roboBAN: A Wireless Body Area Network for Autonomous Robots

Lukas Gisin, Hans Dermot Doran and Juan-Mario Gruber

Institute of Embedded Systems, Zurich University of Applied Sciences, Technikum Str. 9, 8401 Winterthur, Switzerland

Keywords: Wireless Body Area Network (WBAN), Wireless Sensor Network (WSN), Ultra-Low Power (ULP), Energy

Harvesting, Autonomous Robots.

Abstract: In this paper we describe a Wireless Body Area Network (WBAN) designed and implemented for an

autonomous robot. We concentrate in particular on the ultra-low power radio aspects. We compare and

contrast standard and proprietary solutions before deciding on a proprietary radio SoC in the 2.4 GHz. band.

We describe the protocol implementation and tests on both a test jig and on the robot itself at sampling rates

of up to 1 kHz. and conclude that the principle of a robot based WBAN works well. We also show that the

proposed WBAN is suitable for connection by nodes employing energy harvesting.

1 INTRODUCTION

1.1 Motivation

Much work has gone into the research and

development of autonomous robots. The focus of

this research effort is to understand how to build

such robots and get them functioning in real-world

use cases. There are also considerable engineering

challenges to be surmounted to get such robots ready

for market including issues such as power and

weight distribution. If we take the use-case of a

payload carrying robot then the robot needs to be

aware of the dynamics of the payload which requires

communication between the payload, or its harness,

and the robot. For reasons of security, compatibility

and bandwidth, this communication should not be

carried over the in-robot control network but must

be carried over a separate channel. Any power the

payload may require, including that needed to

communicate with the robot, should not be sourced

from the robot. The communication network must

reflect that fact.

In this body of work we propose a Wireless

Body Area Network (WBAN) as a communication

network for such ancillary information providers,

although we use a system critical component as the

first participant. As a second novelty we follow an

aggressive power policy by stipulating that the nodes

find their own power through energy harvesting

techniques. In this paper we restrict ourselves to

describing the networking aspects, physical and data

link layer, of the proposed WBAN.

The remainder of this paper is structured

accordingly: We end this chapter by introducing the

target application. In the next section we discuss

issues around WBAN and the application and

describe state of the art radio devices for the typical

WBAN protocols before describing the concept we

decided on. In Section 3 we discuss the results of the

measurements on both a test jig and on the target

robot and we finish, in the final section, with

conclusions and further work.

1.2 Target Application

The target application for this work is the quadruped

StarlETH (Hutter 2012) developed at the

Autonomous System Lab of the Swiss Federal

Institute of Technology, ETH Zürich. This medium-

dog-sized robot, Figure 1, achieves stable

locomotion especially on rough terrains where other

humanoid or wheel-based robots have difficulties.

StarlETH weighs about 25 kg, can carry a payload

of up to 25 kg and has a maximum speed of > 2

km/h. Energy autonomy of approx. 1 hour is

currently reached at an average power consumption

of 300 W (ASL, 2015.) The StarlETH is a research

object; a further version is currently undergoing

advanced development for use in such environments

as oil rigs or in copper mines as an inspection robot.

Gisin, L., Doran, H. and Gruber, J-M.

roboBAN: A Wireless Body Area Network for Autonomous Robots.

DOI: 10.5220/0005968700490060

In Proceedings of the 13th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2016) - Volume 2, pages 49-60

ISBN: 978-989-758-198-4

Figure 1: Quadruped robot StarlETH with 3D force

sensors in the foot.

To enable adaptive motion control each foot is

equipped with a high precision optical three axis

(3D) force sensor (Opto Force, 2015.) The sensor is

installed within a robust rubber sphere enclosure to

sense contact force, detect ground contact and, at the

same time, dampen the impact of the foot hitting the

ground. Currently only the binary signal denoting

ground-contact as true or false is used but the sensor

was built into the robot with the idea of detecting the

microstructure of the terrain in future work. The

sampling rate is that of the force control loop, 1 kHz.

The force sensor is connected to the motion

controller by a cable with 5 wires. The track of the

cable can be seen in Figure 1. This cable, and its

connectors, is subjected to considerable mechanical

stress during normal operation and, in an

industrialised version, the cable placement would

need more careful consideration; the cable type in all

probability replacement and the connector would

also require re-specification. Experience from the

automotive industry shows that robust connectors

are preferred which generally take up considerable

space, although optimised for weight, whereas in

this application both space and weight are at a

premium. Equally cables tend to restrict movement

so were this sensor to be replaced with a wireless

sensor fewer restrictions would be placed on leg

flexibility. Cognizant of the problems associated

with the use of wireless transmissions we chose the

force sensor as the first target for integration into the

WBAN.

