Servo Tutorial

Phidgets Inc. Servo Tutorial  A Single Servo Controller in Visual Basic Overview In this, tutorial we will cover: • • • Installing Visual Basic Express Create a Graphical User interface Writing code to operate the Graphical User Interface and control a Phidget servo. This tutorial provides an excellent starting point for learning how to use the PhidgetServo controller and helps develop a feel for the way all Phidgets work. It is specifically geared towards those users new to Visual Basic and creating Graphical User Interfaces. To attempt this tutorial, you should understand basic programming concepts. Knowledge of object oriented programming would also be an asset. There are extensive resources available online that deal with this subject. The source code for this application is provided with the tutorial. It is suggested though that you follow the steps outlined here to gain a firm grasp of how Visual Basic works and the steps used to create the application. Visual Basic Express and Initial Setup Visual Basic (VB) Express is a free Windows development package from Microsoft that uses the .NET framework. It provides all the functionality that the vast majority of people will require. Additionally, it contains documentation and help files specifically targeted at assisting beginner and intermediate programmers. VB Express can be downloaded from the Microsoft web site. Important: Once VB Express has been installed, or if you are using a pre-installed .NET programming package, it is necessary to download and install the API from the Phidgets website, Phidget21.msi. Without this file, it is not possible to use Phidgets. Creating the Graphical User Interface (GUI) The GUI is what you actually see when running your program. It allows you to interact with the code and make changes that can affect your Phidget. Open VB Express To begin, open VB Express and create a new project. Select “Windows Application,” name the project ‘ServoControler’ and click “Ok.”  Phidgets Inc. Load Phidget API Under the “Project” menu (1), select “Add Reference.” In the new window, click on the Browse tab (2) and navigate to the directory in which you installed Phidgets21.msi. The default directory is: C:\Program Files\Phidgets. For VB Express, select the Phidget21.NET.dll (3) and click “Ok” (4). This is a compulsory process as it loads the Phidget API or “Application Programming Interface.” The API contains the Phidget library which is a collection of the controls we will need to interact with Phidgets. You must load the library regardless of which Phidgets you are using. GUI Layout A program with a GUI is composed of one or more “forms.” The forms contain all of the elements necessary to interact with the computer in a graphical environment. The form for this program is the grey box on the screen. Most programs that you will be writing for Phidgets, at least to begin with, will only use one form. Applications that make use of more than one are often substantially more complicated to program. The screen you see now is called the “Form Designer” It is useful to first set up your workspace. If the “Toolbox” panel is not already visible, open the “View” menu (1) and select “Toolbox.” To ensure the Toolbox panel does not cover the form you are going to be working on, toggle the “pin” button (2). The various elements of the GUI, known as controls, can be placed in whatever order you wish and in any location you wish. For this example, let us first start by placing the “TrackBar” (3). This control will be used to change the position of the servo. Under the “All Windows Forms” rollout, select TrackBar. Click and drag somewhere in the upper portion of the form. The exact positioning is irrelevant, however, for aesthetic purposes, the trackbar may be repositioned, and re-sized. Edge “snaps” are often useful to create a uniform look (you should discover these when dragging the track bar around the edges of the form). The next step is to place the “Labels” (5) which will be used to display information. ‘Label1’ will present an absolute numerical value for servo rotation, ‘Label2’, the connection status and finally, ‘Label3’, the serial number of a connected Phidget. Under the “Common Controls” rollout in the toolbox, select the label and click and drag in the form. Repeat this action to create 3 labels. Note the names of your labels and the order they appear on the form, they should follow the order shown here. When working with the labels and controls in general, it is often useful to open the layout toolbar by selecting View (1) >> Toolbars >> Layout. The layout toolbar provides many helpful tools for aligning elements on the form. Select the three labels you created and perform a “Left Align” (6). If your labels are spread out vertically you can also try clicking “Make Vertical Spacing Equal” (7). You can move the labels as a group into whatever position you wish.  