Series 191 Analog Explained: Practical Wiring for 4-20 mA and 0-10 V

Series 191 is Germanjet’s analog-output magnetostrictive displacement sensor. It converts absolute position into a continuous 4-20 mA current or 0-10 V voltage signal for direct connection to a PLC analog input card or a controller AI channel. It needs no fieldbus configuration and no device description file: once the wiring is live there is a reading. It is the lowest-integration-effort class of interface. The cost is that analog cannot carry diagnostic information, and the signal is degraded on the transmission path by voltage drop and interference, so whether wiring and shielding are done correctly directly determines reading quality.

How to choose between 4-20 mA and 0-10 V

Comparison	4-20 mA current loop	0-10 V voltage output
Long-cable transmission	Good; current is unaffected by resistive voltage drop	Poor; cable resistance divides the voltage and readings run low
Immunity to electromagnetic interference	Strong; a low-impedance loop is hard to induce a noise voltage into	Weak; a high-impedance input readily couples interference
Open-circuit detection	Yes; a reading below 4 mA indicates a line fault	No; 0 V cannot be distinguished from “position at zero”
Wiring complexity	Two-/three-/four-wire; pay attention to how power is supplied	Usually three- or four-wire; more straightforward
Receiver requirement	PLC needs a current input channel with a burden resistor	PLC voltage input channel, high input impedance
Recommended application	Long distance, strong interference, fault detection required	Short distance, in-cabinet, voltage input card already present

The default engineering recommendation is to prefer 4-20 mA. The core reason is not accuracy but the fault detectability of a “live zero”: in normal operation the minimum output is 4 mA; on open circuit or loss of supply the reading falls below 4 mA and the controller can immediately declare a fault. With 0-10 V at 0 V, the controller cannot distinguish “position at zero” from “the cable is broken”. That is an essential difference on safety-related or unattended duty. The overall analog-versus-fieldbus selection framework is in Analog or CANopen. Separating a broken wire from true zero on site: analogue reading at zero.

Wiring: two-wire, three-wire and four-wire

Most analog wiring errors come from whether “power and signal share a conductor”.

Two-wire (4-20 mA only): power and signal share the same loop. The sensor is in series between the supply and the PLC input, and loop current is the signal. Wiring is most economical, but the total loop impedance must stay within the sensor’s allowed load range. If the load is too high the current loop cannot “push up”, appearing as a capped reading near full scale.

Three-wire: supply positive, signal and common earth — three conductors. Signal earth and power earth are shared; on a long cable the voltage drop in the earth conductor is added to the signal. Voltage-output types are especially sensitive.

Four-wire: two supply conductors and two signal conductors fully separated. Immunity and accuracy are best; recommended for long distance or high-accuracy duty.

Whatever the scheme, first confirm the PLC input-channel type (current/voltage) and whether a burden resistor is built in. Connecting a current output to a voltage input channel most directly appears as a reading stuck at full scale or zero. Voltage drop and repeater schemes for long-cable transmission are in Long-cable transmission: voltage drop, interference and repeaters.

Shielding and earthing: the main cause of analog reading jitter

An analog signal has no checking mechanism; any coupled interference is read as a position change. Three practical rules:

• Use shielded cable and earth the shield at one end. Earthing both ends forms a circulating current when the two earth points are at different potential, which introduces interference. The single end is usually at the control-cabinet side; the trade-off is in Shielded cable: single-end versus both-end earthing.
• Keep signal cables and power cables in separate trunking. Inverter and servo-drive output cables are the main interference sources. Where separate trunking is impossible, keep distance and cross at right angles; avoid long parallel runs.
• Avoid earth loops. If the sensor housing, cable shield and PLC chassis are earthed at several points of different potential, circulating current is added directly to the signal. EMC mechanisms are in EMC electromagnetic compatibility.

If the reading jumps at the instant the machine starts, or fluctuates with inverter frequency, interference rather than a sensor fault can essentially be locked. The troubleshooting sequence is in Common fault finding.

191 D60 6-pin assignment

Series 19 analogue (order codes 190 voltage / 191 current) uses a D60 6-pin male connector, IP67 when the plug is correctly locked. Pin view looking into the sensor socket:

Pin	Colour	Function
1	Black	Output group 1 (position)
2	White	Return for pin 1
3	Yellow	Output group 2 (second magnet or velocity)
4	Green	Return for pin 3
5	Red	+24 V supply
6	Blue	0 Vdc

• Ground the cable shield at the controller end.
• Minimum spacing for two magnets is 76 mm.
• LED: green on / red off = OK; both on = magnet not detected. Lamps: electronics-head LEDs.
• Field start/end set with the 19 analogue programmer, order no. 1700 951 018; see the analogue field programmer. PLC two-point: zero calibration.
• Resolution: 16-bit D/A, 0.0015% (min. 1 μm); repeatability < ±0.001% FS (min. ±2.5 μm); non-linearity < ±0.01% FS (min. ±40 μm).

Series 18 4-pin voltage models add a hard rule: the controller analogue input negative must go to the sensor 0 V (P4), not the machine earth. Swapping power and signal burns the sensor immediately and is not covered by warranty. Use 4-core shielded twisted pair, min. 0.2 mm².

Span calibration and zero setting

The span mapping of analog output usually has a factory set value; after installation the mechanical stroke must be aligned with the signal range. Points:

• First establish the two end-points of the actual working stroke on the machine, and confirm that the magnet ring does not enter the dead zones at either end of the measuring range (see Dead zones at both ends of the measuring range);
• Zero calibration is done once, on the controller or on the sensor. Absolute position output is not lost on power-down; the mechanism is in Absolute versus incremental position;
• After calibration, check readings at 10%, 50% and 90% of stroke. A single good point does not mean the whole stroke is good. Calibration and linearisation principles are in Factory calibration and linearisation.

Note: the final accuracy of an analog chain does not depend on the sensor alone; it also depends on the resolution and accuracy of the PLC analog input card. Sensor non-linearity is <0.02% FS; if the AI card has only 12-bit resolution, whole-chain accuracy is limited by the AI card. The meaning of the three sensor accuracy figures is in The three accuracy figures.

Where Series 191 is and is not suitable

Suitable: single-axis or few-axis position feedback; the PLC already has analog input cards with spare channels; retrofit of existing plant whose controller does not support fieldbus; projects that need fast integration and do not want configuration work.

Not suitable: multi-axis systems (a pair of wires per axis; harness cost and cabinet space rise quickly); need for sensor diagnostic information (temperature, status, error codes); need to read several magnet rings / several positions (one analog channel can output only one position; multi-position reading is in Multi-magnet / multi-position measuring principle); high-speed multi-axis control needing μs-level synchronisation.

These applications should move to digital interfaces: Series 192 SSI, Series 194 CANopen or Series 197 EtherCAT. The full comparison is in Analog vs. digital across the whole range. See the Series 191 analog product page; for matching external mounting see Series 18 external explained.