Displaying Real-time Sensor Data in the Browser with SignalR and ChartJS

In my previous posts on Modding My Rowing Machine, I wired up an Arduino to my rowing machine, and streamed the speed sensor data to an ASP.NET core application.

In this post, I’m going to show you how to take sensor and counter data, push it to a browser as it arrives, and display it in a real-time chart.

If you want to skip ahead, I’ve uploaded all the code for the Arduino and ASP.NET components to a github repo at https://github.com/alistairjevans/rower-mod.

I’m using Visual Studio 2019 with the ASP.NET Core 3.0 Preview for all the server-side components, but the latest stable release of ASP.NET Core will work just fine, I’m not using any of the new features.

Pushing Data to the Browser

So, you will probably have heard of SignalR, the ASP.NET technology that can be used to push data to the browser from the server, and generally establish a closer relationship between the two.

I’m going to use it send data to the browser whenever new sensor data arrives, and also to let the browser request that the count be reset.

The overall component layout looks like this:

Setting up SignalR

This bit is pretty easy; first up, head over to the Startup.cs file in your ASP.NET app project, and in the ConfigureServices method, add SignalR:

Next, create a SignalR Hub. This is effectively the endpoint your clients will connect to, and will contain any methods a client needs to invoke on the server.

SignalR Hubs are just classes that derive from the Hub class. I’ve got just the one method in mine at the moment, for resetting my counter.

Before that Hub will work, you need to register it in your Startup class’ Configure method:

You’re also going to want to add the necessary SignalR javascript to your project. I did it using the “Manage Client-Side Libraries” feature in Visual Studio; you can find my entire libman.json file (which defines which libraries I’m using) on my github repo

Sending Data to the Client

In the MVC Controller where the data arrives from the Arduino, I’m going to push the sensor data to all clients connected to the hub.

The way you access the clients of a hub from outside the hub (i.e. an MVC Controller) is by resolving an IHubContext<THubType>, and then accessing the Clients property.

Pro tip:
Got multiple IO operations to do in a single request, that don’t depend on each other? Don’t just await one, then await the other; use Task.WhenAll, and the operations will run in parallel.

In my example above I’m writing to a file and to SignalR clients at the same time, and only continuing when both are done.

Browser

Ok, so we’ve got the set-up to push data to the browser, but no HTML just yet. I don’t actually need any MVC Controller functionality, so I’m just going to create a Razor Page, which still gives me a Razor template, but without having to write the controller behind it.

If I put an ‘Index.cshtml’ file under a new ‘Pages’ folder in my project, and put the following content in it, that becomes the landing page of my app:

In my site.js file, I’m just going to open a connection to the SignalR hub and attach a callback for data being given to me:

That’s actually all we need to get data flowing down to the browser, and displaying the current speed and counter values!

I want something a little more visual though….

Displaying the Chart

I’m going to use the ChartJS library to render a chart, plus a handy plugin for ChartJS that helps with streaming live data and rendering it, the chartjs-plugin-streaming plugin.

First off, add the two libraries to your project (and your HTML file), plus MomentJS, which ChartJS requires to function.

Next, let’s set up our chart, by defining it’s configuration and attaching it to the 2d context of the canvas object:

Finally, let’s make our chart display new sensor data as it arrives:

With all that together, let’s see what we get!

Awesome, a real-time graph of my rowing!

As an aside, I used the excellent tool by @sarah_edo to generate a CSS grid really quickly, so thanks for that! You can find it at https://cssgrid-generator.netlify.com/

You can check out the entire solution, including all the code for the Arduino and the ASP.NET app, on the github repo at https://github.com/alistairjevans/rower-mod.

Next up for the rowing machine project, I want to put some form of gamification, achievements or progress tracking into the app, but not sure exactly how it will look yet.

Streaming real-time sensor data to an ASP.NET Core app from an Arduino

In my previous posts on Modding my Rowing Machine, I got started with an Arduino, and started collecting speed sensor data. The goal of this post is to connect to the WiFi network and upload sensor data to a server application I’ve got running on my laptop in as close to real-time as I can make it.

Connecting to WiFi

My Arduino Uno WiFi Rev 2 board has got a built-in WiFi module; it was considerably easier than I expected to get everything connected.

I first needed to install the necessary library to support the board, the WiFiNINA library:

Then you can just include the necessary header file and connect to the network:

To be honest, that code probably isn’t going to cut it, because WiFi networks don’t work that nicely. You need a retry mechanism with timeouts to keep trying to connect. Let’s take a look at the full example:

The Server

To receive the data from the Arduino, I created a light-weight ASP.NET Core 3.0 web application with a single controller endpoint to handle incoming data, taking a timestamp and the speed:

Then, in my Arduino, I put the following code in a method to send data to my application:

I just want to briefly mention one part of the above code, where I’m preparing body data to send.