Phidgets Inc. Finally, select the button tool (4) and as in the same manner as the trackbar and labels, create a button at the bottom of the form. It should be sized in such a way so its title, “Button1” can be seen clearly. While the button is selected, click “Center Horizontally” (8) on the layout toolbar. In this stage, experimentation is encouraged to create a satisfactory look. Modify Properties To ensure the proper operation of the program, a few properties of the controls you just placed must be modified. By first clicking on the trackbar to select it, look to the bottom right hand side of the screen. The properties panel should be visible with the name of the selected control at the top of the panel. Here the name is “TrackBar1.” Change the following properties 1. Maximum – Set this to 180. In combination with the minimum value two lines below, these numbers determines the range of values that can be selected on the trackbar. Within the application code (which will be discussed latter), the API for the servo accepts values corresponding roughly to degrees of rotation. Consequently, the range of 0 to 180 on the track bar is used to set the position of the servo within half of one complete rotation. It should be noted that the PhidgetServo accept position ranges from -23 to 231. quite obvious that it does not look right. Where there should be ticks beneath the slider, it probably looks like some sort of solid bar. Changing the TickFrequency property from 1 to 10 should fix this. the slide range. 2. TickFrequency – Once you change the maximum property and view the trackbar, it is 3. Value – The value should be set to 90 to ensure that the slider is resting in the middle of After modifying the track bar, click to select the button that you placed near the bottom of the form. In the properties panel, find Text and set its value to “Close.” You should now observe that the button is now labeled “Close.” As is obvious, there are many different properties that can be set. A fairly good explanation can be found by clicking on each and looking to the very bottom of the screen or by searching the help file which comes with VB Express.  Writing the Code Phidgets Inc. Visual Basic and all visual programming languages are “event driven,” as such, any action you want the program to respond to calls a subroutine. As a general rule, every event requires its own block of code. There are three categories of action that this application must be designed to handle. 1. Program Activities – These are events such as opening or closing the program. 2. User Activities –Including sliding the trackbar and clicking the Close button. 3. Phidget Events – Phidget events include attaching or detaching the Phidget and handling any errors that might occur. Objects and Events While still in the Form Designer, under the “View” menu, select “Code” (1). A more or less blank page should appear. At the top of this section, two drop down menus can be found. The left menu (2) is used for selecting the control or object while the right menu (3) is used to select an event related to that object. The first subroutine that should be written is the Load event that is triggered when the application is opened. First, select “(Form1 Events)” (2) from the left menu, then “Load” (3) from the right menu. A new block of text should appear. If at any time you would like to test code you have written, simply click the green arrow button (4) on the “Standard” toolbar. Loading the form Both subroutines seen here should be created by using the drop down menus or other automatic means within VB (such as double clicking on the control in the form designer). You should never have to type out the basic frame work for subroutines that deal with events. Private Sub, End Sub, and any text on the same line as Private Sub should be created by VB. Making your own subroutines will be covered in the next tutorial. Public Class Form1 Dim WithEvents Servo1 As New Phidgets.Servo 1 Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 2 4 Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: End Sub -" Not Connected" -" 3 5 7 Private Sub TrackBar1_Scroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll 4 Label1.Text = "Position: " & TrackBar1.Value 6 End Sub End Class  Phidgets Inc. In attempting this tutorial, you should already have a concept of how object oriented programming works. As a quick refresher though; object oriented programming is centered around creating virtual “objects” that can both store information and take action using subroutines. Objects are defined by writing a “class” that defines their behavior. Each of the controls that you placed in the form designer (the trackbar, button and labels) are objects. If you will notice the very first lines of this code are “Class Form1.” In writing this program we are actually creating an object. It can perform actions through its subroutines and store data. Generally, in this program, we are using other object, like the labels (6), to store information, such as text (called “strings”). After writing the segment of code shown here it would be prudent to test it by clicking the green arrow button as described previously. All the application allows you to do with this code is drag the slider and have a numerical value displayed. 