A design of a network-based robot control system

Proceedings of the 3rd Asian Control Conference July 4-7,2000,Shanghai

A Design of a Network-based

Robot Control System

using FIP Young Shin Kim, Hyung Seok Kim and Wook Hyun Kwon #003, School of Electrical Engineering, San 56 1, Shillimdong,

Seoul National University,

Kwanakgu, Seoul, 15 l-742, Korea

(E-mail: {kys, hskim, whkwon}@cisl.snu.ac.kr)

Abstract

transmission using analogue methods that were used

This paper suggests an architecture for the robot control system that is based on the network. The main advantage of the architecture is that the weight of the control system is far lighter and the size is far smaller without reduction of the performance. In addition, the cabling cost and the maintenance effort can be drastically reduced. This paper considers the communication latency and the synchronization among multiple joints when a network based robot control system is constructed using FIP fieldbus.

previously. It has been replaced the existing systems using 4 - 20 mA analog signal. Each field device has low cost computing power installed in it. Each device will be able to execute simple functions on its own such as diagnostic, control, and maintenance functions as well as providing

communication

capabilities. As a result, it can report if there is a failure of the device or manual calibration is required, this increases the efficiency of the system and reduces the amount

1.

bi-directional

of maintenance

required.

devices the engineers are able to not only access the

Introduction

field devices, but also communicate A conventional robot axis control system is made up (a motor).

point-to-point

They

connections.

are connected There

with other field

devices.

of a controller, a driver, a sensor (an encoder), and an actuator

With these

One of the main features of the fieldbus is its

through

significant reduction in wiring. Each work cell requires

are connections

only one wire to be run to the main cable, with a

between the controller module and the driver module,

varying

between

installing field equipment in a fieldbus system is thus

the driver

module

and the actuator, and

number

of cells

available.

The

cost

of

between the driver module and the sensor. Considering

significantly

that a common industrial robot consists of six axes in

reduced due to the fact that the fieldbus is a multi-drop

order to support enough freedom[ 11, there are eighteen

rather than point-to-point

or more connections. This is one of main factors that

network can offer a 5:l reduction

make a robot very big and heavy. In addition, it

expense. The simpler system design implies that fewer

increases the cabling cost and, at the same time, it

system drawings will be needed in order to develop a,_

causes the maintenance

fieldbus system. This also has the advantage that the

of the robot to be very

difficult.

reduced.

Installation

costs are further

system and the multi-drop in field wiring

simpler design will result in less complex and faster

The same problem was issued in the manufacturing

bus systems. The fact that the fieldbus system is less

system and process control system [2, 31. In those

complex than conventional

areas, various fieldbuses were proposed in order to

there will be less overall need for maintenance. The

tackle the problems. Fieldbus is a digital, bi-directional,

simplification

multi-drop

reliability of the bus system is increased.

communication

network

used

to

link

isolated field devices, such as controllers, transducers,

of systems means that the long term

With the fieldbus

actuators and sensors. This is far more accurate than

bus systems implies that

system, it is possible for the

operators to easily see all of the devices included in the

2229

system and to also easily interpret the interaction

robot arm as a simple servomechanism.

between

560 series robot arm, the controller consists of a DEC

the

individual

devices.

This

will make

discovering the source of any problems and carrying

LSI- 11/02

out maintenance much simpler, and thus will reduce

microprocessors,

the

digital-to-analog

overall

debugging

time.

The

debugging

and

and

computer each

six

with

converter

For the PUMA

Rockwell

a joint

(DAC),

6503

encoder,

and

a

a current

maintenance of the system will also be enhanced due

amplifier [l]. The control structure is hierarchically

to the fact that fieldbus enables online diagnostics to

arranged. At the top of the system hierarchy is the

be carried out on individual field devices. The online

LSI-1 l/O2

diagnostics

include

detection

functions

and predictive

such as open

maintenance

wire

and simplify

microcomputer

which

serves

as

a

supervisory computer. At the lower level are the six 6503 microprocessors-one

for each degree of freedom

as shown in Figure 1.

tasks such as device calibration. Fieldbus allows the user increased flexibility in the design of the bus system. Some algorithms and control procedures,

which

must

be contained

programs in the conventional

in control

bus systems, can now

reside in the individual field devices, reducing the

--

INTERFACE7

overall size of the main control system. This reduces the overall systems cost and makes future expansion a

>_ Ts = 2.3 ma

simpler prospect. System

performance

simplification devices.

