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ZhuoHe Intrumentation HomePage > Technical article > >The weighing applications of load cell transmitter

The weighing applications of load cell transmitter

2009-09-17 16:16   Article Source:N/A   View Times:
New multifunction field-mount weighing amplifier can be programmed to function as a load cell transmitter, belt weigher, loss-in-weight transmitter, throughput weigher, dynamometer, bag filler or batch scale.

Incorporating the latest Advanced RISC Machines (ARM) microprocessor technology, the multifunction Model 6004MF amplifier is an all-in-one unit which can be used for almost any weighing application at a cost-effective price.

The current microprocessor power, speed and memory capacity allows the 6004MF to provide additional functions such as data-logging, Ethernet and USB connectivity and GPS functionality.

Application selection and configuration has been kept simple (Figure 1). Six digital inputs, six relay (or solid-state) outputs, four function keys and two displays are freely configurable. Function blocks enable selection of analogue outputs, auto-zero maintenance, linearisation, application settings, load cell transmitter settings and digital communications. A simulate function is available to drive analogue outputs and force relay states in order to test external devices during commissioning. Another innovation is the inclusion of proportional plus integral (PI) control, eliminating the overheads associated with a separate PID controller. This has enabled the analogue output to be used for SG control in the load cell transmitter, belt weigher, flow weighing controller and loss-in-weight applications.

Figure 1. load cell transmitter-The intuitive set-up and calibration functionality is illustrated in the flow diagram.

Figure 1. The intuitive set-up and calibration functionality is illustrated in the flow diagram.

weighing Application descriptions

load cell transmitter

A load cell weight transmitter is used to convert load cell outputs into standard analogue instrumentation signals (4–20 mA or 0–10 volts) as well as supplying the required excitation power. The Model 6004MF provides additional features such as displays for mass and configuration programming, digital communications for direct connection to control computers and alarm capability. The incorporated pre-calibrate, zero trim and span trim features provide an intuitive approach to system set-up. Variables such as sensitivity and range are defined in the pre-calibrate section prior to final accuracy trimming. The system is then zero trimmed to offset the dead weight of a weigh-bin, weigh-frame or weighbridge deck. Known test weights are then used for span trimming to the desired accuracy. In some instances, such as very large weigh-tanks where it is not possible to use test weights, pre-calibration using factory test profiles for load cell sensitivity is used to achieve overall system accuracy.

The ARM processor used incorporates a USB port and a realtime clock for time and date stamping. In the design full use has been made of these facilities and an optional, factory fitted, global positioning system (GPS) has been added to cater for such applications as on-board truck weighing, positioning and data-logging.

Some interesting applications are found in refuse removal, some countries are introducing legislation for customers to pay by weight for this service; in agriculture, farmers are starting to monitor crop production and fertiliser distribution by location to improve efficiency; and in logging, contractors desire to load for maximum profit without incurring overweight fines.

Belt weigher

The belt weigher measures the weight of material on a conveyor belt and its speed, from which mass flow can be calculated in kg/min or tonnes/hr. The 6004MF allows users to set or measure a belt speed and also to enter the weigh-section length. Using this information and the pre-calibrate feature described, material flow rates are calculated and displayed. Zero trim time is selectable to allow for one full conveyor belt cycle and, if some form of check weighing is available, a rate correction factor can be inserted. All rates have a main totaliser as well as a re-settable totaliser. The totalisers can be set for remote display through one of the set-point outputs, and the PI controller used with a variable speed drive can convert the belt weigher into a flow controller.

Loss-in-weight transmitter

Loss-in-weight is becoming a popular form of mass flow measurement or control as it requires few moving or wearing parts. Measuring the weight of a container over time allows the material flow rate (either emptying or filling) to be calculated. As standard the 6004MF samples the averaged weight every second, but the interval can be set for any measurement comparison period up to 480?seconds. A short period is ideal for small containers with the interval being extended proportionately for larger bins or hoppers. The system can be selected to run for one weigh-bin cycle then stop or can be set to hold the rate display and output whilst it refills the weigh-bin. Totalisers and selectable PI control are also standard features.

Throughput weigher

A throughput weigher makes use of a small bin to weigh and dump batching systems in quick succession. They are typically used in the bulk handling of cereals. The flexibility of the 6004MF allows setting of high and low levels of the bin and various time delays in order to tune the throughput weigher for maximum efficiency. The main display is normally the total but a flow rate can also be displayed provided the time base is long (kg/hr or tonnes/hr) in order that batch smoothing is achieved.

Dynamometer

The dynamometer application is used to measure the output power of internal combustion engines and electric or hydraulic motors, in conjunction with an appropriate form of braking. It makes use of the load cell input to measure torque normally in conjunction with a lever arm and the encoder input to measure revolutions per minute (RPM). The load cell can be pre-calibrated and the length of the lever arm defined, for various shaft encoders the impulses per revolution can also be set. The 6004MF will then display kW (or horsepower), RPM and torque. Displays are selectable, eg, kW on main and RPM and torque on the auxiliary LCD.


Bag filler

This application provides the control for bag filling machines, used for measuring precise weights of cement or flour, etc. It has set-points for fast and slow fill, inputs for interlock and footswitch and outputs for filling, vibrating and bag clamps. The settle, vibrate and waiting times are adjustable to optimise the filling. In-flight correction is standard which will correct any weighing error over a preset number of bags. The totaliser can record bags filled, total weight or both.

batch scale

Batch weighers are typically used for control of industrial concrete mixers but can also be used in almost any application that requires a repetitive recipe mix. Freely programmable inputs, function keys, displays and outputs, result in a batch weighing system with tremendous design flexibility. The set-up allows for a maximum of five ingredients. Each variable can be named (sand, cement, etc), be a mass or count (e.g. from a water meter), have an interlock, time delay and in-flight correction. The system allows up to four recipes to be programmed, allowing flexible systems to be implemented and customised – even on site if required.

For more information contact Dave Howcroft, Instrotech, +27 (0)11 462 1920,




Respective topics: Weighing Applications   Weighing Accuracy   Load Cells  
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