Vishay BLH
Monday, August 30, 2010
Monday, August 23, 2010
How does a load cell work?
Load Cell is a device that converts force into electrical signal.
A strain gauge based load cell is made of a specially engineered mechanical component called as an element. Four strain gauges are positioned precisely and bonded on to the element using a special adhesive. (strain gauge is a device that changes resistance when it is deformed or stressed).
The force-electrical signal conversion happens in two stages-
The strain gauges are wired in Wheatstone bridge configuration. The bridge is connected to a power supply unit and a signal conditioner through a 4-core cable. When an input voltage (10VDC) is applied to the bridge, the output is a voltage in the range of few millivolts.
The load cell output is proportional to the load applied to the element.
Basic specifications of a load cell-
Below are the commonly used standard values-
Sensitivity = full scale output/excitation voltage
Example: 20mV/10V = 2mV/V
Two articles to you can read-
Introduction to Load Cells
How Does a Load Cell Work?
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A strain gauge based load cell is made of a specially engineered mechanical component called as an element. Four strain gauges are positioned precisely and bonded on to the element using a special adhesive. (strain gauge is a device that changes resistance when it is deformed or stressed).
The force-electrical signal conversion happens in two stages-
- force applied deforms the element which in turn deforms the strain gauges
- strain gauges converts the deformation to electrical signals
The strain gauges are wired in Wheatstone bridge configuration. The bridge is connected to a power supply unit and a signal conditioner through a 4-core cable. When an input voltage (10VDC) is applied to the bridge, the output is a voltage in the range of few millivolts.
The load cell output is proportional to the load applied to the element.
Basic specifications of a load cell-
- Capacity (kg)
- Full scale Output (mV)
- Sensitivity (mV/V)
- Excitation Voltage (VDC)
- Input resistance (ohms)
- Output resistance (ohms)
Below are the commonly used standard values-
- Excitation voltage: 10V or 12V DC
- Full scale output: 10mV, 20mV and 30mV
- Load cell sensitivity: 1mV/V, 2mV/V and 3mV/V
- Input resistance: 350Ω and 700Ω
- Output resistance: 350Ω and 700Ω
Sensitivity = full scale output/excitation voltage
Example: 20mV/10V = 2mV/V
Two articles to you can read-
Introduction to Load Cells
How Does a Load Cell Work?
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Tuesday, August 17, 2010
How does a Digital Weigh Scale work?
Imagine you pick an apple and try to guess it's weight. You would place it in your palm and toss it lightly to feel the weight. After 3 or 4 tosses you would say "200 grams". Your hand sensed the weight and sent a signal to your brain which after some calculations estimated the weight. Right? Similarly, when you place the same apple on a digital weigh scale, the load cell senses the weight and sends the signal to the indicator. The signal conditioner processes the load cell signal and displays weight.
Basically all weigh scales consist of-
Structure is the load bearing part. It transfers the weight to the load cell(s). In low capacity scales (table-top/bench/platform) the structure houses the load cell and electronic components. In higher capacity scales (floor/dormant/weigh bridge) the structure rests on load cells. The weight indicator
Load Cell is basically a transducer which converts force into electrical signal. Every digital weigh scale has one or more load cells. Table-top/Bench/Platform scales are single load cell weighing machines. Floor/Dormant scales have four load cells. Truck scales or Weigh bridges have 4 to 8 load cells depending on the rated capacity and length of the bridge. For all scales load cell is the part which determines the scales rated capacity. Of course the mechanical structure is designed to withstand loads well beyond the rated capacity. Load cells capacities range from 500g to 100T.
Signal Conditioner is the electronics part, it powers the load cell, receives signal from the load cell(s) and converts that signal to readable numbers. The numbers- digital output- can be made to display in desired unit of weight. Signal conditioners have extended features like RS232 or 4-20MA output which can be used for data acquisition or control other electrical equipment. An electronic weigh scale hooked up to a computer can automate many aspects of business operations like materials management or process control or billing automation.
Compare a mechanical scale to an electronic scale. Mechanical scale just shows the weight but an electronic scale does much more than that- it helps automate.
.........
Basically all weigh scales consist of-
- a structure
- one or more load cells
- signal conditioner
Structure is the load bearing part. It transfers the weight to the load cell(s). In low capacity scales (table-top/bench/platform) the structure houses the load cell and electronic components. In higher capacity scales (floor/dormant/weigh bridge) the structure rests on load cells. The weight indicator
Load Cell is basically a transducer which converts force into electrical signal. Every digital weigh scale has one or more load cells. Table-top/Bench/Platform scales are single load cell weighing machines. Floor/Dormant scales have four load cells. Truck scales or Weigh bridges have 4 to 8 load cells depending on the rated capacity and length of the bridge. For all scales load cell is the part which determines the scales rated capacity. Of course the mechanical structure is designed to withstand loads well beyond the rated capacity. Load cells capacities range from 500g to 100T.