We wish to ensure that the WBAN and its

attachments do not burden the robots power budget.

If attachments provide their own power through

batteries then weight is added and maintenance

(battery replacement) is increased. Energy

harvesting is a technique that offers alleviation.

Common energy sources for harvesting are ambient

light, thermal energy and vibration/motion. These

sources allow harvesting from 10 uW to 100 uW. An

additional possible energy source is the weight of

the robot. For example commercially available

energy harvesting switches (push buttons) generate

> 300 uJ out of a single actuation with a force of 13

N (Cherry 2015). The nominal weight of the

unloaded robot being 25 kg, a contact force of ≥ 60

N is applied by a foot making contact with the

ground. Extrapolating the performance of the

commercially available switch, > 1000 uJ could

conceivably be generated.

The force sensor and associated data acquisition

unit (DAQ) have an average power consumption of

340 mW, out of scope of energy harvesting units.

We therefore leave the development of an

appropriate replacement sensor to further work.

2 WBAN TECHNOLOGIES AND

CONSIDERATIONS

2.1 Wireless Body Area Networks

Wireless Body Area Networks are networks of

wearable computing devices and fall into the

category of Wireless Personal Area Networks

(WPAN) which again can be seen as a subset of a

Wireless Local Area Network (WLAN)

(Movassaghi, 2014).

WBANs are often associated with medical

applications, both professional and patient

welfare/fitness, where the system architecture

consists of one central device which receives and

processes data from several wireless sensor nodes in

close vicinity. In WBAN applications the devices

are generally accessible so batteries can be replaced

with ease – despite of which the device client

expects battery life of one to two years – except of

course for medical implants which are difficult to re-

charge. More detailed discussions may be found in

(Movassaghi, 2014, Chen, 2011, Cao, 2009).

2.2 General Considerations for the

Proposed WBAN

2.2.1 Power Savings

Power savings are dependent on the selection of an

appropriate wireless protocol, radio device and on

eliminating unnecessary losses. In (Ullah, 2010)

potential sources of energy waste are briefly

summarised and their impact considered in the

following sections. In order to facilitate the use of

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics

energy harvesting techniques, ultra-low power

(ULP) radio devices must be used.

Table 1: Summary of potential sources of energy waste

(Ullah, 2010).

Source Description

Collisions

Multiple transmitters accessing the channel at

the same time cause packet collisions and

therefore power losses if packets are re-

transmitted.

Overhearing

Receiving packets that are destined to other

nodes and hence must be discarded cause

power losses due to unnecessary active packet

reception.

Idle listening

Listening to an idle channel when no nodes

are transmitting causes power loss due to an

active RF receiver.

Overhead

Additional network control packets from

MAC and upper layer protocols cause power

losses due to additional data transmission and

reception.

Contention

Contention-based protocols add significant

protocol overhead increasing the time a node

requires to transmit its data. Collisions still

occur at the beginning which again increases

power loss at all competing nodes.

Routing

In multi-hop communication the nodes must

relay packets from neighboured nodes to the

destination which increases power loss due to

additional reception and retransmission to

forward packets.

2.2.2 Duty Cycle

One of the key parameters for ULP wireless nodes is

the duty cycle i.e. the ratio between the active-

processing time of a node with respect to the energy-

conserving sleeping time; a low duty cycle is

desirable. The motion control algorithm currently

samples the force sensor every 1 ms therefore the

data acquisition, processing and transmission needs

to be fast enough to ensure that sleep times are of

useful duration. To support concurrent operation of

multiple sensor nodes the transmitted packet must

have needs to be considerably shorter in on-air time

than the interval time.

2.2.3 Reliable Links

As the force sensor data is used in the control loop

of the robot the integrity of control is sensitive to

missing data caused by transmission errors so a

reliable radio link is important to minimise this

source of data loss which might cause the robot to

miss-tread, stop or, in the worst case scenario, tip

over. The WBAN is intended to extend only over the

surface area of the robot but several robots may be

operated concurrently in close proximity which may

lead to cross-interference. Cross-interference and

other EMI-based disturbances may occur randomly

or even permanently and will affect the reliability of

the radio link in terms of increased error rate or even

link loss.

2.2.4 Flexibility

To benefit from the WBAN other sensors like

temperature, position, acceleration or “skin contact”

sensors must be easy to interface. Therefore the

proposed WBAN concept should not be restricted to

fast and interval-based sensors but also support

irregular event-based data transmission and low duty

cycles.

2.2.5 Security Aspects

The security of the WBAN’s is considered vital but

security of wireless networks is a complex discipline

so any radio device should have intrinsic support for

identification, encryption and authentication.

