CONTROL OF A SIX-JOINT ROBOT ARM USING THE TWO-AXIS MOTION CONTROLLER NI PCI-7342_

by Moataz Ghader

ABSTRACT The project consists of a control system for a six-joint robot arm using a motion controller that can control up to two axes only. The description below shows some information about the robot arm and the techniques and the ways used to extend the control to all the joints.

I- THE ROBOT ARM “Pro-Arm RS 2200” is a 6-joint, 5 degrees of freedom robot arm. It is designed to simulate industrial robot operations for laboratory or classroom training and education research. It is a joint-coordinate type robot. It is so named because its movements resemble that of human joints. It has six axes, and each axis imitates the movement of one of the human’s mobility.

Figure 1- the PRO-ARM RS 2200 robot arm

Mechanics of the Robot Arm The robot consists of 6 stepping motors that move the six joints. The following table shows the motion specifications on each of the motors.

Motor No. Joint Degree Step Max movement stepping from zero position

Positive direction Negative direction

M1 Body 0.12 1000

(clockwise)

1000

(counter clockwise)

M2 Shoulder 0.12 600

(Upward)

600

(Downward)

M3 Elbow 0.1 500

(Upward)

500

(Downward)

M4

M5*

Wrist 0.1 + , + (counterclockwise) Max 1800

- , - (clockwise) Max 1800

+ , - (downward) Max 900

- , + (upward) Max 900

M6 Gripper 0.1 Close +1800 & Open -1800

* If M4 and M5 Input is (900, -900), the wrist will move downward 90º.

If the input is (-900, -900), the wrist will turn clockwise 180º.

When M4 > M5 or M4 < M5, movement of bending and turning will act simultaneously.

Controlling the Robot Arm

The Pro-Arm RS 2200 is an old robot that uses the DOS Basic and Assembler programming languages. Its basic controller is the Zilog Z-80A microprocessor. Since now we can barely find computers with the old operating system DOS, the original controller of the robot arm is almost considered useless; and if in anyway such system is found, any addition to the robot that can be done to develop its functions would be a big burden to supply the elements compatible with the old system.

From all these circumstances rose the idea of a new control system that is compatible with nowadays computer systems and platforms, and capable of being updated and added new functions.

The controller that we used is the National Instruments PCI- 7342 motion controller. And for programming tasks, the most suitable choice was the National Instruments LabVIEW.

Robot Arm Kinematics

It is a common practice for the analysis of the robot arm system to define a world coordinate system and a local coordinate system attached to each joint. This latter coordinate frame moves with the link. The world coordinate system may be a Cartesian coordinate system whose origin is fixed to the base. The relationships between different coordinate frames are described by functions that are static in the sense that there are no variable derivatives of time explicitly in the equations.

Since the controller we are using consists of only two axes, it is useless to set coordinate frames for all the joints. Instead, shoulder and elbow joints were used to set the idea of simultaneous motion. Other joints work independently.

Coordinate Frame of the Body Joint (Ground Plane Joint)

This joint, as been said above, works independently. It is given a coordinate frame that lies on the ground plane. Its origin can be considered coinciding with the origin of the base frame shown above. It is positioned at the center of the robot’s width. Figure 2 shows this coordinate frame.

Figure 2- coordinate frame for the body joint

θ is the angle that the body joint makes with the x-axis. In the control software, user inputs the position on the plane in Cartesian coordinates form. It is then changed to angular form through the arctangent function, and input to the body joint.

Coordinate Frames for shoulder, elbow and gripper joints

(Vertical Plane Joints) A world coordinate frame is implemented at the base of the robot arm, and three local coordinate frames were hanged on the two joints, shoulder and elbow, and on the gripper.

The focus lies on the shoulder and elbow joints, and we already know that they rotate about the axes normal to the plane they make and passing by their origins. These axes are parallel and considered the Z-axes according to the D-H algorithm for assigning coordinate frames. In calculation, we only consider the two coordinate axes, X and Y of each joint for the purpose of simplifying the matrices and equations. This idea is clarified more in figure 3.

