Weighing System Diagnostics Made Easy with Integrated Technician (IT)

By Ted Kopczynski

Troubleshooting a weighing system (scale) can be very time consuming and costly. Isolating a scale component causing the problem is difficult to do without specific tools or training, and sometimes a "weighing expert" well need to be consulted. With the Integrated Technician ("IT") feature of Hardy Instruments weighing components, the tools are already built into the weighing system to allow in-house personnel (or contracted weighing experts) to troubleshoot the system from the weight controllers front panel, quickly and easily. This paper will explain the Integrated Technician feature and show you how it will help you save time and money on your scale installation and system diagnostics. This paper was first presented at the 1998 Powder & Bulk Solids Conference/Exhibition. It was printed in the Conference Proceedings and The August 1998 issue of The Journal of Powder/Bulk Solids Technology.

Process scales are among the most reliable measurement systems in your plant. But when one goes down it can stop production. You typically know when there is a problem with your weighing system, but knowing which component is the cause, replacing it and getting your system up and running quickly is the challenge.

Often the problem with a weighing system is related to environmental or external influences. A ladder or bag of material inadvertently leaned up against a weigh vessel will cause binding. Welding on the weighing vessel can damage load cells (sensors). Load cell cables can be damaged by machinery or animals, and moisture in the weighing electronics, or even load cell cable, can alter weight readings significantly.

Weighing System

A process weigh system usually involves a vessel of some size and shape which is transformed into a scale. The vessel sits on or is hung from one to six (usually four) load cells. The load cells convert the mechanical force of a material in the vessel into a minute analog electrical signal. The electrical signal from each load cell is routed to a junction box, where they are summed and then routed over a single cable to a weight controller/indicator. The weight controller digitizes the signal for local display and control. It can also convert it to the proper protocol for communications to other devices.

Troubleshooting

In the past when you had a weighing system problem you would call a "weighing expert" who would arrive with a variety of tools. These tools would include common hand tools as well as test devices such as multimeters and load cell and instrument simulators. Troubleshooting and manually isolating the faulty component consisted of breaking the systems wiring integrity at the junction box to read the millivolts of each load cell signal, then performing a series of calculations to arrive at the respective weight values. This process is slow at best and requires someone with an in-dept knowledge of weighing systems. In addition, these measurements can include climbing ladders and scaffolding, entering dirty or hazardous areas, and attempting to read load cell specifications from worn or missing labels. Mean while production from the scale system is at a standstill.

With Integrated Technician all of these operations can be accomplished from the front panel of the weight controller. There are many "troubleshooters" to choose from within your own staff. When a problem is suspected, trouble shooting can begin immediately without the need for costly test equipment. Reducing the need for test equipment, it's maintenance, and the cost in training for proper operation, increases your profitability.

As I mentioned troubleshooting a weighing system is a matter of isolating each component and verifying its proper operation. With the Integrated Technician feature the process is quick because diagnostics are done from the instruments front panel usually in the comfort of the control room. There is no need to disconnect wires, perform complex calculations, or risk injury while attempting to take measurements from unaccessible junction boxes and load cells. From the front panel of the weight controller, the operator can push keys to isolate portions of the weighing system.

• First isolate the instrument itself from the rest of the system to determine if its functioning properly.

• Next add the cable between the instrument and junction box along with the junction box itself.

• If everything checks good to this point, each load cell can be added one at a time.

Since you are not breaking the integrity of the of the weighing system there is less of a chance of introducing new problems such as broken wires, loose connections, or mis-wiring when reconnecting the weighing system.

These isolated readings are shown in pounds so no conversion from millivolts is needed and anyone familiar with the process can recognize suspected problems. This process pinpoints the faulty or suspected component area. Flow charts guide you through the troubleshooting for many of the symptoms to help determine the problem component.

Continuous Monitoring

A violent storm strikes your facility with high winds, lighting and moisture. Your scale system seems to have come through it unscathed but unbeknown to you damage has occurred. One of the load cells is damaged due to high current flowing through it from a lightning strike resulting in a 10% error reading. This isn't noticed until hours or even days later when your product no longer meets minimum standards.

With the continuous monitoring feature of "IT" the error would have been detected immediately and an alarm displayed. "IT" continuously monitors the excitation current in the system and will alarm if it detects a change in current of 5% or more.