2.2.6 Channel Access Scheme

Various methods such as Frequency or Time

Division Multiple Access (FDMA resp. TDMA) can

be used to access the wireless channel. With FDMA

each node in the network is assigned its own channel

(frequency) which imposes an intrinsic limit on the

number of nodes in the network depending on the

used frequency band and channel bandwidth (e.g. in

2.4 GHz ISM band 83 or 41 channels are available

with a bandwidth of 1 MHz or 2 MHz respectively).

In terms of reliability, if a particular channel is

constantly disturbed then individual nodes may not

be accessible.

TDMA is more flexible in terms of network

scalability and data rate as the number of time slots

and the duration of a timeslot allocated for a

particular node can be adapted. Complexity is

increased as scheduling or contention based methods

must to be implemented to ensure media access

fairness. Scheduling requires the nodes to regularly

wake-up and listen to a network coordinator for

synchronisation while contention-based TDMA

requires collision detection (CD) and/or avoidance

(CA) methods. Both have major drawbacks in terms

of power consumption.

A combined method called frequency hopping

(FH) is used in Bluetooth (IEEE 2005). Here a node

hops between channels based on some function of

time. This method also requires synchronisation so

that all nodes in the network use the same channel

sequence and the correct channel at a given time.

Advertising is used in Bluetooth Low Energy

roboBAN: A Wireless Body Area Network for Autonomous Robots

(BLE - BSIG 2015) but is not explicitly a channel

access scheme. Ignoring the activities of other nodes

a sensor node transmits its data, waits for

acknowledgement and then sleeps until the next

transmission. Consider the scenario of a star

topology of several sensor nodes in close vicinity.

Transmission success due to collisions will be

probabilistic but the energy saving potential on the

part of the sensor nodes is considerable due the

shoot first philosophy, there is no overhead due to

synchronisation contention resolution, protocol

overhead, routing, limited overhearing and idle

listening only occurs at the central receiving device.

2.3 Communication Protocols

There are four protocol options for the proposed

WBAN, the three standard protocols for constrained

devices, IEEE 802.15.4 (IEEE 2011) / ZigBee (IEEE

802.15.4,) Bluetooth (IEEE 2005) and its derivatives

(BSIG 2015), IEEE 802.15.6 (WBAN) and the use

of a proprietary protocol. The standardised protocols

are well known and detailed information can be

gained from the references. Here we just offer Table

4 for comparison of the general characteristics and

consider only the state of research as regards

available radio devices.

2.3.1 Standardised Protocols

In the IEEE 802.15.4 & ZigBee domain currently

available compliant radio devices offer a transmitter

and receiver power consumption of approx. 30 mW

(0 dBm) and 40 mW respectively (Texas, 2013). In

2012 the IEEE Task Group TG15.4q was formed to

draft an alternative ultra-low power physical layer

operating at a power level of less than 15 mW (IEEE

2013). IEEE 802.15.4q has not yet been released.

One proposed radio device from Samsung (Bynam

2013) achieves a transmitter and receiver power

consumption of 5.3 mW (-5 dBm) and 3.2 mW

respectively.

In the Bluetooth Low Energy domain there are

currently no radio devices available but reports from

research projects such as the icyTRX from Centre

Suisse d’Electronique et de Microtechnique (CSEM)

(Raemy, 2014) a transmitter and receiver power

consumption of 9 mW (0 dBm) and 6 mW

respectively is achieved.

2.3.2 Proprietary Protocols

In order to assure inter-compatibility and adequate

scalability, standardised protocols require a certain

overhead which can be quite substantial. In order to

fill the market niche for faster and simpler WBANs

several proprietary solutions have been developed.

Insteon (Insteon 2005), Zarlink (Microsemi 2013),

Z-Wave (Sigma 2013) or ANT (Dynastream 2014)

find frequent mention in the literature. Quite often

the radio device manufacturer will also supply a

proprietary protocol supporting reduced power

usage; examples include the Enhanced Shock Burst

(ESB) basic protocol (Nordic 2015a) and Gazell link

layer protocol (Nordic 2015b).

2.4 Proposed Concept

Since none of the standardised protocols offer the

low power required by this application it was

necessary to rely on a proprietary solution and so a

WBAN concept inspired by Bluetooth Low Energy

was chosen and further developed. The proposed

WBAN concept and underlying evaluations will be

introduced in this section. A comparison of key

parameters of the proposal and Bluetooth LE can be

found in the appendix.

2.4.1 Frequency Band & Wireless Protocol

Global license free availability and an impressive

range of available devices in the 2.4 GHz ISM

frequency band makes its selection almost a

foregone conclusion. Unfortunately the considerable

on-air packet duration and low update rates (of

several ms) make Bluetooth Low Energy, IEEE

802.15.4 (ZigBee) and IEEE 802.15.6 (NB)

unsuitable for this application which means that a

proprietary solution must be used.

