A SIMPLE AND EFFICIENT CONTROL ARCHITECTURE

FOR WALKING ROBOTS

Application to AMRU5

Q. Bombled, O. Verlinden

Department of Theoretical Mechanics, Dynamics and Vibrations (TMDV), University of Mons

Place du Parc 20, 7000 Mons, Belgium

M. Bagein, P. Manneback

Department of Computer Science, University of Mons, Place du Parc 20, 7000 Mons, Belgium

Keywords:

Distributed control, Real-time Linux real-time, PIC microcontrollers, UDP/IP communication, Walking robot.

Abstract:

A decentralized control architecture has been developed on the walking machine AMRU5. The vehicle is

actuated by 18 DC motors, which have to be highly synchronized to produce a smooth motion of the robot

body. Each leg has 3 motors driven by a microcontroller. The six microcontrollers communicate with a PC

running real-time Linux which manages the feet motion generation to produce the desired gait. The complete

control chain has been developed using standard freely available C tools.

1 INTRODUCTION

Data ﬂows between sensors and controllers is a cru-

cial point in robotics which is solved by using ter-

rain buses such as the ISA (Gonzalez de Santos

et al., 2005) or the CAN bus (Berns et al., 1999).

But, a general trends nowadays is the development

of open source and free frameworks, which use the

widely spread ethernet support. Several collaborative

projects are still growing, taking advantages of the

TCP/IP or UDP/IP protocols: see for example YARP

(Metta et al., 2006), Player (Gerkey et al., 2001) or

ROS (Quigley et al., 2009).

Following this trend, we propose here a very sim-

ple but efﬁcient control architecture for walking ma-

chines actuated by DC motors. It is based on a clas-

sical master-slaves relationship: the master is a per-

sonal computer responsible for gait generation algo-

rithm and motors position control. The slaves are

PIC-based boards which simultaneously drives three

motors. Moreover, the currents and voltages are mea-

sured and sent back to the master which saves them

as a data logger. An interesting particularity of this

work is that all the exploited devices used are cheap,

and usable with free development tools.

Master and slaves are presented in Section 2. The

data synchronization and some results are provided in

Section 3. Conclusion is given in Section 4.

2 MASTER AND SLAVE BOARD

AMRU5 is a six-legged robot with hexagonal conﬁg-

uration. It weights about 34kg and its outer diameter

varies from 1.2 to 1.6m. Each leg is actuated by 3 DC

motors equipped with incremental encoders. The dis-

tributed control architecture is highligted in Figure 1.

Each of the six embedded slave boards drives three

joints. Moreover it measures the DC motors supply

voltages, the rotor shafts positions and the motor cur-

rents.

Their synchronization is assumed by the master

PC which computes in real-time the target position

that each foot has to reach to produce the desired gait

(detail about gait generation are out of the scope of

this article). Master and slaves communicate with the

UDP/IP protocol over 10Mbit/s local network.

2.1 The Master

The master is a PC (3GHz, 1.5Go RAM) running

Linux 2.6.31-11 preemptive real-time kernel. The

277

Bombled Q., Verlinden O., Bagein M. and Manneback P..

A SIMPLE AND EFFICIENT CONTROL ARCHITECTURE FOR WALKING ROBOTS - Application to AMRU5.

DOI: 10.5220/0003404802770280

In Proceedings of the 8th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2011), pages 277-280

ISBN: 978-989-8425-75-1

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

Master datagram

, ..., u

, u

, i

, u

mast

... ...

3 × 60

0 5

Figure 1: Global overview of the control architecture.

control loop is rated at 10ms by means of the C-ANSI

select()

function (details are given in Section 3).

When all the data from slaves are received, the gait

algorithm computes the next 18 joint target positions.

Each of them enters in a Proportional Integral (PI)

control loop, calculating the new reference voltage to

be applied to the corresponding motor.

One speciﬁcity of this architecture is that com-

mand is computed on the master, letting ﬂexibility

during the development phase because position con-

trollers are implemented directly on the master in-

stead of re-programming the six slaves. In return

the communication must be very reliable, because the

controllers require the updated position of each joint

for control calculations.

2.2 The Slave

The three DC motors of each leg are commanded

by a SBC65EC from Modtronix

. This single board

computer with Ethernet capabilities is based on a

PIC18F6627 running at up to 40MHz. A TCP/IP

open-source stack written for the Microchip C18

LITE compiler is freely delivered by the manufac-

http://www.modtronix.com

turer. This stack naturally includes the UDP/IP pro-

tocol, and classical socket deﬁnitions similar to the

C-ANSI standard.

Basically, the slave board has four tasks to per-

form within one control time slice: 1) manage local

network communication, 2) collect continuously po-

sitions of the three motor shafts, 3) drive the three

motors by applying the received command, 4) per-

form A/D conversions of three motor currents and the

board voltage supply. The systematic data logging

helps us to validate accurately a dynamic model es-

tablished in (Bombled and Verlinden, 2009). Tasks 1)

and 3) and 4) require less than 1.5ms to be completed.

