Case Study: Choosing a Sensor That Gets the Job Done
A device engineer explains a few key factors to consider when choosing the ideal sensor for a medical device.
Choosing the right sensor is critical to effective medical device design.
Let's consider a situation where a sensor is required to measure the drop in pressure created by a patient's inhalation through an inhaler. This application calls for certain criteria to be met. For instance, the measurement must be made and displayed in near real time. It must also have an accuracy reading of +/- 10% or better.
I made the following assumptions:
- The sensor will be in a handheld wireless device;
- The sensor will be surface mount technology;
- The device will use standard 3.3 volt CMOS logic;
- The sensor needs to cost less than $30 dollars in 10-unit quantities.
Given the above assumptions, which dramatically narrow the field, we can now evaluate the sensor for the application at hand. To choose an effective sensor we evaluated several different sensors in three different configurations. We evaluated the sensors for accuracy, resolution, and bandwidth. These are the steps that led to the final specification.
An analysis of the specific application provides us with a list of performance requirements.
Important parameters to evaluate are:
- Sensor type--digital or analog
- Measurement bandwidth requirements
- Sensor accuracy specification
- Range of measurement
- Resolution requirements
- The sensing technology itself
- Physical constraints such as size and material of construction
This is not an exhaustive list of things to consider but it is the minimum set of criteria that should be examined. Note that there are strong interactions between these parameters. For example, high bandwidth sensors can have low resolution. Sensors need to be chosen to minimize negative interactions.
Choosing the Sensor Output Mode
Integrated Circuit sensors come in two main varieties--analog and digital. There are pros and cons to each type. For example, analog sensors can be less expensive and may be simpler to implement. Digital sensors, on the other hand, are less prone to noise problems but can have complex interfaces.
Determining Bandwidth Requirements
The importance of determining the necessary bandwidth of a sensor cannot be overstressed. Some sensor applications push the upper limits of sensor bandwidth and others require bandwidth limiting. Techniques from sampling theory can be used to determine system bandwidth.
Determining Accuracy Requirements
There is also a tradeoff between bandwidth and accuracy with respect to temperature and pressure sensors. High accuracy often requires long processing times, which is a factor in determining bandwidth. The necessary accuracy of the measurement should be considered.
Range of Measurement
The range of measurement will affect both the bandwidth and the accuracy. In some cases, high dynamic range measurements come with high bandwidth and accuracy requirements. The device design in my case study uses a pressure sensing device that can measure 0.01 kPa with a dynamic range of 6kPa with off-the-shelf sensing technology.
It's important to think about how to determine the optimum resolution for a particular application. Resolution can vary across the sensor range. The choice of sensing technology will have a bearing on the resolution as well.
Sensor Technology Selection
There are many different technologies to choose some. These include piezoresistive sensing elements versus mass flow rate detectors; platinum resistance temperature detectors versus thermistors, and more. The application in this case requires both temperature and pressure measurements as well as atmospheric pressure and temperature mesurements, all with different resolution, accuracy, and bandwidth requirements.
The details of a medical device's design will inform the specific physical constraints that impact sensor selection. The inhalation pressure monitor described at the beginning of this piece is a handheld wireless device. It must be mated with the actual inhaler, making the inhaler disposable and the device reusable. An especially important constraint was that the sensor could not enter the powder flow path.
After evaluating numerous parameters, we chose to use digital sensors with serial interfaces for our medical device. This led to a device that works demonstrably better for a lower cost.