2.4.2 Radio Device

At the time of analysis the SoC device nRF52832

from Nordic Semiconductors (Nordic 2015c) notes

the lowest power consumption and supports

proprietary protocols based on the efficient

Enhanced Shock Burst (ESB) packet format, a high

data rate and very short on-air packet duration.

2.4.3 Channel Access

The advertising method is chosen to transmit the

data on three different frequencies (advertising

channels). Combined with three RF receivers each

of them listening on one of the advertising channels

three redundant radio links can be established.

Packets received by the three RF receivers having a

valid CRC (others are discarded) are passed through

a packet filter which rejects duplicated packets. With

this method the overall reliability of the radio link

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics

can be significantly increased even when a channel

is distorted or packet collisions occur. For added

robustness the three frequencies have been selected

such that they do not interfere with the widely used

WLAN channels 1, 6 and 11 nor overlap with the

BLE advertising channels.

2.4.4 Power Savings

The advertising proceeds in a shoot and forget

manner, the nodes do not wait for an acknowledge

(ACK) from the receiver and it follows that nodes

need not support power-expensive Rx. Whilst ACK-

less operation may lead to undetected data loss, real

time data has low temporal validity so at high update

rates packet retransmission is both unfeasible and

pointless. Upper control layers, as is typical of any

real-time system, need to be robust against minor

data inconsistencies. Other nodes with lower update

rates are not prevented from utilising an ACK signal.

2.4.5 Packet Format

A slim packet structure as shown in Figure 2 is used

to transmit the 3D force sensor data. For test

purposes a 32-bit packet sequence counter value has

been added so receivers may detect if packets have

been lost. This is configurable per software and

disabling reduces the packet length to 15 bytes i.e.

60 us (on air). The content of the packet briefly

described in Table 2.

Figure 2: Packet structure of the proposed WBAN

concept.

Table 2: Elements of the proposed packet structure.

Parameter Description

Preamble 0x55 or 0xAA (depends on first data bit)

Network Address

address of the WBAN (currently

0x4749534E)

CRC CRC-16 polynomial x

+ x

+ 1

Sequence Number

32-bit TX packet counter (for test purpose

only)

Force Fx, Fy, Fz 3D force vector values (signed)

Node Address

256 nodes can be addresses (within one

WBAN)

Node Error

= 1 if any Data, DAQ or Sensor Error is

present

ACK request = 1 if node requests an acknowledgement

Data Error

= 1 if payload data are invalid (for test

purpose)

DAQ Error

= 1 if the sensor data acquisition unit has

an error

Sensor Error = 1 if the sensor itself has an error

Overload (X, Y, Z)

= 1 if sensor is overloaded in X, Y or Z-

axis

2.4.6 Reliability

When multiple nodes are advertising at short interval

times the medium is heavily utilised and collisions

will happen regularly. Bluetooth LE uses its three

advertising channels in the same order, i.e. 1-2-3 so

packet collisions due to concurrent transmission will

affect all three channels equally which leads to the

loss of data from all concurrently transmitting nodes.

Our concept implements a random channel sequence

where one out of six different channel orders (1-2-3,

1-3-2, 2-1-3, 2-3-1, 3-1-2 or 3-2-1) is randomly

selected prior to advertising. A rudimentary

simulation using Microsoft Excel indicated that a

random channel sequence could outperform a fixed

channel sequence by a factor of 4.

2.4.7 Flexibility

The receivers do not recognise packets with invalid

network address but can be configured to listen on

up to 8 different networks. Multiple logical networks

can be instantiated on the robot for different services

e.g. control related data and ambient information.

Each node in the WBAN is further identified with an

8-bit node address so each logical network can

manage up to 256 nodes.

2.4.8 Security Aspects

The radio device supports 128 bit AES encryption

and authentication features. While authentication is

not implemented in this proof-of-concept it is

supported for later use. We expect encryption, and

especially pairing during authentication will make a

significant addition to the power budget.

3 PROOF OF CONCEPT

3.1 Software Application

The software is organised as an interrupt triggered

state machine. The state diagram of the wireless

sensor nodes in Figure 3 describes a simple

MAC PHY

Preamble NetworkAddr. Payload CRC

1byte 4bytes 12bytes 2bytes

Header Seq.Nr. ForceFx ForceFy ForceFz

2bytes 4bytes 2bytes 2bytes 2bytes

NodeAddr. NodeError ACKrequest DataError DAQError SensorError Overload(X,Y,Z)

8bits 1bit 1bit 1bit 1bit 1bit 3bits

PHY

Payload

Header

19bytes at2Mbps=76us(onair)

roboBAN: A Wireless Body Area Network for Autonomous Robots

application that transitions from one state to another,

including a sleep state, in a cyclic fashion.