3 DATA FLOW

3.1 Data Synchronization

The UDP/IP Communication protocol has been cho-

sen to meet real-time requirements. Data integrity

over local network and low latency are reached, de-

spite of the unsecured aspect of UDP, by dedicating

the network to one and only one protocol detailed

hereafter. And, even if risk of data losses exists, the

ICINCO 2011 - 8th International Conference on Informatics in Control, Automation and Robotics

278

Slaves are working

Master is waiting

Slaves are polling

gait algorithm

...

t ≈ 1

∆t

≈ 1

Figure 2: Communication time diagram.

update of the next control cycle should compensate

the previous losses.

The master imposes the control rhythm by broad-

casting every 10ms a UDP global packet containing

the 18 actuator commands. The broadcast ensures a

simultaneous reception of this packet by all the slaves,

who are ready to listen data on a speciﬁed port. Each

slave collects its own three actuator commands from

the broadcasted packet.

The master bases its emission-reception cycles on

the

select()

C standard function. This blocking

function returns in two events: when data are incom-

ing on the socket or when a timeout occurs in the case

of no network activity. The timeout is set to 10ms at

the beginning of the control cycle and is updated each

time data are received on the master port. Each slave

returns data to the master in order, at time t

+ i· ∆t

where i = 1...6 is the number of the i

slave. The

time diagram of the communication process is shown

in Figure 2.

The following pseudo-code describes the master

cycle:

deadline = now() + delay; // delay of 10ms

while(robot_has_to_move())

{

if(slave_cnt == 6)

{ // all slaves data received

targets = compute_motor_command();

slave_cnt = 0;

}

switch(select(socket,deadline-now()))

{

case -1: // network error

return(error);

case 0: // timeout is over

broadcast(targets);

deadline = deadline + delay;

break;

case 1:

data_from_slave[slave_cnt] =

= receive(socket);

slave_cnt = slave_cnt + 1;

break;

}

Note that the CPU load of each slave is a func-

tion of pulse frequency from the encoders, which in-

creases with the motor velocity. Consequence is that

the real slave data emission is slightly delayed. This

variation is noted ∆t

in Figure 2.

3.2 Results

A tripod gait with a leg cycle period of 15s has been

implemented on AMRU5. Figure 3 is a plot repre-

senting the arrival time on the master, of the data sent

by the six slaves (numbered

→

), inside the 10ms

of control time slice, for a gait during 30s. Time 0 on

the Y-axis corresponds to the emission of the broad-

cast packet. In this example, t

and ∆t

have been

ﬁxed to 1.5 and 0.7ms respectively.

First data incomes at 3.5ms on the master, because

A SIMPLE AND EFFICIENT CONTROL ARCHITECTURE FOR WALKING ROBOTS - Application to AMRU5

279

Gait algorithm and PI control

Send broadcast



Time (s)

Reception time from the slaves (ms)

302520151050

Figure 3: Reception times of data coming from slaves.

a propagation delay of 1 ms has been recorded on the

Ethernet network (Figure 2). The network propaga-

tion delay between master and slaves mainly comes

from the switch.

The varying load on the slave CPU is visible on

the receiving times. But in any case, the reception

is well ordered, and the time remaining for the gait

algorithm and control computations is sufﬁcient (8.5

- 10ms).

4 CONCLUSIONS

A distributed control architecture has been developed

for a hexapod robot. The slave element controls a

leg at position level: the PIC-based board allows easy

C programming of the control tasks, and is provided

with an open-source TCP/IP stack compatible with

the C18 Microchip compiler. The master is a PC run-

ning real-time Linux kernel on which an application

has been developed in C-ANSI for leg motion gen-

eration and data synchronization. The resulting con-

trol architecture is very ﬂexible and has several ad-

vantages:

• The slaves have a generic program: they only

drive three motors and collect real-world data.

This avoids several re-programmings during the

development as it had been the case with a po-

sition controller directly implemented on them.

They just have a speciﬁc identiﬁcation number,

which determines their IP address and data they

have to read from the master global packet.

• Any kind of controller and/or gait algorithm could

be implemented on the master, provided that the

computation is fast enough to stay inside the con-

trol time slice. For example, see (Bombled and

Verlinden, 2010) for a ground detection algorithm

from motors currents sensing.

• Communication hardware is made from widely

spread and inexpensive on the shelf materials,

namely: a network switch, an Ethernet adapter,

and microcontroller boards.

ACKNOWLEDGEMENTS

Many thanks to Prof. Y. Baudoin from the Royal Mili-

tary Academy of Belgium for the lending of AMRU5.

Authors are also grateful to the technical staff of the

TMDV Department: R. Berton and K. Nis.

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