4.2Sensors and actuators

When an embedded designer says "drive a motor," what counts as the optimal actuator differs greatly depending on whether positioning accuracy is required, whether simple speed control of continuous rotation suffices, and whether feedback (detecting the current position) is needed. Sensors likewise require choosing the right method based on the physical quantity being measured, the required response speed, and power constraints. This section builds the judgment needed to decide which to choose and how to drive it, given the characteristics of representative sensors and actuators.

4.2.1Sensor types and selection

• Temperature sensors (thermistors, thermocouples, IC temperature sensors, etc.) differ from one another in response speed, measurement range, and output form (analog voltage versus digital). Accelerometers (predominantly MEMS-based) are used to detect vibration, tilt, and shock, and can also derive a tilt angle from the gravitational acceleration measured while stationary. Light sensors (photodiodes, CdS cells, etc.) detect illuminance and are also applied as proximity sensors for object detection.
• What matters in sensor selection is not the oversimplified idea that "higher accuracy is always better," but judging the tradeoffs among response speed, power consumption, and cost against the actual requirements. For example, in a battery-powered device that operates intermittently, a low-power sensor that can be woken only when needed is often more suitable than a high-accuracy sensor that requires continuous power.