Figure 3: State diagram of the wireless sensor nodes for

interval-based transmission.

3.2 Demo Setup

3.2.1 Implementation

For the evaluation of the proposed WBAN concept a

test-jig was built to imitate the metallic construction

of the robot’s legs, joints and the solid body

enclosure and so simulate the operating conditions

as close to the target application as possible (see

Figure 4 and Figure 5).

Figure 4: Demo setup of the robot test-jig.

3.2.2 3D Force Sensor

The 3D force sensor was delivered with a separate

data acquisition unit (DAQ) for converting the

analog sensor signals to a digital format, calculating

the force signal and making the data accessible over

SPI. The mounting of this DAQ is shown in Figure

3.2.3 Radio Transmitter (TX)

The brand new nRF52832 was only available as a

pre-release development board (PCA10036) for

engineering purposes. The development board and

the DAQ are powered over the USB interface (see

Figure 6). The orientation of the TX antenna was

considered when mounting the development board

on the robot leg which was mounted such that the

radiation pattern of the antenna is at right angles to

the leg (in Figure 5 the antenna is parallel to the

indicated edge of the PCB).

Figure 5: Flexible leg construction with the 3D force

sensor as foot and the nRF52 development board as

wireless sensor node.

3.2.4 Radio Receiver (RX)

Acting as receivers for the three advertising channels

were three small and lightweight nRF51822 USB

dongles (PCA10000) from Nordic Semiconductors.

These were inserted directly into a USB hub directly

connected to a test computer for data exchange and

voltage supply (see Figure 6.) The dongles listen to

one of the advertising channels each, buffer the

received data to the internal flash and transmit them

to the computer.

Figure 6: Diagram of the wireless sensor node

implementation.

The receiving nRF51 dongles exhibit a radiation

pattern similar to the transmitters, but weaker so, to

avoid cross-polarisation, the same antenna/PCB

orientation is chosen. Mounted on the middle of the

INIT

IDLE

START

RNG

READ

SENSOR

WAIT FOR

SENSOR

PROCESS

DATA

SEND

DATA

PREPARE

SLEEP

WAKE UP

ERROR

Power

OFF

(reset)

peripherals & sensor

initialised

more random

numbers

required

>2 random

numbers

available

RNG

started

sensor

readout

triggered

readout

completed

radio packet

ready

transmission

completed

(advertising < 3)

transmission

triggered

sleep mode

triggered

sleep timer

timeout

context

restored

any unexpected

error

error handling

defined next state

transmission

completed

(advertising = 3)

WAIT FOR

RADIO

Sensor

Node

Analog signals

& sensor supply

Data acquisition

unit at 1 ksps

SPI at

8 Mbps

nRF52832

at 64 MHz

2.4 GHz radio

at 2 Mbps

3D force

sensor

+5V USB supply

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics

“body” of the robot (Figure 4), the aluminium base

plate of the test jig, they are elevated about 25 mm

from the plate.

3.3 Measurement Results

This sub-section describes measurement results of

experiments conducted using the test-jig.

3.3.1 Preliminary Measurement

Before commencing measuremens on the test jig an

indoor line of sight (LoS) measurement was

performed. With one wireless sensor node

transmitting at 0 dBm it could be shown that, up to a

receiver signal of -60 dBm, individual packet error

rates below 0.1 % could be achieved (see Figure 7.)

This figure could be used as an indicator to adjust

the TX power in real-world applications on the robot

so that the receiver signal is stronger than -60 dBm.

3.3.2 3D Force Sensor

An unexpected issue was found in the

implementation of the DAQ. Sensor data is

packaged in 16 bytes with 6 bytes payload and is

clocked out of a ring buffer implemented such that it

is necessary to read the complete ring of 64 bytes.

CPU time and power is subsequently wasted in

finding and extracting the data packet. In addition

the sensor packet was occasionally incompletely

placed in the 64 bytes data block which resulted in a

negative effect on the packet error rate (PER).

Figure 7: Indoor LoS evaluation of RSSI vs. PER

(transmitter at variable distance between 0m and 3m

sending at 0 dBm).

3.3.3 PCB Antennas

First measurements showed that the received signal

at the USB dongles was suspiciously small even

when positioned close to the wireless sensor node

transmitting at 0 dBm. Comparative measurements

with the nRF52 development board and the nRF51

dongles showed that while the RF front-end and

antenna losses of the nRF52 development board (TX

node) was evaluated to be around 7 dB, the nRF51

dongles (RX node) performed approx. 20 dB worse

i.e. losses of 25 dB to 28 dB were measured. The

reason for this is assumed to be the meander

monopole antenna of the small nRF51 dongles

which is small compared to the inverted L PCB

antenna of the nRF52 development board.

