Weight monitoring with wireless and battery free sensors requires both the designer and the user to understand hardware and software components correctly.
Using wireless and battery free load sensors for weight monitoring solutions is interesting for different solutions – from warehouse inventory to replenishment operations.
The fact that an RFID reader gets information of the unique ID of the tag PLUS weight information of the associated container automatically and without human intervention saves time and increases data reliability.
On top of that, the fact that these solutions can be designed for battery free operation completely cut maintenance costs.
Using battery free weight monitoring tags
RFID weight monitoring tags require two different parts:
- RFID chip that can supply power to the sensor part
- Sensor part that can take weight measurements
For a container weight monitoring solution, the most typical sensor approach is a load cell.
Due to mechanical reasons, you need 3 to 4 load cells per container to guarantee that all weight of the container is actually measured by the load cells.
Electronic circuitry for load cells
Load cells, as analog sensors, require an ADC so that the RFID chip can then send the information wirelessly to the reader.
They will also require extra electronic circuits to match the output of the load cells to the desired weight range.
In many cases, if you want to develop a generic device to be able to work with multiple load cells, you will have to design an amplification circuit that actually allows you to modify components based on the specifications of the load cells to use.
Note that each load cell is typically used in a Wheatstone bridge configuration.
Check an example of a generic RFID tag designed for development of battery free weight monitoring sensor tags:
Gain and calibration of the load cell tags
Different load cells have different specifications, so a load cell with an input resistance of 350 Ohm – quite typical – can have a range from 0 to 3kg while a load cell with the exact same input resistance can have a range from 0 to 100kg.
This means the designer needs to adjust the amplification circuit to the specific load cell being used. This ‘adjustment’ is done in two ways:
- Gain adjustment. For a given amplification circuit, you will have to change the value of just one resistance so that the gain changes. A change in the gain will allow you to increase or reduce the range of your conditioning circuit to match the characteristics of the specific load cell you are using.
The VMeter for example provides the following algorithm to calculate the gain:
- Calibration of the load cells. Once the gain is modified, calibration is needed. The conversion from ADC values to weight values is actually done by using two constants. These two constants are dependent on the gain, so a new calibration is required every time you decide to change the gain of the device.
Understanding the Wheatstone bridge
The key concept to successful battery free RFID weight monitoring sensor tags is the Wheatstone bridge and the associated circuitry.
A good RFID chip will provide you the power supply for the bridge so you only have to focus on developing the correct circuitry to monitor the variations in the bridge.
The real challenge is to design for low power consumption and maintain good accuracy values of course.
The VMeter developed by Farsens was thought for this use amongst others. You can download the BOM, schematic and design files from our website for free but the real key is to understand how to modify the conditioning circuit to match the requirements of your selected load cells.
If you’re interested, share your ideas with us at firstname.lastname@example.org.