Whats A Microcontroller v3

Controlling Motion · Page 93

Chapter 4: Controlling Motion

MICROCONTROLLED MOTION

Microcontrollers make sure things move to the right place all around you every day. If you have an inkjet printer, the print head that goes back and forth across the page as it prints is moved by a stepper motor that is controlled by a microcontroller. The automatic grocery store doors that you walk through are controlled by microcontrollers, and the automatic eject feature in your DVD player is also controlled by a microcontroller.

ON/OFF SIGNALS AND MOTOR MOTION

Just about all microcontrolled motors receive sequences of high and low signals similar to the ones you’ve been sending to LEDs. The difference is that the microcontroller has to send these signals at rates that are usually much faster than the blinking LED examples from Chapter 2. If you were to use an LED circuit to monitor control signals, some would make the LED flicker on/off so rapidly that the human eye could not detect the switching. The LED would only appear to glow faintly. Others would appear as a rapid flicker, and others would be more easily discernible.

Some motors require lots of circuitry to help the microcontroller make them work. Other motors require extra mechanical parts to make them work right in machines. Of all the different types of motors to start with, the hobby servo that you will experiment with in this chapter is probably the simplest. As you will soon see, it is easy to control with the BASIC Stamp, requires little or no additional circuitry, and has a mechanical output that is easy to connect to things to make them move.

INTRODUCING THE SERVO

A hobby servo is a device that controls position, and you can find them in just about any radio controlled (RC) car, boat or plane. In RC cars, the servo holds the steering to control how sharply the car turns. In an RC boat, it holds the rudder in position for turns. RC planes typically have several servos that position the different flaps to control the plane’s motion. In RC vehicles with gas powered engines, another servo moves the engine’s throttle lever to control how fast the engine runs. An example of an RC airplane and its radio controller are shown in Figure 4-1. The hobbyist “flies” the airplane by manipulating thumb joysticks on the radio controller, which causes the servos on the plane to control the positions of the RC plane’s elevator flaps and rudder.

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Figure 4-1

Model Airplane and

Radio Controller

So, how does holding the radio controller’s joystick in a certain position cause a flap on the RC plane to hold a certain position? The radio controller converts the position of the joysticks into pulses of radio activity that last certain amounts of time. The time each pulse lasts indicates the position of one of the joysticks. On the RC plane, a radio receiver converts these radio activity pulses to digital pulses (high/low signals) and sends them to the plane’s servos. Each servo has circuitry inside it that converts these digital pulses to a position that the servo maintains. The amount of time each pulse lasts is what tells the servo what position to maintain. These control pulses only last a few thousandths of a second, and repeat around 40 to 50 times per second to make the servo maintain the position it holds.

Figure 4-2 shows a drawing of a Parallax Standard Servo. The plug (1) is used to connect the servo to a power source (Vdd and Vss) and a signal source (a BASIC Stamp I/O pin). The cable (2) has three wires, and it conducts Vdd, Vss and the signal line from the plug into the servo. The horn (3) is the part of the servo that looks like a four-pointed star. When the servo is running, the horn is the moving part that the servo holds in different positions. The Phillips screw (4) holds the horn to the servo’s output shaft. The case (5) contains the servo’s position sensing and control circuits, a DC motor, and gears. These parts work together to take high/low signals from the BASIC Stamp and translate them into positions held by the servo horn.

Controlling Motion · Page 95

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In this chapter, you will program the BASIC Stamp to send signals to a servo that control the servo horn’s position. By making the BASIC Stamp send signals that tell the servo to hold different positions, your programs can also orchestrate the servo’s motion. Your programs can even monitor pushbuttons and use information about whether the buttons are pressed to adjust the position a servo holds (pushbutton servo position control). The BASIC Stamp can also be programmed to receive messages that you type into the Debug Terminal, and use those messages to control the servo’s position (terminal servo position control).

ACTIVITY #1: CONNECTING AND TESTING THE SERVO

In this activity, you will follow instructions for connecting a servo to your particular board’s power supply and BASIC Stamp.

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Servo and LED Circuit Parts

(1) Parallax Standard Servo

(1) Resistor – 470 Ω (yellow-violet-brown)

(1) LED – any color

The LED circuit will be used to monitor the control signal the BASIC Stamp sends to the servo. Keep in mind that the LED circuit is not required to help the servo operate. It is just there to help “see” the control signals.

CAUTION: use only a Parallax Standard Servo for the activities in this text! Other servos may be designed to different specifications that might not be compatible with these activities.

Building the Servo and LED Circuits

In Chapter 1, you identified your board and revision using the BASIC Stamp Editor Help. You will need to know which board and revision you have here so that you can find the servo circuit building instructions for your board.

9If you do not already know which board and revision you have, open the BASIC Stamp Editor Help and click on the Getting Started with Stamps in Class link on the home page. Then, follow the directions to determine which board you have.

9If you have a Board of Education USB (any Rev) or Serial (Rev C or newer), go to the Board of Education Servo Circuit section below.

9If you have a BASIC Stamp HomeWork Board (Rev C or newer), go the BASIC Stamp HomeWork Board Servo Circuit section on page 99.

9If your board is not listed above, go to www.parallax.com/Go/WAM → Servo Circuit Connections to find circuit instructions for your board. When you are done with the servo circuit instructions for your board, go on to Activity #2: Servo Control Test Program on page 102.

Board of Education Servo Circuit

These instructions are for all USB Board of Education Revisions as well as for the Serial Board of Education Rev C or newer.

9 Turn off the power as shown in Figure 4-3.

Controlling Motion · Page 99

Up until now, you have been using the 3-position switch in position 1. Now, you will move it to position 2 to turn on the power to the servo header.

9Supply power to the servo header by adjusting the 3-position switch as shown in Figure 4-7. Your servo may move a bit when you connect the power.

Power turned on to Board of Education and Servo Header

0 1 2

If you see instructions in this chapter that read “Connect power to your board” move the 3-position switch to position-2. Likewise, if you see instructions in this chapter that read “Disconnect power from your board” move the 3-position switch to position-0.

9Disconnect power from your board.

9Go on to Activity #2 on page 102.

BASIC Stamp HomeWork Board Servo Circuit

If you are connecting your servo to a BASIC Stamp HomeWork Board (Rev C or newer), you will need these extra parts from your kit:

(1) 3-pin male/male header (shown in Figure 4-8).

(4) Jumper wires

Figure 4-8

Extra Part for BASIC Stamp

HomeWork Board Servo Circuit

3-pin male/male header

Figure 4-9 shows the schematic of the servo and LED indicator circuits on the BASIC Stamp HomeWork Board. The instructions that come after this figure will show you how to safely build this circuit.