circular loom LOAD CELL SYSTEM (Installation & Calibration Procedure)

 

 

 

LOAD CELL IN  MACHINE FOR WARP TAPE TENSION CONTROL

•        OBJECTIVE:

To monitor and maintain warp tape tension automatically, load cells have been introduced in the Looms. Load cells are mounted on the dancing roller just after the maxi drag roller & below the platform. The resultant tension of all the warp tapes on the roller is sensed continuously. Any change in the tension on the roller is detected by the load cell and proportional amount of current through amplifier card is fed to the inverter of inlet. Inverter computes the current & gives the output frequency to the inlet motor so as to maintain the set tension in the warp tapes. Weaving at constant tension give the good fabric quality. T is also very much helpful in weaving of low denier tapes.

 

 

Description of Load Cell & Related Parts:

§  Load Cell:

Load cell is used to sense the tension in the warp and supply the feedback parameter for the control loop.

      

 

•        Amplifier Card:

Amplifier Card is used for signal conditioning; it amplifies and converts the Load Cell output signal to variable current signal (4- 20 mA). The amplifier card has adjustment potentiometer for calibration purpose of zero and span. By potentiometer the weight of the roller is calibrated so as to display zero load setting on display unit (HMI). By Span Potentiometer, the desired span of tension can be set. For each roller, individual card is used.

•        Variable Frequency Drive:

The variable frequency drive gets the current signal (proportional to the tension in the warp) from Amplifier card. The drive compares the current signal with the set tension value of HMI and computes the out put frequency so as to maintain the set value of the tension. Output frequency goes to the motor. Motor rotates the Inlet roller so that the set tension in the warp tapes is maintained. For each roller, individual drive is used.

 

•        Inlet Motor:

The inlet motor is used to drive the inlet roller & maintains the set tension of warp tapes by varying the speed of inlet roller according to the VFD output frequency. Thus inlet motor maintains the tension in the warp. For each roller, individual Motor is used.

 

Principle of operation of Load Cell System :

•        The Load Cell consists of very sensitive Strain Gauges. The application of load on load cell result in the tension and compression of Strain Gauges. The resistance of strain gauge increases with tensile force and decreases with compressive force. Due to the changes in the resistance of strain gauge, the bridge becomes unbalanced resulting in generation of a potential difference. The resultant increase/drop in voltage is proportional to the variation in load. The electrical signal output is in few mili-volts and requires amplification by an instrumentation amplifier before it can be used.

 

WORKING OF TENSION CONTROL SYSTEM

•        As the weaving starts the tension over the roller increases or decreases which results in the change of the out put of the load cell. The O/P from the load cell is amplified and converted to 4 to 20 mA DC signal. This signal is input to VFD. The VFD is used in PID loop so that the system is in close loop. The O/P of the VFD is connected to inlet motor which rotates the inlet roller. As the tension increases the O/P of the load cell increase so the speed of the inlet roller also increases. Due to this tension will decrease and reach the desired value. The Inverter varies the frequency of the motor according to the Set-Point and the feedback signal from the Load cell Signal Conditioner and thus the speed of the Inlet Motor to maintain the desired tension in the warp.

 

•        The desired tension can be set from the Prism / Controller and actual tension can also be seen on the Prism/ Controller.

 

Load Cell System Installation Procedure

•        Before mounting the load cell check the  input and output resistance of the load cell which is as follows :-

•        Red & Black – I/P  = 415 ± 15 ohms

•        Green & White – O/P = 350 ± 5 ohms

•        Always mount the load cell with Arrow direction upward.

•        Connect the connector as per the connection diagram.

•        White –             - VE O/P

•        Green –             +VE O/P

•        Red –                +VE EXI

•        Black –              -VE EXI

•        Shield –             Shield

•        Switch ON the machine and check the polarity of the 24 VDC supply.

•        Switch OFF the machine and connect the connector to the amplifier card.

•        Connect analog shielded signal cable connector (brown wire to +VE & white wire to COMMON) to the amplifier card.

•        Switch ON the machine and check the following –

–       Voltage between +VE EXI & -VE EXI  is 12 VDC

–       The mV voltage between + VE O/P & - VE O/P should be increasing when the roller is pulled slowly up-ward. 

 CALIBRATION METHOD:

§  ZERO ADJUSTMENT

Remove all the tapes from the roller so that there is no tape tension on the roller except roller weight.

Switch OFF the machine and set the jumper in the amplifier card to Calibration side.

Switch ON the machine. Measure the DC Current by multimeter at +VE and COMMON (O/P) , then rotate the  Zero Adj. potentiometer and adjust the current O/P at 4 mA.

 

§  SPAN ADJUSTMENT

Method 1-

 

 

 

 

 

 

 

•        Load the roller with 30 KG weight (weight to be hanged by nylon rope so that the weight is free hanging and the 30 kg weight is coming on the load cell roller vertically upward) so that the total weight is pulling the roller upward. During this calibration the chain should be removed between the inlet motor and inlet roller.

•        Switch OFF the machine and set the jumper in the amplifier card to Calibration side.

•        Switch ON the machine. Measure the DC Current by multimeter at +VE and COMMON (O/P) , now rotate the  Span Adj. potentiometer and adjust the current O/P at 20 mA. Switch OFF the machine and set the jumper in the amplifier card to Calibration side.

 

Method 2-

•        Remove the load cell connection from the amplifier card. Connect the mV generator to amplifier card at +VE O/P & -VE O/P. Keep The mV generator at mV source position and switch ON the source. Adjust the voltage to 6.78 mV for 120 Kg and 12.67 mV for 30 Kg.

•        Switch OFF the machine and set the jumper in the amplifier card to Calibration side.

•        Switch ON the machine. Measure the DC Current by multimeter at +VE and COMMON (O/P), now rotate the Span Adj. potentiometer and adjust the current O/P at 20 mA.

•        After calibration remove the connection from the amplifier card and re-connect the load cell connection to the card.

•        After calibration switch off the machine and change the Jumper position to normal side before running the machine.

 

Note: Once the Zero setting is done the pots related to zero setting should not be disturbed to avoid re-calibration.

 

PRECAUTIONS

 

•        During Zero or Span adjustment first rotate the TRIM POT very slowly in one direction and see that current is increasing or decreasing and then accordingly rotate the TRIM POT and adjust the current.

•        Don’t touch the ICs on PCB.

•        Don’t connect or disconnect any connector in power ON condition.

•        Always keep multimeter knob on DC current

•        For calibration change Jumper position to calibration side (As shown in the figure)

•        Put the protection clamp after doing the trial run for 2 to 4 hrs.

•        The gap of 0.5 mm is to be maintained when there is no load the roller.

•        For calibration change Jumper position to calibration side (As shown in the figure).