The Arduino libraries do not support the %f specifier (for a float) in the sprintf method, so I can’t just add the speed as an argument there. Instead, you have to use the dtostrf method to insert a double into the string, specifying the number of decimal points you want.

Also, if you specify %d (int) instead of %lu (unsigned long) for the timestamp, the sprintf method treats the value as a signed int and you get very strange numbers being sent through for the timestamp.

Once that was uploaded, I started getting requests through!

Performance

We now have HTTP requests from the Arduino to our ASP.NET Core app. But I’m not thrilled with the amount of time it takes to execute a single request.

If we take a look at the WireShark trace (I love WireShark), you can see that each request from start to finish is taking in the order of 100ms!

This is loads, and I can’t have my Arduino sitting there for that long.

ASP.NET Core Performance

You can see in the above trace that the web app handling the request is taking 20ms to return the response, which is a lot. I know that ASP.NET Core can do better than that.

Turns out this problem was actually due to the fact I had console logging switched on. Due to the synchronisation that takes place when writing to the console, it can add a lot of time to requests to print all that information-level data.

Once I turned the logging down from Information to Warning in my appsettings.json file, it got way better.

That’s better!

That actually gives us sub-millisecond response times from the server, which is awesome.

TCP Handshake Overhead

Annoyingly, each request is still taking up to 100ms from start of connection to the end. How come?

If you look at those WireShark traces, we spend a lot of time in the TCP handshaking process. Opening a TCP connection does generally come with lots of network overhead, and that call to client.connect(SERVER, SERVERPORT) in my code blocks until the TCP connection is open; I don’t want to sit there waiting for that every time I want to send a sample.

The simple solution to this is to make the connection stay open between samples, so we can just repeatedly sent data on the same connection, only needing to do the handshake once.

Let’s rework our previous sendData code on the Arduino to keep the connection open:

In this version, we ask the server to leave the connection open after the request, and only open the connection if it is closed. I’m also not blocking waiting for a response.

This gives us way better behaviour, and we’re now down to about 40ms total:

There’s one more thing that I don’t love about this though…

TCP Packet Fragmentation

So, what’s left to look at?

TCP segments

I’ve got a packet preceding each of my POST requests, that seems to hold things up by around 40ms. What’s going on here? Let’s look at the content of that packet:

Wireshark data view

What I can tell from this is that rather than wait for my HTTP request data, the Arduino is not buffering for long enough, and is just sending what it has after the first println call containing POST /data/providereading HTTP/1.1. This packet fragmentation slows everything up because the Arduino has to wait for an ACK from the server before it continues.

I just wanted to point out that it looks like the software in the Arduino libraries isn’t responsible for the fragmentation; it looks all the TCP behaviour is handled by the hardware WiFi module, that’s what is splitting my packets.

To stop this packet fragmentation, let’s adjust the sending code to prepare the entire request and send it all at once:

Once uploaded, let’s look at the new WireShark trace:

No TCP Fragmentation

There we go! Sub-millisecond responses from the server, and precisely hitting my desired 50ms window between each sample send.

There’s still ACKs going on obviously, but they aren’t blocking packet issuing, which is the important thing.

Summary

It’s always good to look at the WireShark trace for your requests to see if you’re getting the performance you want, and don’t dismiss the overhead of opening a new TCP connection each time!

Next Steps

Next up in the ‘Modding my Rowing Machine’ series, I’ll be taking this speed data and generating a real-time graph in my browser, that updates continuously! Stay tuned…

Modding my Rowing Machine with an Arduino – Part 2 – Reading the Speed Sensor

If you didn’t see my previous post on setting up an Arduino, then to catch you up, I’ve got a rowing machine, and I am trying to replace the basic LCD display on it with my own; to do this I need to read the sensor data it collects and process it myself.

This is my ‘competition’ right now

Just so everyone is aware, this post could also be titled, “How I struggled with basic electronics” or “How you shouldn’t make assumptions when reverse engineering something”.

When I started this little project, I looked at the existing board that provides the LCD display for the rowing machine, and saw the two connectors. One for speed (which goes to the fan wheel at the front) and one for count (which goes to a sensor under the seat).

The connectors on the back of the PCB.

I made the assumption that because speed is an analog value, then the incoming data should be analog as well. This turned out to not be true, but first I’ll show you the wiring/code that told me it was not true.

Wiring

The connector for the speed sensor is actually a 3.5mm mono female jack:

The jack on the right is the speed sensor, the one in the middle goes to the seat movement sensor.

So, I got a cheap 3.5mm cable, cut the end off it, then wired it to a junction.

The wired mono jack.

I was momentarily confused by the fact there were 3 cables rather than 2 for a mono connector, but it turned out that the yellow wire was wired straight to the red, so I just ignored that one.