ENCODER+

is enhanced

of the collection

Measurement

due

to

the k

of data from field

and device

values

I_

4

PUMA ARM

LSI-I Ilo2

will be

available to all field and control devices in engineering

Figure 1. PUMA robot arm servo control architecture

units. This eliminates the need to convert raw data into the required units and will free the control system for other

more

important

information complication

tasks.

The

reduction

in

The LSI-1 l/O2 computer

performs

two major

functions:

will allow the development

1. on-line user interaction

and subtask scheduling

of better and more effective process control systems.

from

System performance is also enhanced due to the ability

software package from Unimation Inc. for. control

to communicate

of the PUMA robot arm.

directly between two field devices

rather than via the control system. This also enables

the user’s VAL commands.

2. subtask

coordination

with

the

VAL is a

six

6503

several related field devices to be combined into one

microprocessors

device.

on-line interaction with the user includes parsing,

to carry out the command. The

This paper introduces a robot arm control system

interpreting, and decoding the VAL commands, in

used for a case study in Section 2. In Section 3, FIP is

addition to reporting appropriate error messages.

introduced.

In Section 4, the paper suggests three

to the user. Once a VAL command

architectures

for the robot control systems using the

has been

decoded, various internal routines are called to

FIP. Section 5 is conclusion. It is shown that the effect

perform scheduling

of

These functions, which reside in the EPROM of

the

communication

latency

on

the

control

performance is negligible through these procedures. 6-axis Robot Control

functions.

the LSI- 11/02 computer, include: 0

2.

and coordination

System

Coordinate

systems transformations

(e.g., from

world coordinates or vice versa). 0

Current industrial practice. treats each joint of the

Joint-interpolated involves

2230

trajectory

sending incremental

planning; location

this updates

corresponding

to each set point to each joint

the joint-interpolated

every 28 ms. l

Acknowledging

from the 6503 microprocessors

that each axis of motion

has completed

4. Convert the error actuating signal to current using

its

the DACs, and send the current to the analog

required incremental motion. l

Looking

set points and the values

from the axis encoders.

ahead

two

servo board which moves the joint.

instructions

to

perform

continuous path interpolation if the robot is in a

3.

FIP

continuous path mode. FIP is the fieldbus proposed by France, which is At the lower level in the system hierarchy are the joint controllers,

each of which consists of a digital

servo board, an analog servo board, and a power amplifier for each joint. The 6503 microprocessor is an integral part of the joint controller

which directly

resides on a digital servo board with its EPROM and through

an interface

de-multiplex information

that

with the LSI-1 l/O2 computer board which

routes

functions

trajectory

to each joint controller.

set

as a points

The interface

board is in turn connected to a 16-bit DEC parallel interface board (DRV-11) which transmits the data to and from the Q-bus of the LSI-1 l/02 as shown in Figure 1. The microprocessor

(PDC) type

protocol[4].

as

A

arbitrator(BA) producer

distributor

controls

to

the

known

the access

network.

a

right

Periodic

bus

of each

transmissions

consist of four steps. This procedure

is called a

transaction. The BA first broadcasts an identifier frame

controls each axis of motion. Each microprocessor DAC. It communicates

called a producer-distributor-consumer

computes the joint error

signal and sends it to the analog servo board which has

(IDDAT).

Then

the

sole

producer

and

multiple

consumers of the requested information recognize the ID of the information. broadcasts

a

Next,

response

frame

the

sole

producer

(RPDAT).

Finally,

multiple consumers acquire the information. When one transaction has been completed, following

transaction

the

schedule

determined when the system is configured.

