# Selection principle of sensors and instruments in

2022-07-23
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Selection principle of sensors and instruments in weighing instrument system design

problem raised:

in weighing instrument scheme design, how to select sensors and weighing instruments is often unknown after the accuracy grade and other indicators are determined? Sometimes I think that I have correctly selected the weighing sensor and weighing instrument, but in the actual test, the measurement performance index of the weighing instrument is still not guaranteed. Please give me some tips on the selection of sensors and instruments

problem solving:

how do we solve such problems in the weighing apparatus? Today, let's introduce in detail the processing method of inaccurate measurement data of 1 downward tension testing machine. When designing the scheme, it is often necessary to master the following selection principles of sensors and instruments after determining the accuracy grade of the weighing instrument:

1. the measurement parameters related to the weighing instrument in the selection of sensors and instruments

(1) the main parameters of the weighing instrument: maximum weighing capacity, minimum weighing capacity, accuracy grade, maximum verification division number n Operating temperature range, static load of the carrier, peeling weight, etc

(2) error distribution coefficient of weighing instrument: p12+ p22+ p32+ p42+ p52... ≤ 1

where: P1, P2, P3, P4, P5... Pi - error distribution factor of weighing instrument, 0.3 ≤ PI ≤ 0.8

2. the maximum weighing capacity correction factor q

generally speaking, the maximum weighing capacity correction factor Q of the weighing instrument should be greater than 1, mainly considering the possible effects of eccentric load (uneven load distribution), static load of the carrier, initial zero setting range and adding tare weight

Q = (Max+ DL + ISZR + NUD + T+...)/ Max

among them: Max - the design of weighing instrument has a maximum weighing capacity of cup-shaped grinding wheel

dl - static load of the carrier, which shall be determined during the design of the weighing instrument scheme

iszr - initial zero setting range, generally 20% max

nud - load uneven distribution value of the carrier. Generally, the assumption can be made according to the weighing instrument of typical structure

· scales with levers and single sensors, or hoppers with sensors arranged symmetrically, there is no device on the carrier that is easy to cause partial load NUD = 0%max

· other typical scales NUD = 20%max

· for forklift scales, monorail hanging scales and truck scales NUD = 50%max

· multi platform scales, the combination number is fixed NUD = 50%max (cumulative maximum scale)

· multi platform scales, The combination number is variable NUD = 50%max (maximum weighing capacity of single weighing platform)

t - the weight value designed by adding tare weight method in the weighing scheme design

of course, the maximum weighing capacity correction coefficient Q of the weighing instrument can also be added with impact coefficient, overload coefficient, wind pressure coefficient, etc. according to the actual use conditions of different weighing instruments

3. type selection of load cell

accuracy grade, including the temperature range of load cell (LC) and wet heat stability and creep index, must meet the requirements of weighing instrument

*) if the temperature range is wide enough, and the stability and creep index of damp heat can only meet the requirements of lower accuracy level

3.2 the maximum allowable error distribution coefficient of the sensor plc

if the error distribution coefficient of the weighing sensor is not specified in the design of the weighing system, PLC = 0.7. The error distribution coefficient of the load cell can be 0.3 ≤ PLC ≤ 0.8

3.3 working temperature range

if the working temperature range of the weighing sensor is not specified in the weighing instrument system design scheme, the lower limit of the temperature range Tmin = -10 ℃ and the upper limit of the temperature range Tmax = 40 ℃. According to the design scheme of weighing instrument system, the temperature range can also be limited, but the working temperature range shall not be less than 30 ℃

3.4 maximum weighing capacity of the sensor

the maximum weighing capacity Emax of the weighing sensor shall meet the following conditions:

emax ≥ Q × Max × R/n

where: Q - correction coefficient of maximum weighing capacity of weighing instrument

max - maximum weighing capacity of weighing instrument

r - scale of load transfer device of weighing instrument

n - number of supporting points of weighing instrument sensor

3.5 minimum static load of the sensor

the minimum load generated by the carrier must be equal to or greater than the minimum static load of the load cell Emin (many load cells have Emin = 0):

emin ≤ DL × R/n

where: Emin - minimum static load of the sensor

dl - static load of the carrier, which shall be determined during the design of the weighing instrument scheme

3.6 maximum division number of the sensor nlc

(1) for each weighing sensor, the maximum division number NLC of the weighing sensor shall not be less than the verified division number of the weighing instrument n:

nlc ≥ n

(2) for weighing instruments with multiple weighing ranges and division values, The following requirements are applicable to any separate weighing range or local weighing range:

nlc ≥ ni

(3) sensor minimum static load output recovery dr

l for multi weighing range weighing instruments, when the same sensor is used in more than one weighing range, the following conditions shall be met:

dr × E/Emax≤ e1 × R/n

or: Dr/Emax ≤ E1/max

when Dr is unknown, the condition NLC ≥ 0.4 shall be met × Max r/e1。

· for multi division value weighing instrument, the following conditions shall be met:

dr × E/Emax≤0.5 × e1 × R/N is Dr/Emax 0.5 × E1/max

where e = max × R/N is the partial load applied to a single load cell when the scale is loaded to max

when Dr is unknown, the condition NLC ≥ max/E1 shall be met

3.7 the minimum calibration division value of the sensor

the minimum calibration division value Vmin of the weighing sensor shall not be greater than the calibration division value E of the weighing instrument multiplied by the reduction ratio r of the load transfer device, divided by the square root of the number n of weighing sensors:

vmin ≤ E1 × R/

for multi weighing range weighing instrument, when the same sensor is used for more than one weighing range, or for multi division value weighing instrument, e is replaced by E1

and strict polymerization conditions are required. 3.8 sensor input impedance

the input resistance RLC of the sensor is limited by the weighing instrument. RLC/N must meet the input resistance range RL min to RL max of the weighing instrument