Signal Conditioner is the electronics part, it powers the load cell, receives signal from the load cell(s) and converts that signal to readable numbers. The numbers- digital output- can be made to display in desired unit of weight. Signal conditioners have extended features like RS232 or 4-20MA output which can be used for data acquisition or control other electrical equipment. An electronic weigh scale hooked up to a computer can automate many aspects of business operations like materials management or process control or billing automation.
Compare a mechanical scale to an electronic scale. Mechanical scale just shows the weight but an electronic scale does much more than that- it helps automate.
.........
Tuesday, August 10, 2010
Rated Capacity & Accuracy
Digital weigh scales come in various capacities- starting with jewelry scales (to weigh few grams) to weigh bridges (to weigh mining trucks weighing hundreds of tons). The maximum weight a scale can measure is called Rated Capacity.
Accuracy is an important characteristic of any measuring instrument. We have high, medium and low accuracy scales.
Low - kitchen scales and bathroom scales (non-commercial scales)
Medium - scales used for business purposes
High - scales used in labs or for scientific research
For commercial purposes accuracy is considered as 1/10000th of Rated Capacity. While jewelry scales measure grams with milligrams accuracy, weigh bridges weigh tons with kilograms accuracy. Example; accuracy of a 30T weigh bridge is ±5Kg. Accuracy defines the Least Count - the value with which the weight increments or decrements in the display.
Example 1; display value in a 10kg scale increments as shown below-
01.996 Kg
01.997 Kg
01.998 Kg
01.999 Kg
02.000 Kg
Example 2; value in a 30T weigh bridge increments as shown below-
20.080 T
20.085 T
20.090 T
20.095 T
Also there's another important parameter- Minimum Weight - the least possible weight that a scale can measure for the specified accuracy. It's not practical to measure 2kg on a 100kg scale or to weigh 200Kg on a 30T weigh bridge because we loose out on accuracy. It's ideal to use a 10Kg scale to weigh 2Kg and a 500Kg scale to weigh 200Kg. Minimum weight is normally considered 20 times Least Count.
Table below shows standard values of Least Count, Accuracy and Minimum Weight for some Rated Capacities.
However scales with better accuracies are available. Naturally high accuracy scales would cost more than the standard ones.
'Dual Range' is another popular feature in jewelry, table-top, bench and platform scales. These scales measure lighter loads with high accuracy and switch over to lower accuracy for heavier loads.
.........
Accuracy is an important characteristic of any measuring instrument. We have high, medium and low accuracy scales.
Low - kitchen scales and bathroom scales (non-commercial scales)
Medium - scales used for business purposes
High - scales used in labs or for scientific research
For commercial purposes accuracy is considered as 1/10000th of Rated Capacity. While jewelry scales measure grams with milligrams accuracy, weigh bridges weigh tons with kilograms accuracy. Example; accuracy of a 30T weigh bridge is ±5Kg. Accuracy defines the Least Count - the value with which the weight increments or decrements in the display.
Example 1; display value in a 10kg scale increments as shown below-
01.996 Kg
01.997 Kg
01.998 Kg
01.999 Kg
02.000 Kg
Example 2; value in a 30T weigh bridge increments as shown below-
20.080 T
20.085 T
20.090 T
20.095 T
Also there's another important parameter- Minimum Weight - the least possible weight that a scale can measure for the specified accuracy. It's not practical to measure 2kg on a 100kg scale or to weigh 200Kg on a 30T weigh bridge because we loose out on accuracy. It's ideal to use a 10Kg scale to weigh 2Kg and a 500Kg scale to weigh 200Kg. Minimum weight is normally considered 20 times Least Count.
Table below shows standard values of Least Count, Accuracy and Minimum Weight for some Rated Capacities.
| Type of scale | Rated Capacity | Least Count | Accuracy | Minimum Weight |
| Laboratory | 100g | 0.01g | ±0.01g | 0.2g |
| Jewelry | 500g | 0.1g | ±0.1g | 2g |
| Table Top | 10Kg | 1g | ±1g | 20g |
| Bench | 50Kg | 5g | ±5g | 100g |
| Platform | 100Kg | 10g | ±10g | 200g |
| Floor | 5000Kg | 500g | ±500g | 10Kg |
| Weigh Bridge | 30T | 5Kg | ±5Kg | 100Kg |
| Weigh Bridge | 100T | 10Kg | ±10Kg | 200Kg |
However scales with better accuracies are available. Naturally high accuracy scales would cost more than the standard ones.
