Design of a MEMS Based Mass Flow Meter

By AZoSensors.com Staff Writers

Topics Covered

Introduction
MEMS for Thermal Mass Flow Sensors
Measurement Cycle and Flexible Sensor Signals Conditioning
Flow Sensor or Mass Flow Meter?
Conclusion
About iC-Haus

Introduction

Micromachining enables a wide range of different Micro-Electro-Mechanical Systems (MEMS) for a range of sensor types, such as pressure, gyroscope, accelerometer and thermal flow sensors. In flow metering thermal MEMS are used for the measurement of volume, velocity or mass. With this technique, heat transfer in a gas or liquid is measured.

A specific amount of temperature and energy and sensors is supplied by a heating element and the temperature difference is measured at different positions in the pipe. Application system specific integration (ASSP) and thermal MEMS sensors and are well suited for small mass flow sensors and meters such as in industrial and medical applications.

MEMS for Thermal Mass Flow Sensors

The King’s Law for the fundamental calculation of flow velocity helps determine the average flow velocity and density of a gas or liquid using a defined amount of energy applied by a wire, fixed upright mounted in a measurement pipe, over a defined time.

Since the measuring pipe diameter is known, the flow pattern of the medium has to be evaluated once. The pressure needs to be determined in compressed gases. The volume (m3/s) or mass flow rate (kg/s) is calculated from the density.

The measurement cycle may either be a constant power anemometer known as a CPA or a constant temperature anemometer (CTA).

Figure 1 shows a MEMS flow sensor with one heating element including the temperature sensors, as well as one temperature sensor upstream and one downstream.

Figure 1. Thermal MEMS mass flow sensor with and without flow condition

The upper portion of Figure 1 shows the MEMS mass flow sensor in a “no-flow” condition with both temperature sensors measuring the same temperature. In the lower part of Figure 1 the “flow” condition is illustrated in which temperature sensor 2 will measure a higher temperature proportional to the flow of the liquid or gas.

Measurement Cycle and Flexible Sensor Signals Conditioning

Thermal MEMS sensors require more than a normal sensor signal conditioner (SSC), such as in a pressure sensor. A well-regulated heater and controller for the measurement cycle management are required in addition to the SSC. One must develop dedicated universal sensor signal conditioning devices to handle this complex task.

Figure 2 shows a block diagram with the functional blocks for a regulated heating circuit, three constant current temperature sensor supplies and a signal conditioning path. The look-up-table provides linearization. The analog output provides the linearized measurement results as a differential signal of typically +4 V to -4 V.

Figure 2. Controlling and measuring mass flow through a dedicated universal sensor signal conditioning IC

The iC-HO has a slave serial-peripheral Interface and it can be connected to a microcontroller, or being used for an initial calibration of the sensor. Access is provided to all programmable functions by the SPI. Linearization data and parameters can be stored in an external EEPROM through an SPI master interface.

After power-up, the parameter and linearization data is read and validated by a CRC check. The chip temperature is given with a resolution of 1°C by an on-chip temperature sensor.

The signal path includes a Programmable Gain Amplifier (PGA), 11 bit Analog to Digital Converter (ADC), a look-up table (LUT) for linearization and an 11 bit Digital to Analog Converter (DAC) with a differential output driver for the analog flow value.

The heater has a PI-characteristic and determines the heater temperature with a 14 bit resolution. The path’s measurement frequency is 10 kHz. This makes it easy to also record states that change very fast in MEMS sensors with their small dimension and thermal masses.

The two key modes of operation in mass flow metering CPA and CTA are selectable. The signal conditioning path is adapted and suitable to support pressure, gas and mass flow sensors.

Flow Sensor or Mass Flow Meter?

The minimum flow sensor system with an analog output signal includes just three devices as shown in Figure 3:

  • the thermal flow sensor
  • the iC-HO and
  • the EEPROM

The analog output value of -/+4 V is proportional to the measured flow. A +4 V is output on the maximum flow in one direction and -4 V in the opposite direction and 0 V without flow. For mass flow metering applications an optional microcontroller with or without display can be used to do the required calculation for metering and recording

This is also the case in case a digital communication such as IO-Link for parameterization by a HOST controller is needed. The IO-Link slave transceiver also contains the DC/DC-converter to supply +5 V or 3.3 V for the total mass flow meter through the 24 V supply line.

The startup time with an EEPROM is 1.6 ms for the iC-HO. Standard MEMS sensors show thermal time constants in a range of up to 10 ms, which defines the total time of a measurement cycle.

Figure 3. Thermal MEMS Mass Flow Sensor/Meter with analog or digital I/O

The standard delay time for a measurement is 100 µs. In case of an error in the MEMS-sensor the iC-HO will send an error signal to the microcontroller, which then can analyze it and pass on details to the central Host computer/controller.

Conclusion

System integration enables the design of a complete thermal MEMS controller and signal condition for micro mass flow sensors and meters in a single integrated circuit. Various MEMS flow sensors can evaluate bi-directionally flow using the precise CTA method of measurement.

Due to the high level integration of the iC-HO cost-efficient flow meters can be implemented for remote industrial applications with just 4 integrated circuits. Built-in small flow sensors such as for medical applications require just two ICs.

Gas flow metering with pressure measurement in addition to the purpose of volume and mass are supported as well. An evaluation board with an USB- Port and a free GUI software for a Lab-PC supports a fast and simple evaluation.

About iC-Haus

iC-Haus GmbH is a leading independent German manufacturer of standard iCs (ASSP) and customized ASiC semiconductor solutions.

The company has been active in the design, production and sales of application-specific iCs for industrial, automotive and medical technology since 1984 and is represented worldwide. The iC- Haus cell libraries in CMOS, bipolar, and BCD technologies are fully equipped to realize the design of sensor, laser/opto, and actuator ASiCs.

The iCs are assembled either in standard plastic packages or using chip-on-board technology to manufacture complete microsystems, multichip modules and optoBGA™, the latter in conjunction with sensors.

This information has been sourced, reviewed and adapted from materials provided by iC-Haus.

For more information on this source, please visit iC- Haus.

Date Added: Aug 17, 2013 | Updated: Aug 30, 2013
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