Reading an Analog Value

I thought that the speed sensor might be some sort of variable resistor, so that as the speed increased, the voltage allowed through the connection would increase.

So to start with, I connected the 5V line on the Arduino to the red wire on the junction, and the black wire (GND) to the A0 analog input pin on the Arduino:

Wiring to the analog input pin.

Then, I wrote some code to read the analog value (in volts) and write it to the Serial connector:

Before you connect the sensor, you just get analog ‘noise’ because there’s nothing connected to the A0 pin:

Voltage data from a floating analog input

Once I connected the 3.5mm jack to the rowing machine’s speed sensor I got:

So that already tells me that the circuit is complete, and I’m seeing the 5V from the Arduino coming back.

When I pull on the rowing machine, my numbers change, but only very slightly (and not really in a way that relates to the speed):

Not much change in the voltage…

I was definitely expecting more change in values from this; and as soon as the wheel of the rower starts to slow down, the number goes back to 5V.

What’s going on here? It doesn’t so much look like the voltage is changing with speed, it’s more like something is slightly interfering with a stable voltage reading…given the nature of the analog value sampling, it stands to reason that a switch rapidly changing from on to off and back might look like this…

Using a switch to read speed

So how can you use a switch to read speed? Well, I suspect there is a magnetic reed switch inside the wheel of the rowing machine, with a magnet attached to one of the blades of the fan inside the wheel.

Each time the magnet passes the switch, it will cause the circuit to break temporarily. The faster the blades spin, the more often the circuit will break.

To detect this, I need to change my approach a bit, and rather than read analog values, I need to read a digital value.

In order to get this working I had to sort out some basic electronics; I used the guide found here, which gave me the circuit I needed to make it work. It’s not too complicated to wire up, here’s the final diagram:

Circuit Diagram of the switch reader

It looks much less neat than that on the breadboard, but here it is:

The circuit to detect the state of the switch.

Then, I added the following code, to sample the digital input and watch for transitions.

Now, when I plug this into the rowing machine and I give it a good couple of pulls, I get some meaningful speed readings out that correlate with how hard I pull it!

Pulling on the rower gives me increases in speed!

Next post will cover getting the data off the Arduino; connecting up the WiFi on my board and sending an HTTP request with my speed data!

Modding my Rowing Machine with an Arduino – Part 1 – Arduino Basics

I’ve got a rowing machine in my garage that I use pretty regularly, and it displays some statistics on a basic little read-out, including speed, calories burnt, number of ‘strokes’ and so on.

The rowing machine read-out.

My mission is to replace the simple LED read-out with my own board that will capture the sensor data it uses, upload it somewhere and then do some fun stuff with that data.

For background, I have a little past experience in embedded programming, but I haven’t used it in years. I understand the basics of GPIO pins and similar, but I place myself firmly in the ‘beginner’ category with embedded development.

I hadn’t touched an Arduino before today, when I started writing this post, so this is going to cover just getting to grips with Arduino basics, and subsequent updates will go through all the steps needed to mod my rowing machine!

Picking an Arduino Board

My only real criteria for picking an Arduino board is that I can connect the sensors on my rowing machine, and also that I can connect to WiFi to get data off it.

The back of the rowing machine display.

From the connections on my rowing machine, I can tell I need two connections on the board that can read analog sensor values, so I knew I would need two ADCs (Analog to Digital Converters) on my board, one for the speed and one for the counter.

Going through the Arduino website to figure out which board I wanted (turns out there’s a lot of choice), I picked the Arduino Uno Wifi Rev2, which:

  • Has 2 ADCs (for reading the sensors on my rowing machine).
  • A built-in WiFi module for connectivity (so I can upload my data).
  • A pretty attractive price of about £35 (as of writing in May 2019) plus shipping.

Shipping only took a couple of days, and once it arrived I was ready to go. You will also need a USB Type B cable. You can pick one up off Amazon for about £5, but I had an old one lying around.

Setup

After plugging in the Arduino board with the USB connector, I went to the getting started guide for my particular board (the site has different pages for each one).

I believe that a lot of the following content should work for most Arduino boards that don’t have WiFi, but I am not certain.

From there I tried out both the web-based and desktop IDE Arduino provide, but I decided I want to work in a familiar environment. Luckily, it turns out that trusty VS Code can come to my rescue, because it has an extension for working with Arduino, with the added bonus of having decent intellisense.

You will need to download the regular Arduino desktop IDE from here before you can use VS Code with your Arduino; the extension needs to use the tools it supplies.

Go ahead and install the VS Code Arduino extension (by Microsoft), then go and configure the extension if you need to set the installation path for your Arduino installation to the non-default setting.

What is an Arduino Program?

What actually is an Arduino Program anyway, and how is it different from a console application on my desktop?