1. Here, the “Servo1” is wished. It can not start with a number or contain any spaces or special characters such as *, !, &, etc. It must also be unique. These naming rules are applicable for any variable or object within Visual Basic and most other programming languages. 3. As with (3), this portion of the line was automatically created and for most new users is unimportant. In the simplest sense, it generates two objects, the first of which, sender, can be used to identify the object or event responsible for triggering the subroutine. The second object, e, contains other information that the subroutine is given. This structure containing sender, and e objects is very common in .NET programming. for determining when a subroutine is called. In the first instance, the handle is Me.Load. “Me” is used to reference the active form and .Load is the condition for action. Hence the statement is equivalent to “Call this subroutine when loading.” Similarly with the second block, the handle TrackBar.Scroll calls that subroutine when the trackbar is scrolled or moved. Without these handles, neither event of loading the form or moving the trackbar would call a subroutine. While creating an event to respond to trackbar changes it is important to use the .Scroll event and not the .ValueChanged event though both seem similar. labels. The text shown on the Close button was modified in the properties panel when laying out the GUI. Here however, the contents of the labels are modified directly within the code. Most properties found in the properties panel can be modified by the code in this same manner. The structure,object.property or object.variable is a universal way of accessing or modifying information within objects. If the call contains brackets, it is a subroutine, such as object.subroutine(). It is also possible to have layered structures, as in object.property.property.subroutine(). In this case, the property actually refers to another object contained within the main object. is very similar to the statements contained within (6). The highlighted section however demonstrates a way to display variables. The “&” symbol is necessary to separate distinct elements. selecting the two drop down menus. Together, they define the start and end of the subroutine. 4. Handles are very important and are by default, responsible object is created. Note that the position of this line is outside of any subroutine (as explained in (7)) and therefore it remains globally accessible (every subroutine can access it) within the scope of this application, or more specifically, this form. It may be obvious that this line is not strictly necessary as Servo1 is not used. Later, it will allow us to interact with the Phidget. Notice the structure of this declaration. The keyword Dim is responsible for declaring the object, creating a reference for the object and reserving appropriate memory. It does not create the object. Objects or variables created in memory always have a reference, also known as a pointer. If you are unfamiliar with references, you can simply think of them as the objects name. When using an object in code, such as Servo1, you are actually not using the object but the reference to that object. For example, it is possible to have two objects (A and B) and to swap their references. In such a case, calling object A would actually access object B. 5. This three line block of code is responsible for changing the WithEvents allows the object to use its own handles. If this term is neglected, you would have to manually define subroutine names for the events the object can trigger. In tutorial 3, this will be discussed in deeper detail. For now, it is important for simplicity. As New Phidgets.Servo creates this object as a Servo object. The term New is important as it is responsible for actually creating the object in the memory defined by Dim. Finally, If you were to create a program using Phidget encoders, you would use As New Phidgets.Encoder to create an encoder specific object. 2. The entire line shown here was automatically created 6. It should be obvious that the precursor to this statement 7. These two statements were created automatically when and is used to define the subroutine. This portion is the name of the subroutine and can be changed to whatever  Phidgets Inc. Private Sub Servo1_Attach(ByVal sender As Object, ByVal e As Phidgets.Events. AttachEventArgs) Handles Servo1.Attach Servo1.servos(0).Position = 90 TrackBar1.Enabled = True Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: End Sub Handling the Phidget It is now time to interface with the Phidget. As you can see in the code to the left, there 4 are three important events common to 3 1 all Phidgets, these are, attach and detach, occurring when a Phidgets USB cord is 90" connected or removed from the computer. Connected" " & Servo1.SerialNumber Additionally, there is the error event which is called when the Phidget malfunctions. 