Potentiometer measurement

The potentiometer should be supplied via the integrated power supply unit (max. 5 V, configurable). The slider voltage is then measured in relation to the supply voltage and given as a percentage. Technically, the measurement process is similar to a half-bridge strain gauge.

In the following, the specification is given for a 5-wire configuration; external line resistances are compensated for by the 5-wire configuration and the potentiometer is recorded directly in the measuring channel. In the 3-wire configuration, the measuring channel essentially meets the same specification, as it continues to measure internally in 5-wire configuration. However, the lead and contact resistance levels in cables and plugs, which are usually unclear and temperature-dependent, must be taken into account by the connected potentiometer, as they affect measurement. The entire "potentiometer + supply lines + measuring channel" system in the 3-wire configuration will therefore not be able to achieve the specification values listed below.

Potential diagnoses at full-scale deflection or 0 display:

slider break
interruption in supply

Potentiometers from 500 Ω can be used.

The specification and the manufacturer's calibration were determined with a 5 kΩ potentiometer. If a different potentiometer resistance is used, the measurements will be incorrect due to the changed parallel circuit ratios, see "Supply, potential groups" chapter.

The distortion can also be recorded mathematically:

	Using the sample programs This document contains sample applications of our products for certain areas of application. The application notes provided here are based on typical features of our products and only serve as examples. The notes contained in this document explicitly do not refer to specific applications. The customer is therefore responsible for assessing and deciding whether the product is suitable for a particular application. We accept no responsibility for the completeness and correctness of the source code contained in this document. We reserve the right to modify the content of this document at any time and accept no responsibility for errors and missing information.

The following example code can be used to theoretically correct a different potentiometer:

VAR

    R_Poti_calibration : LREAL := 5000.0; // Ohm (vendor calibration case)

    R_Poti_actual : LREAL := 500.0; // Ohm (user application case)

    Rin : LREAL := 2.06E6; // Ohm

    PDO : LREAL; // RAW Value

    PDO_Ratio : LREAL;

    R2_act : LREAL;

    R1_act : LREAL;

    R2_act_Rin_parallel : LREAL;

    R2_calibration : LREAL;

    R1_calibration : LREAL;

    R2_calibration_Rin_parallel : LREAL;

    PDO_corrected : LREAL;

END_VAR

PDO_Ratio := PDO / 7812500.0;

R2_act := R_Poti_actual / 2.0 - PDO_Ratio * R_Poti_actual / 2.0;

R1_act := R_Poti_actual / 2.0 + PDO_Ratio * R_Poti_actual / 2.0;

IF (R2_act + Rin) <> 0 THEN

            R2_act_Rin_parallel := (R2_act * Rin) / (R2_act + Rin);

ELSE

            R2_act_Rin_parallel := 0.0;

END_IF

R2_calibration := R_Poti_calibration / 2.0 - PDO_Ratio * R_Poti_calibration / 2.0;

R1_calibration := R_Poti_calibration / 2.0 + PDO_Ratio * R_Poti_calibration / 2.0;

IF (R2_calibration + Rin) <> 0 THEN

            R2_calibration_Rin_parallel := (R2_calibration * Rin) / (R2_calibration + Rin);

ELSE

            R2_calibration_Rin_parallel := 0.0;

END_IF


IF (R1_act + R2_act_Rin_parallel) <> 0 AND R_Poti_actual <> 0 AND (R1_calibration + R2_calibration_Rin_parallel) <> 0 AND R_Poti_calibration <> 0 THEN

            PDO_corrected := (PDO_Ratio-

                        (((2.0*(0.5-R2_act_Rin_parallel/(R1_act+R2_act_Rin_parallel)))-

                        (2.0*(0.5-R2_act/R_Poti_actual)))-


                        ((2.0*(0.5-R2_calibration_Rin_parallel/(R1_calibration+R2_calibration_Rin_parallel)))-

                        (2.0*(0.5-R2_calibration/R_Poti_calibration)))))*7812500.0;

ELSE

            PDO_corrected := 0.0;

END_IF

Measurement mode	Potentiometer (3/5-wire)
Operation mode	The supply voltage is configurable via CoE, 0.5...5 V
Measuring range, nominal	-1 … 1 V/V
Measuring range, end value (FSV)	1 V/V
Measuring range, technically usable	-1 …1 V/V
PDO resolution	24 bit (including sign)
PDO LSB (Extended Range)	0.128 ppm
PDO LSB (Legacy Range)	0.119… ppm