Basically, all Arduino ‘programs’ run C++ code you write. Each program fundamentally consists of a setup and a loop.

You can sort of think of setup as your ‘main’ method, but you exit it immediately and start looping.

One thing that you need to remember with an embedded program is that your program never stops; it runs until the device doesn’t have power, you reset the board, or you upload a new program. There is no ‘exit’.

In an Arduino program you can do a lot of the things you’d expect from a C++ program, like having additional C++ code files (files with a .cpp extension work just fine). You do have a lot less memory to play with though; the processor on my Arduino only has 6KB of RAM. As someone who tends to work on web applications that consume hundreds of MB, it’s a bit jarring, but 6KB is actually plenty for my needs.

When you want to run your program, you compile it as you would with a normal program; the Arduino components on your desktop then upload the program to the device (over the USB connection) and store it in Flash memory. Again, you can’t have massive applications; my board has 48KB of Flash to fit the program in.

My First Arduino Program

First off, I’m going to make an LED flash. Big stuff, I know. My Arduino (and most of them I think), have a built-in LED that you can turn on and off from your program.

Let’s make a new project in VS Code. Create a new folder somewhere and open it in VS Code.

Then, to start a new ‘project’, run Arduino: Initialize from the command palette. This created a blank app.ino file and let me select my board, which got me started.

I found that I got an intellisense error in the created ino file:

Clicking on the ‘fix-it’ bulb took me to the C++ include path settings:

After a quick search of the Arduino install directory for ‘pgmspace.h’, turned out I was missing an additional include path:

C:\Program Files (x86)\Arduino\hardware\tools\avr\avr\include\**

After I added that to the list with the necessary extra backslashes, closed and then re-opened my app.ino file, no more squiggles, and I get nice intellisense.

Controlling an LED

My built-in LED is on PIN #13 of the processor (I used the Arduino Board reference to check what was available); in your program there is an LED_BUILTIN constant already supplied that references it.


A bit of embedded basics here; everything you do in embedded programming is basically turning pins on and off on your processor.

The big arrow is pointing to a pin

Want to turn on an LED? Set the ‘level’ of the pin connected to the LED to HIGH (which means it has voltage on it). To turn it off, set the level to LOW (which means it has no/negligible voltage on it).


I had to configure the LED pin to be an output pin in my setup method, and then set it HIGH and LOW in my loop. Put some delays in there and you have a flickering LED!

You can upload the program from the command palette (or CTRL+ALT+U).

GIF of an LED blinking (very exciting!)

You might encounter one or more problems getting that program downloaded (like I did):

COM Port

I had problems getting the COM Port (the Arduino connects as a virtual COM Port over USB) to work first time, so my code wouldn’t upload. I had to go into the main Arduino IDE and install the package it recommended for my board. This installed different drivers, which changed the COM Port to Atmel rather than Microsoft.

I also had to go into Windows Device Manager, go into the settings, and change the COM Port to not be the default COM3. Only then would it actually upload.

Select the Right Board!

Pro Tip – make sure you pick the right board in the VSCode footer, and select the processor.

Mine is the Arduino Uno WiFI Rev2, with the A4809 processor.

Hello World

Next step, I want to do the tried and tested programming exercise of writing Hello World to the ‘Console’.

The Arduino doesn’t have a console to write to in the normal sense you might be familiar with; what it can do however is write to the Serial connection you use to upload your programs, and the Arduino extension in VS Code can display it.


More embedded basics; Remember how I said that everything in embedded programming is turning pins on (HIGH) and off (LOW)? Well, that’s true of writing text to a Serial connection too, but we do it extremely quickly.

Luckily, we don’t have to do all the pin changes ourselves (phew); there are functions already supplied by the Arduino libraries that will do this for us.

When we configure our Serial port, we need to set its ‘baud’, or speed. This tells the Serial code how fast we want to send characters. 9600 baud means that we are sending approximately 9600 bytes/characters per second. Whatever is ‘listening’ on the other end needs to know the speed, otherwise you’ll just get junk out.


So once we’ve setup our Serial Port, in our loop, once per second we print our Hello World to the Serial port.

Once you’ve copied that code into your app.ino, upload the program as you would normally.

Once you’ve uploaded your program, it’s initially a bit anti-climatic, nothing is obviously happening. To see the output, we need to open the Serial Monitor. Click on the little connection logo in the VS Code footer:

This will bring up the Serial Monitor. At this point, you will probably just be getting nonsense, so you will want to change the ‘baud’ for the Serial Monitor to 9600.

Once you’ve done that, you should get your Hello World being displayed!

Next Steps

Ok, so that’s it for now. Subsequent posts will cover:

  • Wiring up my sensors and reading analogue values.
  • Using the WiFi module to sent HTTP requests
  • Using the data I collect!