Figure 3- coordinate frame for shoulder, elbow and gripper joints

II- HARDWARE ARCHITECTURE The system of the Robot Experiment consists of the following hardware parts:

- The 6-joints Robot Arm

- A computer with PCI slot

- NI PCI-7342 Motion Controller Board

- UMI 7764 accessory (Universal Motion Interface)

- 68VHDCI Cables for motion an digital I/O connectors

- 68VHDCI-50F SSR Cable Adapter

Figure 4 shows the hardware components and connections as an architecture overview of the system.

Figure 4- Hardware Architecture

III- DESIGN OF THE DRIVER BOARD The motion controller features only two axes, each outputs two lines: step and direction. But looking for the robot, it is actuated by 6 stepper motors, each having 4 phase lines. Here we find ourselves in front of a challenge of controlling 6 actuators with two control axes. On the other hand, the controller does not feature any amplifier or power driver, making it impossible to count on the controller to power the robot actuators.

For all these reasons, we had to implement an interface between the controller and the robot arm’s motors, which at the same time supplies the robot with the required power. Below are the steps followed to do the design of the driver board.

The driver board consists of 6 stepper driver units, each connected to one motor of the robot arm. Since the 7342 controller board consists of only two axes, the board is designed to receive only two step input lines coming from the step output of UMI 7764, each connected to three driver inputs.

To prevent any unexpected error in the driver’s voltages and currents that may damage the controller, the two circuits are isolated using optocouplers.

Stepper Driver Design The stepper driver has the following inputs:

- One select line to activate or deactivate the motor.

- One step input that receives the required number of pulses at a required frequency depending on the set distance and speed respectively.

- One direction input indicating whether to rotate clockwise or counter-clockwise.

The stepper motor has 4 phases (2 center-tapped inductances); meaning that the step pulses must be distributed to 4 sequenced lines in order to move the motors. For this we needed to program a microcontroller that reads the pulses and direction, and output them in the correct sequence.

Therefore, each driver consists of the following parts:

- Three optocouplers to isolate the input circuit from the driver circuit

- PIC16F630 microcontroller

- Four power transistors

Figure 5 shows the block diagram of a driver circuit.

Figure 5- driver block diagram

Figure 6- The driver board

IV- SOFTWARE ARCHITECTURE Our software is designed using the Client-Server architecture, where the server is at the local place of the experiment and hardware, and the client could be any user connected to the server through the network.

The network technology used is the DataSocket server technology, supplied with LabVIEW.

Figure 7 shows the general block diagram of the software architecture for the robot experiment program.

Figure 7- software architecture of object transport application

V- Graphical User Interface Figure 8 shows the user interface of CLIENT VI:

Figure 8- Graphical User Interface

As the figure shows, the user interface consists of five parts.

a- Control and Status

This part includes three control buttons and two status leds.

The first button, “Start Job” orders the robot arm to start doing the require moves.

The second button, “Change Parameters”, browses a new VI where the user can change input data for the experiment concerning velocity, acceleration, object and obstacles (discussed below).

The third button is the stop button used to halt the motion of the robot.

Concerning the leds, the first led tells whether the robot is busy or not. If it is, the led blinks and shows the clause “Currently in Job”. Otherwise, it is off and shows the word “Ready”.

The second led is the job status led. When the robot is in job, the led is lit and shows what step the robot is performing actually. Else, it is off and shows “No Job”

9 steps constitute the job of the robot arm:

1- Rotating body towards object

2- Moving gripper down to object

3- Picking object by gripper

4- Lifting object and avoiding obstacles

5- Rotating object towards final position

6- Getting object down

7- Releasing object

8- Lifting arm to initial height

9- Rotating body to initial position

b- Gauges Velocity and position of each joint of the robot can be monitored through the gauges on the left. They are updated at every instance so that they show the behavior of motion in real time.

c- Robot Arm Image an image showing the robot arm consisting of the two links and gripper. It is updated with the position of the robot arm to simulate its motion in real time.

d- Graph Chart

This chart plots the speed profile and trajectory of body, shoulder and elbow joints at each step of the job. It shows one plot at a time. The user can choose what plot to show through the control shown at the upper right of the screen.

e- Velocity Data

Simultaneously with the graph chart, shows the estimated peak velocity, measured peak velocity, and velocity override of each of the mentioned joints at each step. The same controls at the upper right of the screen choose the joint and step to show the velocity data for. Velocity override is the ratio of the measured velocity to the estimated velocity.