3.9 rated output of sensor

after the sensor is loaded with Emax, the change of output signal under the corresponding input voltage is generally expressed in mv/v

in order to facilitate calculation, the following relative values are introduced into OIML R 60:

y = e max/V min

z = e max/(2 · DR)

4. selection of weighing instruments

accuracy grade of weighing instruments, including temperature range and hygrothermal stability index, must meet the requirements of weighing instruments

*) if the temperature range is wide enough and the stability index of damp and heat can only meet the requirements of lower accuracy level

4.2 maximum allowable error coefficient of weighing instrument

if the maximum allowable error coefficient of weighing instrument is not specified in the design of weighing instrument system, then PIND = 0.5. According to the design requirements of weighing instrument system, the coefficient can be 0.3 ≤ PIND ≤ 0.8

4.3 working temperature range

if the weighing instrument temperature range is not specified in the design of the weighing instrument system, the lower limit of the temperature range Tmin = -10 ℃, and the upper limit of the temperature range Tmax = 40 ℃. According to the design requirements of the weighing instrument system, the temperature range can be limited, but the working temperature range shall not be less than 30 ℃

4.4 maximum calibration division number

for each weighing instrument, the maximum division number nind shall not be less than the calibration division number of the weighing instrument n:

nind ≥ n

for multi weighing range or multi division value weighing instrument, this requirement applies to any separate weighing range or local weighing range:

nind ≥ ni

if it can be used for multi weighing range or multi division value weighing instrument, These functions must be included in the calibrated weighing instrument

4.5 electrical parameters of weighing instrument related to weighing instrument

uexc (V) excitation voltage of weighing sensor

umin (MV) normal minimum input voltage of weighing instrument

Δ UMIN (MV) minimum input voltage of each calibration division value of weighing instrument

umrmin (MV) minimum voltage of measuring range of weighing instrument

umrmax (MV) maximum voltage of measuring range of weighing instrument

rlmin (W) minimum output impedance of connected sensor

rlmax (W) maximum output impedance of connected sensor

4.6 minimum input voltage of each calibration division value of weighing instrument

(1) Normal minimum input voltage of weighing instrument (no load of weighing instrument) UMIN checking calculation

umin = C × Uexc × R × DL/(Emax × N)

where: uexc - excitation voltage of the sensor

c - rated output of sensor

r - scale of load transfer device of weighing instrument

dl - static load of weighing instrument carrier, determined during weighing instrument scheme design

emax - maximum weighing capacity of the sensor

n - number of supporting points of weighing instrument sensor

(2) minimum input voltage per calibration division of weighing instrument Δ UMIN checking

Δ umin = C × Uexc × R × e/(Emax × N)

where: C - rated output of the sensor

uexc -- excitation voltage of sensor

r - scale of load transfer device of weighing instrument

e -- Calibration division value of weighing instrument. For weighing instruments with multiple weighing ranges or division values, e = E1

emax - maximum weighing capacity of the sensor

n - number of supporting points of weighing instrument sensor

5. type selection of connecting cable

the additional cable between the weighing instrument and the sensor or sensor junction box (the weighing instrument using six wire system shall have the sensor feedback compensation function) must be specified in the weighing instrument manual

the simplest method is to specify the value of cable length per unit cross section (3.3 m/mm detected and analyzed by ferrography microscope) of cable materials (copper, aluminum, etc.) in the weighing instrument manual. Otherwise, the cable length (m), sectional area (mm2), conductive material parameters and maximum resistance per core (W) must be calculated

5.1 calculation of cable resistance change

the range between zero point and maximum load shall be measured. First, calculate the corresponding cable resistance change of the weighing instrument within the maximum operating temperature range:

drtemp = rcable × a × (Tmax – Tmin)

where: drtemp - cable resistance change corresponding to the maximum operating temperature range (unit: Ω)

rcable - resistance value of single strand cable, rcable = (R × 50)/a

where: ρ —— Resistivity of the material (e.g. copper: ρ Copper = 0.0175 Ω mm2/m)

l - cable length (unit: m)

a - cross section of single strand cable (unit: mm2)

a - temperature coefficient of cable material resistivity 1/k, for example: Copper α= 0.0039 1/k

tmax – Tmin - maximum service temperature range

5.2 weighing change caused by cable temperature

in order to determine the limit of weighing change caused by temperature influence on cable, the temperature test results of weighing instrument shall be considered. The maximum weighing error caused by temperature shall not be greater than one third of the absolute value of the maximum allowable error of the weighing instrument multiplied by PI

Dspan(DT) ≤ pi × MPE - Emax (DT)

where: dspan (DT) - the weight change value caused by the influence of temperature change on the cable. Dspan (DT) must meet the following conditions:

dspan (DT) ≤ 1/3 (PI ×| Mpe|)

if the weighing instrument cannot meet these conditions, the maximum resistance of the cable, that is, the maximum length of the cable, must be reduced, or a large cross-section cable must be selected. The following provisions shall be made for the additional cable between the weighing instrument and the weighing sensor or the junction box of the weighing sensor (only applicable to the weighing instrument using the six wire system, such as the feedback compensation system):

material (copper, aluminum, etc.), length (m)

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