Design and fabrication of artificial lateral line flow sensors

Underwater flow sensing is important for many robotics and military applications, including underwater robots and vessels. We report the development of micromachined, distributed flow sensors based on a biological inspiration, the fish lateral line sensors. Design and fabrication processes for realizing individual lateral line sensor nodes are discussed in this paper, along with preliminary characterization results. 1. Underwater flow sensing The fish uses lateral line sensors to monitor surrounding flow fields for maneuvering and survival under water [1]. A lateral line system, shown in figure 1, consists of an array of distributed sensor nodes (the so-called neuromasts) that span the length of the fish body. Each sensor node, in turn, consists of a cluster of hair cells embedded in protective, gellike domes. An individual biological hair cell, a fundamental mechanoreceptor, consists of a vertical cilium attached to a neuron. If the cilium of the hair cell is bent by the local fluid flow, the neuron attached to the cilium stretches and produces action potentials. In certain fish species, the lateral line sensor nodes may be directly exposed at the surface of the skin. In others, the sensor nodes may be hidden in subdermal channels in order to minimize wear and damage. Underwater vehicles and robots require reliable flow sensing methods. Engineered flow sensors have been developed in the past based on a number of sensing principles, including thermal (hot-wire) anemometry [2], Doppler frequency shift and indirect inference from pressure differences [3, 4]. Hot-wire anemometers use fine heating elements that double as temperature sensors. The local flow rate is inferred from the extent of forced convective heat transfer from the hot wire. The majority of existing hotwire anemometers are conventionally made using macroscopic 3 Present address: 313 Micro and Nanotechnology Laboratory, 208 North Wright Street, Urbana, IL 61801, USA. machining methods. Micromachined hot-wire anemometers have also been developed in recent years [5–8]. Sensors based on Doppler frequency shifts consist of an acoustic launcher and a receiver. The overall size of the device is generally large. Existing flow sensors based on pressure distribution measurements also have large sizes and are generally not suitable for forming distributed arrays. In the past two decades, several research groups have developed micromachined flow sensors that are based on a variety of sensing principles including the three principles mentioned above. Microfabrication offers the benefits of high spatial resolution, fast time response, integrated signal processing and potentially low costs. Microsensors [9] based on various principles (including thermal transfer [10–12], torque transfer [13–16] and pressure distribution [17–19]) have been demonstrated. In addition to flow sensors, boundarylayer shear stress sensors have been realized using floating element methods [20] and thermal transfer principles [21]. However, shear stress sensors are located directly on the fluidsolid boundary and do not provide adequate information about mean stream flow velocity. This work offers an alternative method for underwater flow sensing. The sensors can have small footprint and high integration density, if necessary. An array of sensors can be integrated monolithically. The sensor does not involve thin fragile wires as in hot-wire anemometry. A proposed systemlevel implementation of artificial lateral line sensors is shown in figure 2. In this paper, we focus on the development of individual lateral line sensor nodes. The principle of sensing, 0960-1317/02/050655+07$30.00 © 2002 IOP Publishing Ltd Printed in the UK 655