The Siesta Datalogger

In previous years, Davies and McGowan built their own data-loggers from scratch [3,4]. Through connections with Dr. Brown, the Australian company Compumedics^5.1 kindly supplied a mobile digital datalogger called the Siesta, shown in Figure 5.1. This product was undergoing final stages of development prior to being released and marketed as a commercial medical datalogger intended for use in sleep diagnosis. Projects such as this one supervised by Dr. Brown and featuring the Siesta (see Section 1.4), provided an opportunity to field-test Compumedic's new product.

Although this project was not interested in recording signals related to sleep diagnostics - and as a consequence could not test the Siesta's full range of capabilities - its signal recording and communications features were put to effective use. The primary use of the datalogger in this project was to take readings from various analog sensors (see Chapter 6), digitise the sensor signals and transmit them to a host computer for storage and processing.

Compumedics supplied one Siesta datalogger unit, assorted cables and connecting leads, a number of sensors specifically designed for use with the datalogger and an installation of their own software for interfacing with and recording data from the Siesta.

  

Figure 5.1: Siesta datalogger from front, showing waist strap and connecting leads (eight signal, four ground connectors)

Features

Table 5.1 summarises the Siesta datalogger's main features, even though some of these were not particularly applicable to the area of ambulation monitoring.

 

Table 5.1: Siesta datalogger features summary

Data channels	Number	32
	Sampling rates	4 to 512 Hz per channel
	Resolution	16 bits per channel
	Coupling	4 D.C./A.C. channels 4 A.C. with special low-frequency, high pass filters 24 A.C. only
	Gain ranges	10 $\mu$V to 500 mV peak-peak.
	Oximeter	Channel for oximeter probe (SaO2, pulse rate, blood-oxygen level)
Communications	Serial	Infra-red (IRDA) port
	Radio	Spread-spectrum Radio LAN
Storage	Flashcard	Removable compact flashcard storage (currently capacity 144 Mb)
	Remote	Storage on host computer via Radio LAN
Power supply	Batteries	Nickel Metal Hydride (Ni-MH) Rechargeable or 4 Alkaline `AA' Cells
	External power supply	10 V, 1.0 A max
	Recharger	On-board recharger for Ni-MH
Dimensions	Length$\times$Width$\times$Height	$132\times80\times40$ mm
	Weight	300 g (with Ni-MH battery)
[52,53]

 

   
Data Communications and Storage

As discussed in the proposal [51], the Siesta provided three primary means of communicating its readings to a computer for processing; Flashcard, infra-red and radio LAN. The radio communications capabilities of the Siesta datalogger were of particular interest to this project, as it provided a means of radio telemetry. This allowed recordings to be made from a subject who is able to move freely as opposed to being restricted by a tethered datalogger.

   
Radio LAN

The Siesta contains a compact radio transceiver for transferring data using the Radio LAN^5.2 wireless networking protocols. The maximum radio transmission range varied between 50 and 100 metres depending on battery power and environmental or electrical interference.

In order to communicate, the Siesta's Radio LAN transceiver required an equivalent `base station' or host radio transceiver tuned to the same (configurable) radio channel. As the Siesta could communicate with one host at a time, no other radio LAN devices within range could be operating on the same channel.

For this project the host came in the form of a radio network interface card (NIC) and antenna installed in a desktop computer, or a PCMCIA radio card and laptop computer (used for recording outside).

In terms of the OSI networking model [54], the radio LAN provided the physical and data link layers of communication. A the transport layer, the Siesta communicated using standard Internet protocols [55].

   
Infra-red

The Siesta included a serial `IRDA' infra-red port, for use in short range, line-of-sight communication. The designed purpose of the IRDA port was to enable direct configuration of the Siesta's on-board `firmware' and Radio LAN without having to use radio networking.

   
Flashcard

The flashcard port, situated at the bottom of the datalogger and partly shown in Figure 5.1, allowed the Siesta to make long recordings without the need for a host computer to store data. A flashcard consists of compact, high-capacity memory devices contained within a small, flat package or `memory stick'. These can be interested into the Siesta and used to store recordings, then later removed so its memory can be down-loaded to a computer via a flashcard reader.

Datalogger interface software

Two software packages were supplied by Compumedics for use with their datalogger. The most useful of these for this project was the `SeistaView' program. This was used to communicate with the Seista over the radio LAN in order to configure its channels and receive sensor readings. An integrated review and analysis software package was also provided, but was not of any great benefit to this project.

   
SiestaView

The SiestaView program produced by Compumedics provides a Windows-based graphical user interface for interacting with and controlling the Siesta datalogger. Through SiestaView the Siesta's input channels could be disabled or set to a configuration appropriate for the attached sensor. Each channel's sample rate, gain (peak-peak input voltage range), coupling mode (AC or DC), and impedance checking could be individually programmed.

Once the Siesta's channels were configured, the SiestaView program could be used to start the Siesta recording then graphically display the sensor readings received, which was useful for testing sensor calibration and response. The SiestaView software could also be used to query the Siesta's general status, such as battery voltage.

The raw recordings received from the datalogger were stored to a binary file by the SiestaView program. This file was the primary medium by which the DBN software built for this project collected the sensor readings for processing (see Chapter 7).

Compumedics sensors

Compumedics supplied some of their own sensors, specifically designed for use with the Siesta, which could be used to record a range of physiological signals. These are summarised as follows:

EMG, EOG, EEG and ECG electrodes

Sensors as used in standard medical diagnostics for detecting the body's electrical signals emanating from muscles (EMG and EOG) and brain (EEG) and cardiovascular system (ECG).

Leg sensors

Piezoelectric accelerometers (see Section 6.1) designed to give qualitative assessment of limb movement.

Respiratory bands

Thoracic and abdominal bands used to measure expansion and contraction of the thorax and abdomen respectively.

Oximeter

Small sensor mounted on the subject's finger in order to detect blood-oxygen levels and pulse rate.

Microphone

To be attached to throat and used in sleep diagnostic recordings for detecting breathing and other noise.

Position sensor

For detecting orientation of body (lying on left side, right side or back) during sleep diagnostic recordings.

Again many of these sensors were not applicable to ambulation monitoring^5.3, with the exception of the leg sensors. The application of the piezoelectric leg sensors is discussed further in Chapter 6.