Types of Recorders used In Biomedical Instrumentation

Introduction

The recorder is required in any instrumentation system to record data which has been acquired. Data can be in analog or digital form hence the 2 types of recorders namely:

• Analog recorder
• Digital recorder

Analog recorders can be:

• Graphic
• Oscillographic
• Magnetic tape recorder

The graphic recorder is a device which records some physical event on chart paper with stylus as tracing which is the variation of input signal along time on moving chart paper.

Digital recorders use a linear fixed array of small recording elements under which the paper moves. This is in contrast with the conventional recorders that use a moving pen or stylus. The stylus in the digital recorder is a large number of fixed styli, each one of which corresponds to one amplitude of signal to be recorded. Signals are thus reproduced as discrete values at discrete times. Analog as well as digital signals can be processed. In the case with analog signals, sampling and digitization are part of the recording process.

Let us now look at each of the recorders used in medical instrumentation systems in details:

Strip Chart Recorder

A strip chart recorder consists of:

• A long strip of chart paper which can move vertically
• A drive system to move the chart paper at some selected speed
• A stylus for making trace on the moving chart paper
• Stylus drive system to move the stylus horizontally on a moving chart paper or in proportion to the quantity or input to be recorded.
• A range selector switch to limit the horizontal move of the stylus

An electronic stepper motor or synchronous motor or a spring wound mechanism is used for driving the strip chart paper. There are many ways to move the stylus to make the marks on the paper. The marking can be done with:

• Ink filled stylus
• Heated stylus
• Chopper bars
• Electric stylus marking
• Optical marking

An Ink filled stylus contains a capillary connection between the pen (stylus) and ink reservoir. However this type of stylus is unsuitable at high speed and it also develops clogging of ink when the stylus is set. The heated stylus melts a thin coating of white wax on a black recording paper while working. In a chopper bar, the stylus marks on a paper with special coating which is sensitive to current. The optical stylus uses a beam of light to write on a photosensitive paper.

Galvanometric Recorder

The principle of working of galvanometric recorder is that when a current flow as per the input signal through the coil kept in a strong magnetic field, the coil with the pen or stylus deflects proportionally to the input quantity as shown in the diagram below:

There are 3 forces which act on the moving system i.e.

• The deflecting force which results from the current flowing in the coil.
• The controlling force as applied by the spring action to control the limit of deflection.
• Damping force to bring the pen or stylus to rest as quickly as possible.

The set-up of galvanometric recorder is shown below:

The chart paper is driven at a constant speed by an electric or stepper motor. The ink recording method is widely used in which pen or stylus marks on the moving chart paper. The demerits of a galvanometric recorder include:

• A low input impedance
• limited sensitivity

Potentiometric Recorder

Potentiometric recorders are used for recording of low frequency events or physiological signals. The working principle of potentiometric recorders is based on balancing input voltage signal with some reference voltage on the potentiometer and the balance is achieved by moving a sliding contact on the potentiometer resistive wire with the help of a servomotor.

In any unbalance these two result into a voltage difference which is given for a chopper (DC amplifier) which converts this voltage into a square wave signal. The difference voltage signal is amplified and then applied to the control winding of a servomotor and the motor moves as per the difference voltage signal. The second or reference winding of the servomotor is connected to the main supply. The control and reference windings are perpendicular to each other to generate torque for the movement of motor. The motor is mechanically connected to the sliding contact of the potentiometer. The servomotor rotates and moves the sliding contact in such a way that the difference voltage signal arriving at the servomotor becomes zero which enables the motor to stop.

The schematic diagram of a self-balancing potentiometer is shown below:

The potentiometer has a uniform wire AB which is supplied with a constant current from a battery S. The moving contact on the potentiometer resistive wire is slider C. The signal (unknown) voltage is applied between points A and C. The slider C is moved by the servomotor so that the current flowing through the servomotor is zero and the servomotor can stop. At that point the unknown signal voltage is proportional to the length of the resistive wire AC. The moving contact C of the resistive wire is attached with a pen or stylus that can write on a moving chart paper. As the paper moves, a trace of physiological signal is obtained.

A tachogenerator is coupled to the servomotor with a shaft to provide proper damping to the servomotor so that it can come to rest quickly after each motion.

Ultraviolet Recorder

Ultraviolet recorder can record events or physiological signals with frequencies in the range of zero to several kHz. Note, galvanometric and potentiometric recorders are unsuitable when the signal has high frequencies.

The ultraviolet recorder consists of a number of moving coils mounted in a single magnet block as shown in the diagram below:

A silvered mirror is attached to each galvanometer coil. A paper sensitive to ultraviolet light is used for producing a trace for the purpose of recording. The ultraviolet light is projected on this paper with the help of the mirror attached to each coil. The coil is deflected if any current flows through the coil as coils are subjected to magnetic field. The ultraviolet light is also deflected by the mirror in proportion to the deflection of coil. As the current flow in the coil depends upon physiological inputs, hence the deflection of the ultraviolet light through mirror is in proportion to physiological inputs.

The moving ultraviolet light falling on the moving light sensitive paper forms a trace of variation of input signal on the paper. The trace of variation on the paper is the record of the variation of input signal with time. The ultraviolet recorder may be single channel or multichannel. After recording, the ultraviolet sensitive paper has to be chemically treated before any storage.

Inkjet Recorder

The principle of working of Inkjet recorder is that a very fine Inkjet is made to move on the paper per the physiological events or signals. The schematic layout and working of an Inkjet recorder is shown below:

The recorder has a glass capillary tube placed between the poles of an electromagnet. The coil of the electromagnet is connected to the amplified physiological signals. A small cylindrical magnet is attached to this ink capillary tube. The variation of current corresponding to physiological signals in the electromagnetic coil produce a varying magnetic field in it which interacts with the field of the cylindrical magnet attached to the capillary. The interaction of the magnetic field deflects the cylindrical magnet and the capillary tube attached with it as per the strength of the physiological signals. The capillary tube is supplied with ink at high pressure, and the ink comes out of the nozzle provided on the capillary tube in the form of a jet. The waveform is traced on the paper. Using more capillaries of different colours, the inkjet recorder can work as a multichannel recorder. The inkjet uses normal paper. As it does not have any stylus it can work at much high frequencies.

Related: Basics of A medical recording system

Electrostatic Recorder

The electrostatic recorder consists of:

• Imaging or writing head
• Toning head containing positive charged ink particles
• Vacuum knife

The imaging head has a linear array of 1000 wire elements spaced 4 mm apart. There are 32 copper bars called shoes on each side of the array. As the paper moves over the imaging head, a negative charge is applied to a selected wire element while a positive charge is applied to the closest shoes. The paper is then made to pass over the toner head where positively charged ink particles adhere to points on the paper where there is a negative charge. The paper is further moved to vacuum knife which removes all excess toner and other particles thereby obtaining a proper image of charged ink particles.

When exposed to air, the image formed by ink particles, permanently bonds to the paper and the record emerges from the recorder completely dry.