Figure 2

shows

periodic

the

medium

according

the BA begins the

access

to

control

for

transmission.

a current feedback designed for each joint motor. There are two servo loops for each joint control as shown in Figure 1. The outer loop provides position error

information

and

is updated

by

the

6503 Step 1: BA broadcasts

microprocessor

ID

Step 2: P. C recognize

the ID

about every 0.875 ms. The inner loop

consists of analog devices and a compensator

with RP_DAT

derivative feedback to dampen the velocity variable. Both servo loop gains are constant perform as a “critically

and tuned to

damped joint system” at a Step 3: P broadcasts

data

Step 4: All C’s acquire

data

speed determined by the VAL program. The main of the microprocessors

Figure 2. medium access control of FIP

include:

1. Every 28 ms, receive and acknowledge trajectory

set points

from the LSI-1 l/02

perform interpolation

computer

and

is made up of a set of

elementary schedule tables that is called a macro cycle.

between the current joint

The BA gives access rights to producers of periodic

value and the desired joint value.

transmissions according to their order enrolled in the

2. Every 0.875 ms, read the register value which stores the incremental

The message scheduling

table. The BA then gives access rights to the producer

values from the encoder

which

mounted at each axis of rotation.

periodic

3. Update the error actuating signals derived from

needs

an asynchronous

transmissions.

Padding follows

2231

Finally,

transmission

after

a synchronization

in order to make the neriods of

periodic

transmissions

deterministic.

After

all

interface

is replaced

with the FIP network.

This

elementary cycles are performed, the BA repeats the

replaces the point-to-point parallel cables with a single

network schedule from the first elementary cycle. A

bus. The rest control architecture is the same as before.

schedule for each type of transmission is stored in its

Using this configuration

specific window; a periodic window and an aperiodic

system can be changed easily because the parallel

window, respectively.

A synchronization

interfaces

appended

the synchronization

to control

window is padding.

the original robot control

have only to be replaced

with the FIP

network.

These three windows constitute an elementary cycle.

4.

Evaluation of Robot Arm Control Systems

This paper considers three kind of architectures for the network-based robot control system. There has been great advancement in robot control systems, as DSP technologies and advanced control algorithms are applied. However, the structures suggested in this section are basically based on the PUMA robot control architecture. This is because they can be applied to the almost all robot control architectures, since the current robot control architectures have many thing in common with the PUMA robot control architecture though there have been great improvement on the robot controller due to the digital microprocessor technology. So, the terms LSI-11/02 and the 6503 microprocessors are not used at the following subsection. Instead, the main control processor and the joint control processors are used in order to generalize the results of this paper. The main idea of the suggested architectures is to

Figure 3. Network-based

robot control architecture:

Case 1. Table 1 Evaluated data transmission time: Case I Joint controller 16

Data size (bits)

Main controller 96

Overhead size (bits)

(

162

I

162

Frame size (bits)

I

178

I

258

6

1

apply the network to the robot control system. Though many kinds of networks can be applied, this paper selected

the FIP network,

one of the PDC type

networks. The architecture can be simplified and the

No. of data Total transmission (ms)

scale and the weight of the robot control system can be decreased, which will be shown later. In particular the spatial consistency

of the data exchanged

1

time

0.4272

1

0.1032

is easily

guaranteed by using the multicasting or broadcasting

The

communication

capability that can not be obtained in the conventional

processor

robot control system.

performed every 28 ms. The states of the actuators that

and

the

among joint

the

control

main

control&

processors

are

are detected from the sensor data are transmitted and 4.1 Case 1 Conceptually the first architecture keeps the original

the new desired angle value (reference value) for each joint

is transmitted.

