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Build a Machine Learning Application with a Raspberry Pi

Developers looking to evaluate machine learning methods find a growing array of specialized hardware and development platforms that are often tuned to specific classes of machine learning architecture and application. Although these specialized platforms are essential for many machine learning applications, few developers new to machine learning are ready to make informed decisions about selecting the ideal platform.

Developers need a more accessible platform for gaining experience in development of machine learning applications and a deeper understanding of resource requirements and resulting capabilities.

As described in the Digi-Key article “Get Started with Machine Learning Using Readily Available Hardware and Software”, development of any model for supervised machine learning comprises three key steps:

Preparation of data for training a model
Model implementation
Model training
Data preparation combines familiar data acquisition methods with an additional step required to label specific instances of data for use in the training process. For the final two steps, machine learning model specialists, until recently, needed to use relatively low-level math libraries to implement the detailed calculations involved in model algorithms. The availability of machine learning frameworks has dramatically eased the complexity of model implementation and training.

Today, any developer familiar with Python or other supported languages can use these frameworks to rapidly develop machine learning models able to run on a wide array of platforms. This article will describe the machine learning stack and training process before getting into how to develop a machine learning application on a Raspberry Pi 3.

Machine learning stack
To support model development, machine learning frameworks provide a full stack of resources (Figure 1). At the top of a typical stack, training and inference libraries provide services for defining, training, and running models. These models in turn build on optimized implementations of kernel functions such as convolutions and activation functions such as ReLU, as well as matrix multiplication and others. Those optimized math functions work with lower level drivers that provide an abstraction layer to interface with a general purpose CPU, or take full advantage of specialized hardware such as a graphics processing unit (GPU) when available.

Diagram of typical machine learning stack

Figure 1: In the typical machine learning stack, higher level libraries provide functions to implement neural networks and other machine learning algorithms, drawing on specialized math libraries that implement kernel functions optimized for CPUs and GPUs in the underlying hardware layer. (Image source: Google)

With the availability of machine learning frameworks such as TensorFlow that provide these stacks, the development process for implementing machine learning in an application has become largely the same regardless of hardware targets. The ability to leverage TensorFlow across different hardware platforms allows developers to begin exploring model development on relatively modest hardware platforms, and then leverage that experience in developing machine learning applications on more robust hardware.

It’s expected that specialized high performance artificial intelligence (AI) chips will eventually provide developers the ability to implement sophisticated machine learning algorithms. Until then, developers can begin to evaluate machine learning and create real machine learning applications using general purpose platforms, including the Raspberry Pi Foundation’s Raspberry Pi 3, or any of the readily available development boards based on general purpose processors such as Arm® Cortex®-A series MCUs or Arm Cortex-M series MCUs.

The Raspberry Pi 3 offers some immediate advantages as a development platform for machine learning applications. Its Arm Cortex-A53 quad core processor provides significant performance capabilities, and the core’s NEON single instruction, multiple data (SIMD) extensions are capable of performing a certain level of multimedia and machine learning type processing. However, developers can easily extend the base Raspberry Pi 3 hardware platform with any number of available compatible hardware add-ons.

For example, to create the kind of machine learning image recognition system described below, developers can add a camera such as the 8 megapixel Raspberry Pi Camera Module v2 or its low-light Pi NoIR camera (Figure 2).

Image of Raspberry Pi 3

Figure 2: Low-cost boards such as the Raspberry Pi 3 provide a useful platform for machine learning development, supporting add-ons such as camera modules for developing image classification applications. (Image source: Raspberry Pi Foundation)

On the software side, the Raspberry Pi community has created an equally rich ecosystem in which developers can find distributions including complete pre-compiled binary wheel files for installing TensorFlow on a Raspberry Pi. TensorFlow provides these wheel files for Python 3.4 and 3.5 on the Raspberry Pi wheels repository piwheels.org. Alternatively, because Docker now officially supports the Arm architecture, developers can use suitable containers from dockerhub.com.

Implementing a machine learning model
Using this combination of software, a Raspberry Pi 3, and a camera module, developers can build a simple machine learning gesture recognition application using sample code from Arm. This application is designed simply to detect when the user has made a specific gesture: in this case, throwing hands up in the air in a sort of celebratory gesture.

To start, use a Python script (record.py) to record short video clips of someone performing the same gesture a few times. Because this application is meant to be as simple as possible, the next step begins training using the Keras machine learning application programming interface (API) embedded in TensorFlow. In this example, the training process is defined in another Python script (train.py) that includes the Keras model definition and training sequence (Listing 1).