the counter decrements by 1.
5. Cycle the power. Confirm the count is retained (retentivity enabled).
6. Drive the mechanism through 5 full revolutions in each direction and verify the count returns to zero (or to the expected absolute position if the count is offset).
7. Open the web server page in a browser (http://<LOGO IP>). Verify the revolution counter updates in real time.

Troubleshooting Matrix

Alternate Platform Notes

The same algorithmic structure (sampling clock + dual threshold with hysteresis + edge evaluator + Up/Down counter) maps directly to S7-1200 / S7-1500 in TIA Portal using the IEC counter CTUD with a single boolean pulse derived from a comparator on %IW input words. On Allen-Bradley CompactLogix, the equivalent is a CTUD Add-On Instruction driven by a rising-edge transition bit from a GRT comparison. The LOGO! 8 implementation is unique in that the analog memory problem is solved without explicit storage by using two thresholds as 1-bit “was high / was low” memory cells.

Field-Proven Caveats

• The two-threshold method assumes the analog value is monotonic over the range between the two thresholds. If the pot taper is non-linear (audio-taper, log-taper), the direction can become ambiguous in the flat region of the curve. Use a linear-taper (B-taper) pot for this application.
• Counting on threshold crossings gives a revolution resolution equal to the threshold band size in volts, not in degrees. If the user needs 360°/1024 ≈ 0.35° resolution, they must use a multi-turn pot and accept that one threshold crossing = one sub-revolution.
• For a true multi-turn application where the pot value never wraps (e.g., a 10-turn pot), the same algorithm works directly: the threshold at 500 fires once per turn, and the analog value does not wrap, so the counter tracks net revolutions unambiguously.

FAQ