DMS load cell with measuring amplifier HX711AD

fig. 1: DMS load cell with measuring amplifier HX711AD

Autor:	Onyesi John Abiagam
Sprache:	DE EN

Requirements

Table 1: Requirements for the Unit

Req.	Description	Priority
1	The mass must be determined using the HX711AD load cell via Arduino and Simulink.	1
2	The measuring range must be determined.	1
3	The measurement uncertainty (1σ) must be determined for the measuring range and displayed as a confidence interval.	2
4	The on/off button starts the system. The sensor must first be calibrated and display 0 g.	1
5	The weight must be referenced for the measuring range.	1
6	The measured values ​​must be filtered/smoothed over time.	1
7	A beep must indicate when the measured value is stable/constant.	1
8	The sensor system must display the weight in g on a display.	1
9	The tara button resets the current weight to 0 g (recalibration).	1

Working principle

The working principle describes how the DMS load cell and the HX711AD convert an applied force into a digital signal.

DMS Load Cell

Introduction

A DMS load cell is a force transducer that converts mechanical force into an electrical signal. It consists of a deformable metal body with strain gauges attached to its surface.

Figure 1: Structure of the weight scale

Figure 2: Structure and deformation of a load cell. Source: https://learn.sparkfun.com/tutorials/getting-started-with-load-cells

Figure 1 shows the structure of a load cell and its deformation under load. The strain gauges detect small deformations and convert them into changes in electrical resistance.

Working Principle

The operation of the load cell follows these steps:

• A force is applied to the load cell
• The metal body undergoes elastic deformation
• The strain gauges are:
  • stretched: resistance increases
  • compressed: resistance decreases
• The strain gauges are arranged in a Wheatstone bridge

Figure 3: Wheatstone bridge configuration. Source: https://learn.sparkfun.com/tutorials/getting-started-with-load-cells

Figure 4: Loadcell Wheatstone bridge configuration. Source: https://learn.sparkfun.com/tutorials/getting-started-with-load-cells

Figure 2 shows the Wheatstone bridge configuration used in the load cell.

• In the unloaded state:
  • the bridge is balanced
  • output voltage is zero
• When a load is applied:
  • the bridge becomes unbalanced
  • a small differential voltage is generated

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HX711AD Measuring Amplifier

Introduction

The HX711AD is a measuring amplifier designed to process the small output signal from a load cell. It combines signal amplification and analog-to-digital conversion in a single device.

Figure 5: HX711 module and pin configuration. Source: https://justdoelectronics.com/load-cell-hx711-module/

Figure 5 shows the HX711 module and its pin configuration.

Working Principle

The HX711 processes the signal from the load cell as follows:

Figure 6: Internal structure of HX711

Figure 6 shows the internal structure of the HX711.

• The load cell provides a differential voltage signal (A+ and A−)
• The signal is very small (millivolt range)
• The internal amplifier (PGA) amplifies the signal
• The amplified signal is converted using a 24-bit sigma-delta ADC
• The output is a digital value proportional to the applied force
• The data is transmitted to the microcontroller via:
  • DOUT (data)
  • SCK (clock)

Related Links

• HX711 based weight scale

SVN-Repository

https://svn.hshl.de/svn/HSHL_Projekte/trunk/Arduino_Sensorsammlung

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