'Dual Range' is another popular feature in jewelry, table-top, bench and platform scales. These scales measure lighter loads with high accuracy and switch over to lower accuracy for heavier loads.
.........
Tuesday, August 3, 2010
Weighing Scales
Basically there are three types of weigh scales-
Balances: These are the earliest form of weighing equipment. A known weight is placed on one side and the object to be weighed on the other side. Method is comparison of weights and approximate.
Analog Scales: Scales are advanced compared to balances. The object is placed on the scale and the pointer shows the weight on the graduated dial. Weight measurement is precise compared to balances.
Digital scales: Precision is what these scales are known for. Electronic scales have extra features such as display weight in multiple units (kilograms and pound), cost calculation, counting number of pieces, or can be hooked up to a computer to store time/material/weight information which can be used to generate reports.
Mechanical scales are almost obsolete. Digital scales cost lesser. Unlike mechanical scales, electronic scales do not have moving parts hence maintenance cost is lesser. Also electronic scales have self calibration feature. Calibration is the act of checking or adjusting (by comparison with a standard) the accuracy of a measuring instrument.
There's a fourth type called Hybrid Scales or Electromechanical scales. These are mechanical scales converted into electronic by inserting a load cell in the weight transfer lever. These scales have an electronic scale's advance features but also have the disadvantages of a mechanical scale- moving parts & maintenance issues.
Commonly used units of weight are-
Grams (g) and Ounces (oz) - 10g of an expensive spice or 500g of wool
Kilograms (kg) and Pounds (lb) - 5kg rice or 20lb coal for your barbecue or your own body weight
Quintal - a small car weighs around 8 quintals or a large horse could weigh around 4 quintals
Ton (T) - a sport utility vehicle weighs about 1.5T or Indian elephants weigh between 4T and 7T
Weigh scales are built for a certain range of weight called the rated capacity. An object weighing 80kg cannot be weighed on a 50kg scale. So how can one know the weight before weighing?? Of course scales are over-load protected. Most scales are built to take 150% of the capacity. However accuracy cannot be guaranteed beyond the rated capacity.
Accuracy is defined as "the ability of a measurement to match the actual value of the quantity being measured".
Precision is defined as a. the "ability of a measurement to be consistently reproduced" and b. "the number of significant digits to which a value has been reliably measured".
Click Accuracy and Precision to know more.
.........
- Balances
- Analog scales (mechanical scales)
- Digital scales (electronic scales)
Balances: These are the earliest form of weighing equipment. A known weight is placed on one side and the object to be weighed on the other side. Method is comparison of weights and approximate.
Analog Scales: Scales are advanced compared to balances. The object is placed on the scale and the pointer shows the weight on the graduated dial. Weight measurement is precise compared to balances.
Digital scales: Precision is what these scales are known for. Electronic scales have extra features such as display weight in multiple units (kilograms and pound), cost calculation, counting number of pieces, or can be hooked up to a computer to store time/material/weight information which can be used to generate reports.
Mechanical scales are almost obsolete. Digital scales cost lesser. Unlike mechanical scales, electronic scales do not have moving parts hence maintenance cost is lesser. Also electronic scales have self calibration feature. Calibration is the act of checking or adjusting (by comparison with a standard) the accuracy of a measuring instrument.
There's a fourth type called Hybrid Scales or Electromechanical scales. These are mechanical scales converted into electronic by inserting a load cell in the weight transfer lever. These scales have an electronic scale's advance features but also have the disadvantages of a mechanical scale- moving parts & maintenance issues.
Commonly used units of weight are-
Grams (g) and Ounces (oz) - 10g of an expensive spice or 500g of wool
Kilograms (kg) and Pounds (lb) - 5kg rice or 20lb coal for your barbecue or your own body weight
Quintal - a small car weighs around 8 quintals or a large horse could weigh around 4 quintals
Ton (T) - a sport utility vehicle weighs about 1.5T or Indian elephants weigh between 4T and 7T
Weigh scales are built for a certain range of weight called the rated capacity. An object weighing 80kg cannot be weighed on a 50kg scale. So how can one know the weight before weighing?? Of course scales are over-load protected. Most scales are built to take 150% of the capacity. However accuracy cannot be guaranteed beyond the rated capacity.
Accuracy is defined as "the ability of a measurement to match the actual value of the quantity being measured".
Precision is defined as a. the "ability of a measurement to be consistently reproduced" and b. "the number of significant digits to which a value has been reliably measured".
Click Accuracy and Precision to know more.
.........
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