VI- CHANGING PARAMETERS As been said above, user can press the “change parameters” button to change any data concerning the input parameters for the experiment. Once pressed, CLIENT DATA OUTPUT VI is open. Figure 9 shows the front panel for this VI.

Figure 9- front panel for CLIENT DATA OUTPUT

As shown in the figure, user can choose any of the five buttons to change parameters according to the category concerned. When configuration is finished, the OK button is pressed and the Change Parameters front panel closes.

Changing parameters of each category is discussed below.

Change Object Parameters

Figure 10- Changing object parameters

Object parameters consist of:

- Dimensions: its values are used in calculating the path in case of existence of obstacles.

- Initial Position: where the object initially.

- Final Position: where the user wants to place the object finally.

Change Obstacle Parameters

User can set more than one obstacle, and the software calculates the path so that all obstacles are avoided in condition that all of them are entered in the “obstacle parameters”.

As seen in figure 11, obstacles are entered in an array where each element consists of data concerning their position and dimensions.

Change Joint Parameters

Whatever the joint is, user selects its category in the options list and parameters to be changed will appear as in figure 12.

Figure 11- Changing obstacle parameters

Figure 12- Changing Joint Parameters

Just by entering the maximum velocity and acceleration of a joint, the motion controller calculates its profile parameters depending on the distance to be traveled by this joint.

In the case of Shoulder and Elbow parameters, velocity and acceleration values are not for each of the two joints, but for the vector space composed of the two joint variables, shoulder and elbow.

Server Name

In addition to adjusting the parameters of the experiment, the CLIENT DATA OUTPUT also features a data field labeled “Server Name” where we must enter the URL of the server in case of connection to network. In the case of accessing the experiment on the server computer, ‘localhost’ must be entered.

Description of Software A job of the robot arm consists of 9 steps listed above. Each step concerns a part of the robot. Specifically, steps 1, 5 and 9 concern the body joint; steps 2, 4, 6, and 8 concern shoulder and elbow joints; finally, steps 3 and 7 concern gripper joint.

The basic idea of the server program is using an incremental counter, called a status counter, as an indicator of the job phase. It is incremented every time one of the four events occur: Start Job is true, move complete 1 is true, move complete 2 is true, or move complete 3 occurs, where move complete events represent the end of motion for body, shoulder & elbow, and gripper respectively.

This counter controls a case structure: cases 1, 5 and 9 activate body motion; cases 2, 4, 6, and 8 activate shoulder & elbow motion; cases 3 and 7 activate gripper motion.

Since each of the controller’s axes runs more than one motor, absolute position mode is impossible to use. Instead, relative position mode is chosen; and the axis is reset to 0 position at each time it is called to move. We then used variables that memorize the last stop of each joint. Thus, the last stop value is subtracted from the target position and the result is loaded to the axis.

Figure 13 shows the flow chart describing briefly the behavior of the robot arm job.

Reset Axis1 & Axis2 positions Read shoulder & elbow target positions

Read Body velocity & acceleration

Begin

Job Status = Ready Inc: = 0

Start Job = true?No

Inc ≥ 10

Job status = currently in job Inc++

Yes

Yes

Inc =?

Reset Axis1 position Read body target position

Read Body velocity & acceleration

Loaded target position1: = Body target position – last stop 1

Start Motion

Move complete1: = True?

No

1, 5, 9

No

Yes

Last stop1: = Last stop1 + current position1

2, 4, 6, 8

Loaded target position2: = Shoulder target position – last stop 2

Loaded target position3: = Elbow target position – last stop 3

Start Motion

Move complete2: = True?

No

Yes

Last stop2: = Last stop2 + current position2

Last stop3: = Last stop3 + current position3

3, 7

Reset Axis2 position Read gripper target position,

Velocity & acceleration

Loaded target position4: = Gripper target position – last stop4

Start Motion

Move complete3: = True?

No

Yes

Last stop4: = Last stop4 + current position4

Figure 13- flow chart of the robot arm job