This feature is also useful during a scale installation or repair. If the load cells being used contain the C2® Second Generation Calibration feature ( a memory device is added to the cell which stores the cell's performance parameters), the weight controller will read the resistance (base R) and voltage. It will then determine the correct current and automatically turn on the excitation monitor feature. If the current measured does not match, the display will alarm immediately. This will allow you to fix a problem that may not be detected till much later in the installation process. Installation steps will not have to be repeated saving you time and money.

If the weigh system does not contain C2® load sensors, the controller will record the measured system resistance (base R), when the zero calibration value is being set during scale calibration. The current is calculated and this becomes the standard, right or wrong. The excitation monitor feature is automatically turned on at this time. If required, the base R and the current resistance can be displayed and compared manually at the weight controller's front panel to aid in routine maintenance.

System Diagnostics

Now that the excitation monitor feature has detected a problem, we can use the built in system diagnostic tools to isolate it.

The simplest test to use is the "Return TO Zero Test". This test requires that the scale be empty. The present zero voltage reading of each load cell is compared to a value which was read and stored during the zero calibration process. If the two values compare an "Ok" is displayed, if they don't "ERR" is displayed. Each load cell which displays an "ERR" isn't necessarily bad, but has a problem associated with it or with some portion of the weigh system connected to it. This can be a build up of material stuck to the inside of the vessel above the cell.

It can also indicate:

• mechanical binding within the vessel

• loose load cell mounting hardware

• Supporting structural problems, such as broken welds, sagging I beams, missing shims or deteriorating support slabs

• poor or corroded electrical connections in the junction box wiring
  
  

To perform any of the tests the front panel multi function key pad labeled -/Test/Clr is pressed. The weight controllers display changes from a weight reading to "SLFST"(self test). The 4/up arrow and 9/down arrow keys are used to toggle between the self test mode and the "ITSEC" (Integrated Technician Security Code) mode. The numerics keys are used to enter a four digit security code and the down arrow key is pressed until the "RTNTO0" (Return to Zero) test menu appears. If C2 load sensors are being used the number of sensors found is displayed. If non-C2 load cells are used the number of cells in the system must be entered. The "TestTE" (compare voltage) mode is entered and each load cell number is displayed followed by an "OK" or "ERR" message each time the enter key is pressed. In addition a "LS ALL" result is displayed. An "OK" can be displayed here even if there are multiple errors which cancel each other ( one plus, the other minus by the same amount). Next the "Base E" (stored voltages) of each cell can be displayed. Write these down so that they can be compared with readings in the next test.

Digital Voltmeter (DVM)

We can also display the signal voltages of each of the load cells in the weigh system. We can choose to display the signals in millivolts with one digit to the right of the decimal point or in millivolts per volt with four digits to the right of the decimal. Both values displayed have an accuracy of two percent. A displayed voltage from one of the load cells that is extremely different to the others will indicate a problem associated with components attached to it. As discussed previously it does not necessarily indicate a bad load cell.

Frequently a process weigh scale will have multiple piping connections for material to enter and leave the vessel. If these pipes are not distributed equally and do not have flexible connections, the load cells will each generate a different signal voltage. Motors, impactors, mixers and other auxiliary machinery mounted to one side of the vessel will also cause an imbalance in the signal voltages. When this is encountered all the piping and the auxiliary equipment could be removed and the voltages compared. In the real world this not practical to do. By comparing the voltages recorded in the return to zero test we can look for relationships between the cell voltages. Which ones read high then? Does it still read higher than the others with material in the scale?

An alternative to verify that a cell is not responding correctly would be to sequentially add a known test weight directly above each cell. Note the change in signal voltage and compare these to calculated voltages to determine the problem area.

The built in DVM is handy for determining if the scale is level or os load sharing during an installation. Prior to attaching any piping, visually inspect the scale for mechanical soundness, tight mounting connections, binding, etc. Record the signal voltages of each load cell (if possible place a small weight sequentially over each cell). For the load cell with the lowest signal adjust the mounting hardware (if this feature is available) or add shims until the measured signal is equal to the highest displayed value. Repeat this for each cell until they are within 0.5 mv. Of each other.

