Power Electronics

Robotic Arm Simulated Version

Purpose

The purpose of this project is to implement a simplified version of a robotic arm. This arm uses three motors: one to control the horizontal rotation (about z-axis) of the arm, second and third to control the vertical rotation (up & down movement). Servo motors are used in robotics whose position is controlled by varying the duty cycle of the waveform applied. This project uses the Hitec HS-311 servos which operate between 4.8-6V, require a 50 Hz (20ms time period) square waveform, and are controlled using Pulse Width Modulation. These motors can rotate through an angle of 180 degrees

max. A positive cycle duration of 0.9ms corresponds to an angle of -90 degrees and approx 2.1ms positive cycle corresponds to +90 degrees. The position or angle in between varies linearly with the duration of positive part in between 0.9-2.1ms. Hence, for example, a 50 Hz square wave with peak duration of 1.5ms would move the servo to 0 degree.

6 buttons are connected to 6 input pins such that 2 buttons each control the direction of rotation of one servo, i.e. one button moves the motor in one direction and the other moves the motor in opposite direction

Main Components Used

1: Microcontroller 89C51

The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4Kbytes of Flash programmable and erasable read only memory (PEROM). The deviceis manufactured using Atmel’s high-density nonvolatile memory technology and iscompatible with the industry-standard MCS-51 instruction set and pinout. The on-chipFlash allows the program memory to be reprogrammed in-system or by a conventional

nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flashon a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which providesa highly-flexible and cost-effective solution to many embedded control applications. The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, three 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The Power-down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset.

Pin Configurations

2: Servo Motor

There are a wide variety of servos on the market today.  Ranging from micro servos with 15oz-in torque to powerful heavy-duty sail boat servos, they all share several common characteristics.  

A servo is essentially a positionable motor.  The servo "knows" two things: where it is (the actual position) and where it wants to be (the desired position).  When the servo receives a position, it attempts to move the servo horn to the desired position.  The task of the servo, then, is to make the actual position the desired position

The first step to understanding how servos work is to understand how to control them. The servo is controlled by three wires: ground (black), power (red), and command (typically white).  Power is usually between 4v and 6v and should be separate from system power (as servos are electrically noisy).  Even small servos can draw over an amp under heavy load so the power supply should be appropriately rated.  Though not recommended, servos may be driven to higher voltages to improve torque and speed characteristics

Servos are commanded through "Pulse Width Modulation," or PWM, signals sent through the command wire.  Essentially, the width of a pulse defines the position.  For example, sending a 1.5mS pulse to the servo, tells the servo that the desired position is 90 degrees.  In order for the servo to hold this position, the command must be sent at about 50Hz, or every 20mS.  The following table shows the timing constraints of one servo

Position Pulse Width Example Pulse

Minimum 0.5mS

Center 1.5mS

Maximum 2.5mS

If you were to send a pulse longer than 2.5mS or shorter than 0.5mS, the servo would attempt to overdrive (and possibly damage) itself.  

Once the servo has received the desired position (via the PWM signal) the servo must attempt to match the desired and actual positions.  It does this by turning a small, geared motor left or right.  If, for example, the desired position is less than the actual position, the servo will turn to the left.  On the other hand, if the desired position is greater than the actual position, the servo will turn to the right.

In this manner, the servo "zeros-in" on the correct position.  Should a load force the servo horn to the right or left, the servo will attempt to compensate.

To download the simulation and other files of the above project click the link given at the end of the page