2 1. As with the previous section of code, Private Sub Servo1_Detach(ByVal sender As Object, ByVal e As Phidgets.Events.DetachEventArgs) Handles Servo1.Detach Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: TrackBar1.Value = 90 TrackBar1.Enabled = False End Sub -" Not Connected" -" 1 6 5 4 these are the handles to the subroutines. If you modify the name of the subroutine and later can not remember what it does, simply look to the handle. All three handles, and therefore subroutines, are called when something happens regarding the Phidget, and since we are using the Servo1 object to interact with it, the Servo1 object is responsible for triggering these events. Private Sub Servo1_Error(ByVal sender As Object, ByVal e As Phidgets.Events.ErrorEventArgs) Handles Servo1.Error actually telling the Phidget to take action. Here, we are giving it a command to center its position (90 degrees out of 180). Notice MessageBox.Show(e.desc) the structure of this command. Servo1 is 6 the object responsible for controlling this Me.Close() 8 1 Phidget. Next, servos(0), is accessing 7 the subroutine for servo motor with End Sub index 0. On the 4-Motor PhidgetServo, the 0 shown here could be 0, 1, 2 or 3 corresponding to each individual motor channel. Finally, .Position= 90 is responsible for setting the position of the servo. 3. This line ensures that when the servo becomes active, the trackbar is enabled so that the user can modify its position. 4. These two blocks of code are very similar to those shown in (6) from the previous explanation and are responsible for 2. This is the first instance where we are changing the labels information. 5. The Value property of the trackbar can be changed by both the program and the user. You have already seen how this value 6. This expression creates the object, e, of type Phidgets.Events.ErrorEventArgs. The type of object should be of 7. Following from (6), here, a message box is created and the desc variable of the e object is displayed. The e object only 8.Me refers to the active form and the .Close() subroutine initiates the processes of closing the form can be read by the application and while being modified by the user to simply display a value. Here, the value is set by the application and then the control is disabled to prevent its value from being changed when there is no Phidget attached to the computer. Since this is necessary only when there is no Phidget, it is located within the Servo.Detach subroutine. no concern as it is already defined by the nature of the event. The call to this subroutine occurs when there is an error and the e object gets passed the details of that error. contains two variables, the error code and its corresponding description (or desc). It is always a good practice to show what error has occurred when writing code. If you like, you can once again test the code you have written. At this stage, the trackbar still will not have any effect on the servo motor however you should see changes to the lables on the form when you plug in and unplug the Phidget.  Phidgets Inc. Linking the Phidget to TrackBar Return once more to the first two blocks of code you wrote. 1. Adding this line will disable the trackbar Public Class Form1 Dim WithEvents Servo1 As Phidgets.Servo Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Servo1 = New Phidgets.Servo Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: TrackBar1.Enabled = False Servo1.open() End Sub -" Not Connected" -" immediately when the program starts. At this point, it is unknown whether or not a Phidget is connected to the computer. If no servo Phidget is available to the Servo1 object and you try and change its position value through the code at (3), an error will occur. Since there is an enable command for the trackbar in the Attach subroutine, this line provides a measure of protection. 1 2 2. Calling the .open() subroutine Private Sub TrackBar1_Scroll(ByVal sender As Object, specifically tells Servo1 to search ByVal e As System.EventArgs) Handles TrackBar1.Scroll the computers USB ports and try and 3 Label1.Text = "Position: " & TrackBar1.Value detect any Phidgets already connected. If it finds any, it then proceeds to call Servo1.servos(0).Position = TrackBar1.Value the Attach subroutine that you have already written. The .open() End Sub subroutine called here is in fact an element common to any Phidget object that you create. Just as Phidgets.Servo was used to create the Servo1 object used here to interact with the servo motor, a Phidgets.Accelerometer object would interact with an accelerometer. Both objects would contain a number of common properties and subroutines, including .open(). A complete list can be found in the .NET API manual. servo to reflect the position of the trackbar when it is moved. 