3.3.4 Power Consumption of the Wireless

Sensor Node

The power consumption measurements of the

wireless sensor node yielded excellent results which

indicate that power supply from energy harvesting is

viable for the node.

For transmissions at 0 dBm the nRF52832

requires a supply voltage of 3.0 V and consumes 6.9

mA during transmission which results in a TX

power consumption of around 21 mW. The total

energy used during the active state of duration 493

us is approximately 8.6 uJ.

For transmissions at 0 dBm the nRF52832

consumes 5.2 mA during transmission which results

in a TX power consumption of about 16 mW. The

total energy used during the active state of 493 us is

approximately 6.9 uJ.

The random advertising delay affects the

duration of the sleep state but not the active state.

The smallest possible interval times are between

1000 us and 1255 us.

During the shortest sleep state of approx. 500 us

to 760 us around 0.7 uJ to 1.1 uJ is used at an

average power of around 1.4 mW corresponding to

an average current of 0.5 mA at 3.0 V supply (with

DC/DC converter enabled.)

If the DC/DC converter is disabled during the

sleep state the average current drawn from the 3.0 V

supply will increase to 0.7 mA which yields an

average power of around 2.1 mW and therefore an

energy requirement of 1.1 uJ to 1.6 uJ for the same

sleep time.

The unused peripherals on the SoC are

permanently disabled and on entering the sleep state

the CPU is suspended to be woken by a sleep timer

interrupt. The SoC device delivered by the

distributer was a beta engineering sample where the

system OFF mode of the device was not operational

(Nordic, 2015d). With this operational and,

according to the product specification (Nordic

2015c), a power consumption of around 3 uW i.e. 1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

‐70 ‐65 ‐60 ‐55 ‐50 ‐45 ‐40 ‐35 ‐30

PER[%]

RSSI[dBm]

IndoorLineofSightPacketErrorRate

PER[%]atRX1 PER[%]atRX2 PER[%]atRX3

PERborder

roboBAN: A Wireless Body Area Network for Autonomous Robots

uA at 3.0 V can be expected including the real time

clock (RTC) to wake-up the device (DC/DC

converter disabled.)

3.3.5 Radio Link Quality

A core quality criterion of the WBAN is the packet

error rate (PER) so some care was taken to evaluate

this. We distinguish between “individual PER,” the

PER of each individual channel and the “overall

PER,” the PER of the individual node. For each

node three individual PERs (each advertising

channel) were measured as well as the four overall

PER values.

The test jig was in an indoor laboratory

environment where many wireless devices in the 2.4

GHz ISM frequency band (WLAN, Bluetooth LE)

were operated in the close vicinity to better show

operation in “real-world” conditions. Several

measurement cycles were performed to establish the

PER at cycle times of 1, 10 and 100 ms and TX

power at 0 dB and -12 dB.

Figure 8 shows the PER for a cycle time of 1 ms.

@ 0 dBm and the random channel sequencer. The

individual PER, at 20% to 40% is quite large, even

considering the ammount of traffic and that the

nodes are not synchronised to each other, but the

overall PER for the sensor nodes is only 5-10%,

largely due to the proposed random channel

sequence.

To cross check the performance of random

channel sequencing the test was repeated with the

exception that the random channel sequencer was

disabled and a fixed channel sequence used instead.

The results are shown in Figure 9. The individual

PER is slightly larger but the overall PER increases

drastically and lies between 15 % and 35 % which is

a direct result of the fact that packets from randomly

synchronised nodes collide on all three channels

with the result that all data is lost. The average value

of the overall PER for fixed channel sequence is

25.6 % compared to 7.3 % for random channel

sequence which is a factor 3.5 higher. This is close

to the factor of arround 4 derived from the

simulation (Section 2.4.6).

The first experiment was repeated with interval

times of 10 ms and 100 ms, the results are shown in

Figure 10 and Figure 11 respectivly where it can be

seen that both the individual and overall PER

decreases substantially with increasing interval time

confirming that packet collisions are less likely to

occur sporadically.

Figure 8: PER evaluation based on 60’000 packets from

four nodes transmitting with 0 dBm at an interval time of

1 ms. Random advertising delay and random channel

sequence are enabled. The “robot” is in standing position

i.e. the legs are not moved.

Figure 9: PER evaluation based on 60’000 packets from

four nodes transmitting with 0 dBm at an interval time of

1 ms. Random advertising delay is enabled but random

channel sequence is disabled (fixed channel sequence is

applied). The “robot” is in standing position. i.e. the legs

are not moved.

