Conditioning of hot-wire sensors · MEMS sensors for flow measurement in harsh environments

/2 Development of a batch fabrication process for 3C-SiC wall shear stress sensors/2.4 Conditioning of hot-wire sensors

#2.4 Conditioning of hot-wire sensors

#2.4.1 Development of a CTA suited for SiC hot wires

Due to larger resistance values, SiC wires have to undergo large voltage drops be sufficiently heated,For SiC calorimeters presented above, a … mW heating power leads to 35 V drops.

meaning that each active component should properly function at these voltages. This forbids the use of commercial CTA circuits, which are designed to work with ~ 1 – 20 \Omega resistances.Typically 4 \Omega for hot wire, 15 \Omega for hot-film probes.

#CTA loop

A Perry-type CTA control circuit has been designed to drive high-value SiC resistances, with components able to work up to a 35 V working point, as shown in figure 2.7 (a). This enables the bridge to function at a high enough voltage to properly power the SiC heater. The bridge voltage is mitigated by a 2N2222A BJT driven by the controller output. The main differential amplifier is an ADA4522 which output is amplified by a variable gain INA128 instrumentation amplifier. An offset voltage can be applied independently of the main supply though a second INA128, to conduct square and sine wave tests.

#Lateral wire measurement

For calorimeter probes, a complementary circuit to use the lateral wires has also been designed, as shown in figure 2.7 (b). A constant current source REF200 drives 100 µA through the lateral wires mounted in series. The wire voltages are measured by two INA128 instrumentation amplifiers.

(a) CTA control loop.

(b) Lateral wire measurement circuit.

Figure 2.7.
Conditioning circuit for SiC calorimeters.

#Circuit board realization

PCB

#2.4.2 Frequency response assessment

Determining the true bandwidth of the hot wire in a closed-loop configuration is quite complex: the ideal way is to perform a measure of a direct fluid step, i.e. a sudden velocity change. Because it is very difficult to achieve in practice, a workaround has been found to mimic a flow step as closely as possible. Under steady flow, a voltage step (i.e. a square wave) is sent into the feedback loop to create a sudden unbalance. It has been by shown by Freymuth (1977)¹⁴ that it is equivalent to a flow velocity step, under the assumption that the wire-controller system behaves like a third order.

#Static square-wave test

#Resonance issue

Summary