This same procedure is used for corner adjusting the corners in multi-cell floor or platform scales. Instead of shimming a potentiometer is adjusted in the scales junction box bringing the highest values within tolerance to the lowest for each cell.

To enter the DVM mode of operation the "IT" menu is entered and the up or down arrow keys are used until the HI 2151/30WC's front panel displays "DVMTST". Pressing the enter key displays the number of load cells in the system. If load sensors with C2 capabilities are being used the number of sensors are displayed, other wise enter the number of load cells in the system. The Up/Down arrow keys can now be used to choose between millivolts ("DVMMV") or millivolts per volt ("DVMMVV") Press the enter key for the desired unit of measure. "LS1" is displayed for the first load cell. Pres enter again to view the voltage. Continue for "LS2", "LS3", "LS4" and "LSALl"(system voltage). Whenever the "LS_" (not signal voltage) is displayed, the up arrow key can be used to access a specif cell. Press the Exit key to leave this menu and display the the Last "IT" menu item.

Note: A neative signal voltage at "LSALL" will generally indicate a system miswiring.

System Test

The final diagnostic "IT" provides "SYSTST" (system test) is used to display individual weights seen by the load cells and to isolates the weigh system component and compare them to some built in reference circuits. The same tests used in the DVM mode can be performed displaying weight values in pounds or kilograms rather than voltages. Weight displays can be less intimidating to some troubleshooters.

To display weights enter the "SYSTST" portion of the "IT" menu. Us the up arrow key until "NUNLS" appears. Again if C2 is in use, the number of load sensors is displayed other wise the number of load cells must be entered. Press the Enter key and "LS1" appears. As explained under the DVM procedure individual weights can now be displayed.

The Excitation monitor will usually not detect a drifting or unstable signal as displayed on the instruments front panel. This can be caused by any of the following:

• A fault in the instrument

• Lost grounds or ground loops

• Moisture in the load cell cable or junction box

• Loose mounting hardware

• Damage or overloading of the scale supporting structure

• Loose strain gauges with the load cell
  
  

To test the instrument a built-in switch is provided to an internal reference circuit.

When energized the weigh system is disconnected from the instrument and the reference circuit is engaged. If the observed symptoms persist, check the instruments grounding and disconnect any communication ports. Replace the instrument if necessary. If the problem goes away the instrument is functioning properly and another component is causing the problem.

 If the instrument has to be replaced the HI 2151/30WC contains a removable (from rear panel) secure memory module (SMM) which contains all of the set up and calibration parameters of the instrument. This SMM can be plugged into the replacement instrument which downloads this data, eliminating the task of re-configuring it. This saves time, eliminates the need to find the data and the chance of making an entry error.

The systems signal is switched back in and the instability re-appears. Next a switch in the junction box is told by the instrument to disconnect all the load cells and engage a reference circuit within the junction box.

The cable and instrument are now isolated from the load cells. If the symptoms are present check the shielding of the cable between the junction box and the instrument. The cable shield should only be grounded at the instrument. Examine the cable for cuts, breaks or kinks where moisture could have entered the cable. Ensure the cable is not bundled with any high voltage AC cable and is at least three inches from any. Maintain a fourteen inch separation from any magnetic fields. Also inspect the junction box for any moisture. Replace or reposition the cable and replace the junction box if necessary.

If the signal is stable the junction box reference circuit can be disengaged and each individual load cell can added to the system noting where the instability reappears. Test all load cells in case their are multiple problems.

To enter the system test mode press the up arrow key until the "SYSTST" is displayed. Press the enter key and the "INTREF" (internal reference) sub menu is entered. Next the Junction box reference "REFBOX" sub menu can be entered, followed by sequentially switching in individual load cells. Press int exit key twice returns you to the normal operational mode.

If the problem is subtle and you should need the help in troubleshooting, your local weighing expert will find the "IT" tools of great value. If you choose to work with someone who focuses on process weighing systems 100% of their time you can contact our factory service toll free. Our service department can work efficiently with you since we deigned the trouble shooting tools you'll be using.

* This article is provided by Hardy Instruments, www.hardyinst.com. Ted Kopczynski is Product Marketing Manager for process weighing products, Hardy Instruments, Inc., 3860 Calle Fortunada, San Diego, CA 92123-1825; 858-278-2900, fax 858-278-6700, tkopczynski@hardyinst.com