A patient monitor two-channel stripchart recorder

Hewlett-Packard Journal, Oct, 1991 by Leslie Bank

Small enough to fit in a double-width HP Component Monitoring System parameter module, this recorder embodies simplicity of design, a highly tooled mechanism, and sophisticated printhead power management.

The medical environment requires a record of the care that has been given to a patient, both for the patient's file and as a legal document. For patient monitoring equipment like the HP Component Monitoring System, the record has traditionally been a continuous strip of paper of various widths. An example of a recording from the Component Monitoring System's two-channel recorder is shown in Fig. 1. Fig. 2 is a photograph of the recorder.

In the past, the hospital had three options to provide recording capability for a patient, each of which was less than ideal:

* Purchase a recorder for every bedside. This is very expensive in these days of cost containment.

* Use a central, shared recorder. With much of the patient care given at the bedside, not having a recorder nearby is a distinct disadvantage.

* Mount the recorder on a cart and wheel it to the bedside when needed. This takes up too much of the available room at the bedside and is also inconvenient.

The Component Monitoring System philosophy of allowing the monitor configuration to change with the patient's needs extends to the recording function. The two-channel recorder can be moved around like any other parameter module. This approach, along with the requirements for ease of use, high reliability, high performance for many types of applications, low manufacturing cost, and low power led to the following set of major specifications:

* Size: Double-width parameter module

* Power consumption: Approximately 6 watts maximum

* Number of waveforms: 3

* Lines of character printing: 3

* Paper: 50-mm-by-30-m rolls (fan-fold paper would not fit in the desired package size).

These specifications resulted in a number of major technical challenges.

Size. Fitting the paper, motor and drive mechanism, electronics, and supporting structure into a package of this size was a major accomplishment.

Power Consumption. Chemical thermal paper is used in this recorder. A printhead consisting of a linear array of resistors is in constant contact with the paper. When power is applied to one of these resistors, the resistor gets hot and a mark is made on the paper. This, combined with the power requirements of the motor and electronics, normally would require much more power than the 6 watts that are available. In addition, there can be no ventilation in the housing. Meeting the high-temperature specifications was difficult because of the internal heat generated by the power-consuming components.

Ease of Use. Recorder operation should be flexible to meet the various medical applications. It should be intuitive for the occasional user. Most of the Component Monitoring System recorder operation is part of the normal control structure. The difficulty for the recorder design team was to make the paper loading easy while not using any power to aid paper feeding.

Reliability. Recorders, which have moving parts that wear, tend to be less reliable than equipment that does not have moving mechanical parts. A simple mechanical design along with high-quality components and a severe testing program resulted in a highly reliable product.

Manufacturing Ease. This recorder was designed for high-volume assembly. Much effort was spent in minimizing part count, in using the molded parts to perform multiple functions, in designing adjustments out, and in making the instrument easy to test.

Mechanism

Paper is loaded by opening a door and inserting the roll of paper into the paper compartment. The paper is then threaded around a drive roller and pulled taut, and the door is closed. As the door is closed, a cam is engaged which lowers the printhead. The roller is driven by a stepper motor which is connected to the drive roller by a drive belt. The roller is driven when the motor turns. The paper has enough wrap around the drive roller to ensure that it can be driven under the printhead. Enough back tension must be provided to make the paper track properly, yet too much tension increases the motor torque requirements, which in turn increases the power required. This turned into an interesting design trade-off. Sealed ball bearings are used on the drive roller to minimize power requirements while keeping paper dust out of the bearings.

Two injection-molded frames form the chassis. The printhead and drive roller are captured between the chassis halves, while the motor, paper door, power supply board, and digital board are all mounted to the outside of the chassis. The entire assembly is enclosed in a double-width module case.

Electronic Hardware

The digital board contains two Intel 80C196 16-bit microcontrollers which communicate with each other via a shared RAM. Each microcontroller contains a serial port. The I/O processor uses its serial port to communicate with the monitor's computer module via the parameter module interface (see article, page 19). It receives digital commands, waveforms, and text data from the monitor. It interprets the commands and transforms the data into a format compatible with the printhead. It also monitors the front-panel and door-open switches and the paper-out sensor. The I/O processor communicates the recorder status to the monitor.