3. This is the line that links the rotation of the servo to the position of the track bar. It updates the Position property for the At this point the application is nearly complete. If you choose to test it at this stage, dragging the slider bar when a PhidgetServo is connected should result in it rotating. Private Sub CloseButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click Me.Close() End Sub Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms FormClosingEventArgs) Handles Me.FormClosing Servo1.close() End Sub End Class Closing Subroutines The final thing that must be accounted for is exiting the program and clicking the close button. 1. As has been mentioned, these handles tell you what the 2 1 subroutines are responding to. They should be starting to look familiar by now. The lower subroutine is called only when the program is closing. the application to shut down. It causes the program to call the subroutine below it. 2. This is exactly the same as the line shown above and tells 3 1 3. Though not essential, it is recommended to use the .close() subroutine when a given Phidget object is no longer required. It does not delete the object, rather it disconnects the current Phidget from the program. If you wanted to use the Phidget after this command (which is not applicable in this case as the program is closing), you would need to once again call the .open() subroutine.  Conclusion You are now done creating this application. You should now be able to run it (the green arrow). Connect a Phidget and drag the trackbar around. The servo motor should move in relation to the position of the bar. Hopefully you now have a better understanding of Phidgets and how to begin programming with them. Every Phidget works in the same fundamental way so if you understand the creation of the Servo1 object, how it is used and how to modify its properties, you are well on your way to being able to use almost any Phidget. It is very important to note what you have learned in this tutorial is suitable for dealing with only one Phidget connected to the computer. If you have two or more of the same type, you will get unreliable results and not know which one the program will control. A simple method for dealing with this issue is discussed in the next tutorial. If you do have two Phidgets of different types and wish to try creating your own program now, you must remember that each Phidget requires its own object to control it. That object must also be of the same type as the Phidget. Phidgets Inc.  Code 'This application demonstrates the use of the Phidget 'servo controller. Please note that if you are using the '4-Motor version, this application is suitable for 'controlling only motor 0. For reliable results, esure 'that only one PhidgetServo is attached to the computer Public Class Form1 Dim WithEvents Servo1 As New Phidgets.Servo 'Creates the Servo1 Object used to control the Phidget Phidgets Inc. Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 'Subroutine called when the form is created. Sets the initial conditions. Label1.Text = "Position: -" Label2.Text = "Status: Not Connected" Label3.Text = "Serial Number: -" 'Sets the inital text to display TrackBar1.Enabled = False 'Disables the TrackBar to prevent errors with the Servo1 object Servo1.open() 'Will attempt to detect Phidgets already attached to the computer. 'If detected will call the attach subroutine (below). End Sub Private Sub TrackBar1_Scroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll 'A subroutine to deal with changes made to the scroll bar Label1.Text = "Position: 'Updates the position display " & TrackBar1.Value Servo1.servos(0).Position = TrackBar1.Value 'Updates the servo motor to reflect the position of the track bar End Sub Private Sub Servo1_Attach(ByVal sender As Object, ByVal e As Phidgets.Events. AttachEventArgs) Handles Servo1.Attach 'A subroutine activated when a Phidget is detected attached to the computer Servo1.servos(0).Position = 90 'Sets the initial position of the Phidget servo to 90 degrees TrackBar1.Enabled = True 'Enables the TrackBar to allow for changing the servo position Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: 'Updates the text and shows the End Sub Private Sub Servo1_Detach(ByVal sender As Object, ByVal e As Phidgets.Events. DetachEventArgs) Handles Servo1.Detach 'Subroutine called when the Phidget is detached from the computer 90" Connected" " & Servo1.SerialNumber serial number of the connected Phidget  Phidgets Inc. Label1.Text = "Position: Label2.Text = "Status: Label3.Text = "Serial Number: 'Text Changes TrackBar1.Value = 90 TrackBar1.Enabled = False 'Resets the track bar End Sub Private Sub Servo1_Error(ByVal sender As Object, ByVal e As Phidgets.Events. ErrorEventArgs) Handles Servo1.Error 'A subroutine that deals with error conditions MessageBox.Show(e.desc) 'Show the error that has occurred Me.Close() 'Close the program End Sub Private Sub CloseButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click 'A subroutine to close the program Me.Close() End Sub Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms. FormClosingEventArgs) Handles Me.FormClosing 'The subroutine that is run as the program closes Servo1.close() 'This block of code ensures that the Phidget is disabled. It is 'not strictly necessary but recommended End Sub End Class 'Signals the end of this program -" Not Connected" -" 0