Measurement mode		Potentiometer (3/5-wire)
Basic accuracy: Measuring deviation at 23°C, with averaging, typ. ^{1) 2)}	without Offset	< ±0.0025 %_FSV < ±25 ppm_FSV < ±25 µV/V
	incl. Offset	< ±0.0075 %_FSV < ±75 ppm_FSV < ±75 µV/V
Extended basic accuracy: Measuring deviation at 0…55°C, with averaging, typ. ^{1) 2) 6)}	without Offset	< ±0.0055 %_FSV < ±55 ppm_FSV < ±55 µV/V
	incl. Offset	< ±0.009 %_FSV < ±90 ppm_FSV < ±90 µV/V
Offset/Zero point deviation (at 23°C) ¹⁾	E_Offset	< 70 ppm_FSV
Gain/scale/amplification deviation (at 23°C) ¹⁾	E_Gain	< 20 ppm
Non-linearity over the whole measuring range ¹⁾	E_Lin	< 15 ppm_FSV
Repeatability, over 24 h, with averaging ¹⁾	E_Rep	< 1 ppm_FSV
Temperature coefficient, typ. ¹⁾	Tc_Gain	< 1 ppm/K
Temperature coefficient, typ. ¹⁾	Tc_Offset	< 1 ppm_FSV/K < 1 µV/V/K
Common-mode rejection ratio (without filter) ³⁾		DC: tbd. typ.	50 Hz: tbd. typ.	1 kHz: tbd. typ.
Common-mode rejection ratio (with 50 Hz FIR filter) ³⁾		DC: tbd. typ.	50 Hz: tbd. typ.	1 kHz: tbd. typ.
Largest short-term deviation during a specified electrical interference test		tbd. %_FSV = tbd. ppm_FSV typ.
Input impedance (internal resistance)		typ. approx. 2.06 MΩ \|\| 100 pF against AGND typ. approx. 20 nF against SGND

¹) valid for ELM3504-00x0 since HW04, ELM3502-00x0 since HW03

²) A regular offset adjustment with connected potentiometer is recommended. The given offset specification of the terminal is therefore practically irrelevant. Therefore, specification values with and without offset are given here. In practice, the offset component can be eliminated by the functions Tare and also ZeroOffset of the terminal or in the controller by a higher-level tare function. The offset deviation over time can change, therefore Beckhoff recommends a regular offset adjustment or careful observation of the change.

³) Values related to a common mode interference between SGND and internal ground.

⁶) Calculated value according to equation in chapter "General information on measuring accuracy/measurement uncertainty" for quick estimation of usability over the specified ambient temperature range in operation (T_ambient). In real use, for example at a relatively constant ambient temperature T_ambient, a lower (better) achievable uncertainty is attained. A specific calculation according to chapter "General information on measuring accuracy/measurement uncertainty" is recommended, especially if the instrument allows a wider ambient temperature range in operation than 0...55 °C.

ELM3502 (20 ksps)

Measurement mode		Potentiometer (3/5-wire)
Noise (without filtering, at 23°C)	E_{Noise, PtP}	< 105 ppm_FSV< 820 digits < 105 µV/V
	E_{Noise, RMS}	< 18 ppm_FSV< 137 digits < 17.5 µV/V
	Max. SNR	> 95.1 dB
	Noisedensity@1kHz	< 0.18
Noise (with 50 Hz FIR filter, at 23°C)	E_{Noise, PtP}	< 9 ppm_FSV< 70 digits < 9 µV/V
	E_{Noise, RMS}	< 1.5 ppm_FSV< 12 digits < 1.5 µV/V
	Max. SNR	> 116.5 dB

ELM3504 (10 ksps)

Measurement mode		Potentiometer (3/5-wire)
Noise (without filtering, at 23°C)	E_{Noise, PtP}	< tbd. ppm_FSV< tbd. digits < tbd.
	E_{Noise, RMS}	< tbd. ppm_FSV< tbd. digits < tbd.
	Max. SNR	> tbd. dB
	Noisedensity@1kHz	< tbd.
Noise (with 50 Hz FIR filter, at 23°C)	E_{Noise, PtP}	< tbd. ppm_FSV< tbd. digits < tbd.
	E_{Noise, RMS}	< tbd. ppm_FSV< tbd. digits < tbd.
	Max. SNR	> tbd. dB

Potentiometer measurement range

Note: In Extended Range Mode the Underrange/Overrange display in the PDO status has the character of an information/warning when the nominal measuring range is exceeded, i.e. no Error is displayed in the PDO status and LED. If the technical measuring range is also exceeded, Error = TRUE is also displayed. The detection limit for Underrange/Overrange Error can be set in the CoE.

In Legacy Range mode, an Underrange/Overrange event also leads to an Error in the PDO status.