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Developing a liquid level control/delivery system

Posted: 16 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:pressure sensors? data acquisition? delta-sigma ADC?

Although pressure sensors are sensitive to temperature, many contemporary industrial processes as well as commercial and medical applications do not need an extended temperature range. Moreover, some of these applications operate in an air-conditioned environment where the temperature range is quite restricted. Temperature-compensated silicon pressure sensors are well suited for these applications, as they maintain their ratings better as the ambient temperature changes.

This article describes a cost-effective, low-power, liquid-level control and delivery system based on data acquisition systems (DASs) that use a compensated silicon pressure sensor and a high-precision delta-sigma ADC. This reference design is useful for a wide variety of precision sensing and portable applications that must measure and distribute industrial liquids using a noncontact measurement approach. The system uses a compensated silicon pressure sensor to measure and distribute most of the industrial liquids.

The article offers ideas for resolving high-current solenoid valve and pump controls without jeopardizing the high-accuracy delta-sigma ADC-based DAS. It also presents system algorithms, analyses noise, and offers calibration ideas for improving system performance while reducing complexity and cost. The design described here makes use of the MPX2010 series silicon piezoresistive pressure sensor from Freescale Semiconductor, theMAX11206 ADC, and the MAXQ622 microcontroller.

System design
A simplified block diagram of the development system for this reference design is shown in figure 1. This system features a controlled-liquid reservoir consisting of a vertical plastic water-filled tube equipped with 100mL measurement marks on the side. The thin inner measurement tube is located inside the controlled reservoir and connects directly to the sensor's positive pressure port, while the reference pressure port is exposed to the atmospheric pressure.

A small DAS PCB directly attached to the pressure sensor provides dynamic control of liquid level measurements. It generates control signals from the PC-based control-and-dispense GUI to activate the valve-driver PCB and pump-driver PCB, and then to deliver a prescribed volume of the liquid to the controlled vessel. The DAS also provides a control signal to the water pump.

The external main liquid reservoir provides large storage capacity for the liquid needed to replenish the controlled-liquid reservoir. It ensures a steady pressure. The water pump turns on whenever the level at the controlled liquid reservoir drops below a defined mark. This action maintains a constant liquid height in the controlled reservoir.

In this reference design pressure applied to the sensor's positive pressure port is transferred by air trapped in the measurement tube, thereby providing a barrier between the liquid in the reservoir and the sensor. This design makes it possible to use a cost-effective generic pressure sensor in industrial applications with chemically aggressive or corrosive liquids.

Basic system operation
This system (figure 1) measures volume by measuring the height of the liquid, which is determined by the pressure inside the sealed tube with liquid pushing the air inside it. The pressure will be directly proportional to the height of liquid in the large container. The air is trapped inside the inner tube, thus causing the pressure to build there. The higher the liquid rises, the more the pressure builds.

The system produces a very good reading of the height of liquid present in the large vessel. With a fixed-diameter outer vessel, the total volume can be calculated by using a simple equation: radius radius H.

Figure 1: A simplified block diagram of the development system for this reference design.

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