It seems like every week’s tasks have (gratefully!) included learning and applying something new in the eGreenhouse project. In the last few days this has been setting up the movement commands for the CNC controller which moves the camera and sensor module in the x and y directions along the tracks. The structure of this project and the hardware being used has been set up so that we can send movement commands in a pre-defined format. The CNC drivers have already been set up and subscribe to the GCodeFeed topic. The message format for this topic is a single string element. A controller node which accepts the strings and sends them through the specified serial port also exists. An ESP32 running Grbl is connected via USB to the port that we want to send instructions to and expects to receive new line separated G-code strings.
There are many G codes available for various processes which include movement direction, path geometry, tooling control, and the list goes on and on. For this project we are mainly interested in linear motion to make the end effector with the camera and sensors travel along a list of destinations to collect data. The two G codes for linear movement are G00 and G01. G00 is a rapid movement which just tells the controller to move to the destination as fast as possible without performing any work. G01 tells the controller to move to the destination at a specified feed rate and allows for concurrent operations such as milling or extrusion in the case of a CNC mill or 3D printer. For now I’ve been primarily concerned about sending properly formatted codes to a mock node. Since I’m working with a mock, the code at this point is arbitrary and I’ve been using G00, but ultimately this code could change.
The point of today’s post is to talk about the requirements that we have been given for sending instructions to the ESP32 and a couple of iterations I’ve gone through to get to the point I’m at right now. Our goal is for the user to be able to select a series of locations in the interface. The interface should convert those into absolute x-y coordinates measured in millimeters and store them in the database. The ROS program can then read the set of locations (referred to as waypoints), tell the controller to move to each waypoint and perform the associated action at each. Although we could send the waypoint coordinates to the controller with no intermediate locations with the end effector arriving at the specified destinations (assuming no errors), we have been tasked with splitting up the distance to provide some more flexibility like inserting additional commands into the path or interrupting the movement operation.
The first step that I took was to make the end effector mock “move.” Although it is just a mock, it stores a location and has a means of moving about virtual space. First, to make sure that the mock would substitute the ESP32, I made it subscribe to the same topic that the ESP32 will use, parse the G Code, and use its controller function to move to the specified location. This is just a simple x-y coordinate and it just increments or decrements until the current location is the destination. While the mock is “moving” it is also publishing its location to the topic that the actual ESP32 will be reporting the real location to. Success! In iteration 1, the communication channels are functioning, the mock is working as needed, and the MotionNode is telling the ProgramNode when it reaches the destination.
Next was to create some intermediate G Codes. In this step I wanted to get the destination split up into a series of steps, each 5 linear units in length. When sending codes to the actual ESP32, units will be in millimeters, but since this is just a mock, they are arbitrary. In this iteration the virtual end effector moves in the x direction first, then in the y direction. If the case were that we wanted to move from (0, 0) to (21, 14), the MotionNode would publish the destinations (5, 0), (10, 0), (15, 0), (20, 0), (21, 0), (21, 5), (21, 10), (21, 14) as formatted G Codes (G00 XX, YY, ZZ) to the GCodeFeed. Once again, success! The ESP Mock can move in virtual space from its location to the destination passing through each location published to the GCodeFeed.
Although now we’re “moving” along a segmented path, this still seems somewhat inefficient. Why should we waste the time of traveling to the user’s selected destination in an L-shaped path? Next is to move 5 cm at a time directly toward the destination. Or, in other words, time to throw in a little bit of geometry. In this iteration we are going to calculate each intermediate x-y coordinate as a 5 unit increment along the direct path from the current location to the destination. To do this, we’ll just use right triangle properties to calculate the angle between the x-axis and the destination as Θ = tan-1(Δx/Δy) and the length of the hypotenuse as √(a2+b2). The x component of the distance will be the length of the segment (5 units) times cos(Θ). Similarly, the y component will be the length of the segment times sin(Θ). We’ll then take the floor of the length of the direct path divided by the segment length. Each intermediate coordinate will be each integer from 1 to the calculated floor times the x component and times the y component. We’ll then publish each of these destinations as G Code formatted strings to the GcodeFeed to be read by the ESP Mock.
So, the question now is… does it work? All I can say right now is we’ll see. The concept should be sound, and as long as I haven’t mixed up any calculations when writing the code, it should work, or at least within the tolerances of the system. More to follow next week!
