handbook of modern sensors

8.5 Thermal Accelerometers 311
forced convection. As described in Chapter 3, heat can be transferred by conduction,
convection, and radiation. Convection can be natural (caused by gravity) or forced
(by applying an artificial external force, like that produced by a blower). In a HGA,
such force is produced by acceleration. The sensor measures the internal changes in
heat transfer of the trapped gas. The sensor is functionally equivalent to traditional
inertial mass accelerometers. The inertial mass in the sensor is gas that is thermally
nonhomogeneous. The gaseous inertial mass provides some advantages over the use
of the traditional solid inertial mass. The most important advantage is a shock survival
up to 50,000g, leading to significantly lower failure rates.
The sensor contains a micromachined plate adjacent to a sealed cavity filled
with gas. The plate has an etched cavity (trench). A single heat source, centered
in the silicon chip, is suspended across the trench (Fig. 8.8). Equally spaced are four
temperature sensors that are aluminum/polysilicon thermopiles (TP) (i.e., serially
connected thermocouples). The TPs are located equidistant on all four sides of the
heat source (dual axis). Note that a TP measures only a temperature gradient, so that
the left and right thermopiles in fact is a single TP, where the left portion is the location
of “cold” junctions and the right portion is that of “hot” junctions (see Section 16.2
of Chapter 16 for the operating principle of a thermocouple). A thermopile instead
of a thermocouple is used for a sole purpose: to increase the electrical output signal.
Another pair of junctions is used for measuring a thermal gradient along the y axis.
Under zero acceleration, a temperature distribution across the gas cavity is symmetrical
about the heat source, so that the temperature is the same at all four TP
junctions, causing each pair to output zero voltage. The heater is warmed to a temperature
that is well above ambient and typically is near 200 ◦ C. Figure 8.8A shows two
(A)
(B)
Fig. 8.8. (A) Cross-sectional view of the HGAsensor along the x axis. Heated gas is symmetrical
around the heater. (B)Acceleration causes heated-gas shift to the right, resulting in a temperature
gradient.