Figure 10: PER evaluation based on 60’000 packets from

four nodes transmitting with 0 dBm at an interval time of

10 ms. Random advertising delay and random channel

sequence are enabled. The “robot” is in standing position.

A most interesting effect can be observed when

the four legs of the robot test jig are moved in a

walking pattern. The “walking” robot performs

better than when “standing” which could be

Occurrence[#]

PacketErrorRate[%]

PERat0dBmand1msIntervalTime

OverallPER

IndividualPER

Occurrence[#]

PacketErrorRate[%]

PERat0dBmand1msIntervalTime

(fixedchannelsequence)

OverallPER

IndividualPER

Occurrence[#]

PacketErrorRate[%]

PERat0dBmand10msIntervalTime

OverallPER

IndividualPER

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics

explained with multi-path propagation causing

position dependant channel fading (Figure 12).

Figure 11: PER evaluation based on 60’000 packets from

four nodes transmitting with 0 dBm at an interval time of

100 ms. Random advertising delay and random channel

sequence are enabled. The “robot” is in standing position.

Figure 12: PER evaluation based on 60’000 packets from

four nodes transmitting with 0 dBm at an interval time of

10 ms. Random advertising delay and random channel

sequence are enabled. The “robot” is in walking position

i.e. all four legs are moving with a “walking” pattern.

3.4 Assembly on StarlETH

roboBAN was assembled on the StarlETH, a

transmitter on each foot (Figure 13) whilst the array

of three USB-dongle receivers was mounted on the

bottom of the robot The robot was then made to

walk and two sets of measurements were made, the

first of the PER and the second of the force applied

by the robot leg to the floor.

The PER measurement turned out to be

problematic due to an issue with the Rx dongles.

This had the effect that measurements transmitted to

the PC via a UART->USB converter on the Rx

dongle were corrupted by lost bytes and framing

errors cause by poor buffer handling in the UART.

These additional errors could not be separated from

those occasioned by wireless interference and so

pressured the PER to an unknown amount.

Figure 13: StarlETH with transmitter boards taped to the

feet.

Figure 14: PER evaluation based on packets from four

nodes, mounted on the StarlETH, transmitting with 0 dBm

at an interval time of 10 ms. Random advertising delay

and random channel sequence are enabled. The robot is

walking.

The second set of measurements made was the

force applied on the foot of each robot during the

walking action. The purpose was to confirm the

amount of energy an energy harvester could

potentially capture during the walking activity. The

force applied to each leg was captured via the force

sensor on the foot of each leg and an example is

shown below in Figure 15. The force was not

absolutely measurable, the calibration information

for each of the four sensors is not available, but by

estimating the force and assuming that the sensor

membrane is compressed by 5 mm when the foot

hits the floor, a measure could be arrived at. We

estimate, for this foot, an average of approx. 500 mJ

harvestable energy/step. While we show only one

profile in Figure 15 the profiles of the force applied

to the other feet vary considerably from foot to foot

indicating an imbalance in the distribution of mass

across the robot. The results are only useful as a

general indicator as the robot is currently undergoing

Occurrence[#]

PacketErrorRate[%]

PERat0dBmand100msIntervalTime

OverallPER

IndividualPER

Occurrence[#]

PacketErrorRate[%]

PERat0dBm,and10msIntervalTime

("robot"iswalking)

IndividualPER

OverallPER

roboBAN: A Wireless Body Area Network for Autonomous Robots

a redesign so a final figure for the harvestable

energy will only be known at a later date.

4 CONCLUDING REMARKS

4.1 Contribution

The focus of this body of work lies on the

conceptual design of a WBAN employing ultra-low

power wireless sensor nodes for use of devices

mobile robots. It has been shown that the application

of the WBAN concept to a robot is feasible. It has

been shown that standardised technologies

conceived for the WBAN market are unsuitable for

this kind of application, especially if high data rate

sensors are to be integrated. It has also been shown

that the sensor interface can be built to ultra-low

power expectations such that power via energy

harvesting is feasible. This means that sensors can

be added to the WBAN without burdening the

robot’s power budget.

Figure 15: Estimated energy harvestable from front left

foot.

The packet size and hence the on-air time, could

be reduced to allow 1 ms sampling rates – a non-

trivial exercise for wireless systems. Using

redundant advertising channels assigned pseudo-

randomly has been shown to be effective in reducing

the packet error rate making our implementation a

superior solution to standard protocols like

Bluetooth LE with which a complete comparison is

given in Table 3.