The advantage

of using the

one, which consists of the main control processor and

network instead of point-to-point

the joint control processors. It is depicted in Figure 3.

support spatial consistency to the system. The control

The roles of the main control processor and the joint

data from the main control processor is broadcast to all

control processors are not changed. Only the parallel

the joint control processors at the same time. The data

2232

parallel cable is to

transmission time is evaluated in Table 1. From this

sensor value of each actuator, the communication from

table the total data transmission

the joint

time is 0.5304ms

control

processors

to the main control

which is far less than the required time 28ms.

processor may not be necessary. So, this paper does

4.2 Case 2

not consider this communication. The third one is from

The another architecture

is shown in Figure 4. In

the sensor nodes to the main control processor and the

this architecture the sensor values are shared among

joint control processors. The position data of each joint

the joint

are transmitted every 0.875 ms. The data transmission

processor.

control

processors

There

communications

are

and the main control

three

kinds

of

data

time is evaluated

among the joint control processors,

network

the main control processor, and sensor nodes. One is

utilization

in Table 2. From this table the is calculated

as 0.4919, which

seems to support the possible network scheduling.

from the main control processor to the joint control processors.

The new desired angle value (reference

value) for each joint

is transmitted

4.3 Case 3

every 28 ms.

The last case this paper considers is shown in Figure

Another one is from the joint control processors to the

5. As the microprocessor

main control processor. The states of the actuators that

advances, the performance of the processor have been

are detected from the sensor data are transmitted.

drastically

enhanced.

and digital

Therefore

technology

one

powerful

microprocessor

can handle

what several

microprocessors

have handled. In this vein this paper

suggests the third configuration

previous

for a robot control

system. The powerful microprocessor

is assumed that

it handle what is needed in this configuration.

Figure 4. Network-based

robot control architecture:

Case 2. Table 2 Evaluated data transmission time: Case 2.

Data size (bits) Overhead size (bits) Frame size (bits) Transmission rate (Mbps) Calculated transmission time (ms)

Sensor node 16 162 178 2.5

Main controller 96 162 258 2.5

0.0712

0.1032

Figure 5. Network-based

robot control architecture:

Case 3.

The configuration is simple and the maintenance of the system seems to be easier than the original robot control system in Figure 1. The main control processor receives six sensor data., compensates the errors and transmits six actuator inputs every 0.875msec. At the same time the main control

processor

refresh the

incremental path plan every 28 ms and reflects the results to the joint control algorithm. However this operation is the internal matter of the main control Since the main control processor can receive the

processor. The data transmission time is evaluated in

2233

TELECOMMUN.,

Table 3 From this table the total data transmission time, 0.5304ms, is necessary every 0.875 ms.

Vol. 43, No. 9-10, pp. 435-448,

1993 [3] P. Pleinvaux and J.D. Decotignie, “Time Critical Communicaiton

Table 3 Evaluated data transmission time: Case 3.

Networks:

Field Buses,”

IEEE

Network, Vol. 2, No. 3, May 1988. Sensor node Data size (bits) 16 I Overhead size (bits) 1 162 Frame size (bits) 1 178 Transmission rate 2.5 (Mbns) Calculated 0.0712 transmission time (ms) 1 0.875 Period of data (ms) No. of data 6 Total transmission time 0.4272 (ms) 5.

Main controller 1 96 1 162 258 1 2.5

[4] Road

information

0.875 1 0.1032

Conclusion

This paper suggested a new architecture

for the

robot control system that is based on FIP. The main advantage of using FIP in connecting

modules in a

robot is that the weight of the control system is far lighter and the size is far smaller without reduction of the performance.

In addition, the cabling cost and the

maintenance effort can be drastically reduced. When a network

based robot control

constructed using FIP, the communication the synchronization

system is latency and

among multiple axes in a robot

have been considered. This paper has shown that the effect of the communication

latency on the control

performance is negligible through analytical results. It also shows the performance of the network based robot control system is at least as good as that of the conventional robot control system through simulation results.

References [l] K.S. Fu, R.C. Conzalez, C.S.G. Lee, “Robotics: Control,

Sensing,

Vision,

and

Intelligence,”

McGraw -Hill, 1987 [2] Jean-Dominique

Decotignie,

Patric Pleinevaux,

“A survey on industrial communication networks”, Industrial

Communication

Networds,

--

- Controller

Interchange

of

digital

area network (FIP) for

high-speed communication, IS0 11898, 1993.

0.1032 1

vehicles

ANN.

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