4.2 Discussion

In this paper a new concept of a wireless body area

network (WBAN) applied to a quadruped robot has

been presented that shows sufficient potential to be

engineered into a working model of the robot. One

of the aims was to reduce the cabling requirements

for the robot. It has been shown that energy

requirements can, with careful engineering, be

reduced as to make energy harvesting a feasible

option and thus the requirement to minimise cabling

can be achieved. Wireless sensors in critical

machine parts are not without their problems. The

packet error rate of 5-10% is high so some work

needs to be done first on optimising the antennas,

the Rx antennas have been shown to be weak, and

for any remaining packet error rate it also needs to

be shown that the motion control loop can handle the

resultant data omission rate. Wireless

communication is notoriously fragile especially in

the face of jamming attacks and further

experimentation needs to establish to what extent the

operation of the WBAN can be maliciously

disrupted.

It might be considered unfortunate that a

standard protocol could not be found for what will

ultimately become a common issue, the interfacing

of third party data and information providers to a

mobile robot. A comparison of the proven design,

shown in Table 3, with Bluetooth LE shows that this

technology will simply never fulfil the requirements

that a robot WBAN poses so specification work is

required to ensure some form of interoperability

between application data providers and the robots.

The on-time of a sensor is just under 500 us, the on-

air time of its packet is just under 80 us which, at 1

ms sample time allows a theoretical maximum of 12

nodes, assuming no collisions, so while the practical

limits of high data rate nodes are close to being

reached there is ample space for a number of low-

data rate nodes, even those using ACKs, to ensure

transmissions are successful.

Estimates show that the weight of the robot is

sufficient to provide an energy source for powering

the wireless transmissions.

4.3 Outlook

Since it has been shown to be feasible that the

movement of the robot will generate enough

harvestable energy to power the connection to the

WBAN, the development of a suitable harvester and

force sensor would be appropriate next steps. Whilst

the half-sphere construction of the currently used

sensor allows some detection of micro-terrain

features one could speculate that a sensor based on

the features of a dogs paw might be more

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics

appropriate. Secondly a combined sensor/harvester,

that is a harvester whose harvested energy levels

also indicate the force applied, would be of

particular interest for this kind of application.

ACKNOWLEDGEMENTS

We are thankful to Erich Ruff and Tiziano Fabbroni,

both of InES, for their support. Further thanks go to

Mark Hoepflinger and Marco Hutter of the

Autonomous System Lab (ASL) of ETH Zürich for

the access to the StarlETH and to Ákos Tar and

Szabi Fekete from Opto Force for their helpful

discussions.

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roboBAN: A Wireless Body Area Network for Autonomous Robots

APPENDIX

Table 3: Comparison of Bluetooth Low Energy and the Proposed WBAN Concept.

Parameter Bluetooth Low Energy Proposed Method

Frequency band 2.4 GHz (ISM) 2.4 GHz (ISM)

Number of advertising channels 3 3

Frequency of advertising channels 2’402 , 2’426 , 2’480 MHz 2’407 , 2’422 , 2’474 MHz

Sequence of advertising channels Fixed (1 – 2 – 3) Random (6 possible combinations)

Minimum advertising interval 20 ms 1 ms

Random advertising delay 0 … 10 ms 0 … 255 us

On air data rate 2 Mbps 2 Mbps

Packet duration (10 byte payload) 160 us 64 us

Packet reception at receiver Successively scanning channels Parallel scanning of all channels

Packet acknowledgement Yes (optional)

Table 4: Comparison of discussed wireless protocols operating in the 2.4 ghz ISM frequency band.

Parameter Unit

IEEE 802.15.4

(w/o ZigBee)

Bluetooth

Low Energy

IEEE 802.15.6

Narrow Band

Proprietary

Nordic ESB

Proprietary

Nordic SB

Frequency Band MHz 2’400 (ISM) 2’400 (ISM) 2‘400 (ISM) 2’400 (ISM) 2’400 (ISM)

Modulation - O-QPSK GFSK π/4-DQPSK GFSK GFSK

Spreading Sequence - DSSS A-FHSS 1 non non

Gross data rate kbps 2000 1000 600 2000 2000

Information data rate kbps 250 1000 971.4 2000 2000

Maximum payload Byte 118 36 255 32 32

Packet payload Byte 10 (budgeted for packet size calculation)

Packet size Byte 25 20 34.125 17.125 16

Packet duration us 800 160 281 68.5 64

Min. cycle time

(no ACK)

 1.4

20…30 unknown

 0.2  0.2

Min. cycle time

(with ACK)

 2.2

20…30 unknown

 0.4

unknown

Error detection - FCS CRC FCS CRC CRC

Error correction - Yes No Yes No No

Radio device availability - good good poor good good

ICINCO 2016 - 13th International Conference on Informatics in Control, Automation and Robotics