Mechatronic design and manufacturing of an affordable ...

Abstract

The design and manufacturing of affordable medical assistive devices represents a major challenge for developing countries where resources are much more limited than in rich countries. The engineering design process focuses on developing better devices and systems with a low impact on the environment and the most functional and efficient performance, at the lowest possible price. In this context, the mechatronic design is perhaps the most complete way of thinking about performing concurrent design tasks which provide fully of mechanical, electronic, informatics and intelligent control systems integration. This design process can take advantage of many computer-aided tools, which play a key role in the modern methods of optimization and reduce the cost of prototyping. This work presents the mechatronic design process of an affordable assistive robotic bed, from the main requirements through the mechanical design and up to the integration of the electronics and the embedded control, while industrial design suitably merges to all the modules of the robotic bed. The work was developed in cooperation with a set of experts at the Hospital Juarez de Mexico who provided their very specific necessities for the designers and more important for the patients.

Keywords:

Mechatronic design, assistive robotic bed, healthcare

Introduction

Mechatronics dates back to 1969, when the Yaskawa company in Japan coined the term to describe the way mechanics and electronics are merged to develop intelligent devices and systems. Steady advancements in technology have vastly expanded its use. Currently, a major focus area for mechatronics is the design of assistive devices. On the one hand, by integrating mechanical and electrical hardware with information technology and embedded control, designers can deliver highly sophisticated functionality. Therefore, mechatronics tends to improve the design and operation of assistive devices, enabling its use in a growing range of medical applications. On the other hand, this functionality has paid the price of higher costs of production which has a severe impact on the affordability of such devices, especially for developing countries which are importers of foreign technology.

The growing demand for medical devices and the lack of experienced human medical resources represents a major impediment to effective treatment of patients, particularly in developed countries with aging populations. In the specific case of Mexico, recent studies by the National Population Council reveal that the life expectancy will increase from 73.6 years in 1995 (71.3 for men and 75.9 for women) to 80.4 (78.4 for men and 82.3 for women) in 2020 and, finally, 83.7 years (82.0 for men and 85.5 for women) in 2050¹. In addition to this, the continued demand for rehabilitation therapies, mainly for motor rehabilitation due to accidents affecting the limbs of humans or for the elderly population, emphasizes the lack of adequate medical staff. Therefore, in the next years assistive devices will support medical staff by reducing the intense load of physical interaction with patients.

Assistive devices are always priority to demanding fields to satisfy hospitals needs and more importantly to improve patient comfort and the quality of the devices that assist his rehabilitation. Those devices must be designed to fulfill successful healthcare services.² In this context, we have recently proposed a diagnosis methodology that aims for technology project development and represents a framework that might be used to assess the feasibility of the development of a device or system by analyzing its impact in a specific environment.³ Once it is decided that it is a feasible project, it can be suitably developed. By applying our methodology to the Hospital Juarez de Mexico (HJM) it was found that the device that has most impact on the medical institution is the hospital bed.³ As explained in “Mechatronic design approach,” this diagnosis established the main criteria for the mechatronic design of the robotic bed.

Several designs of hospital beds have been proposed to alleviate very intensive labor and make up for the lack of qualified personnel (nurses and stretcher-bearers). The designs consider different criteria ranging from mechanism design,^4-6 to control system design,⁷ intelligent system design⁸ and the mechatronic design approach⁹. Moreover, only a few meet the needs of a patient as well as those of the medical staff.¹⁰ In the commercial plane, many kinds of hospital beds are available for the same purpose as ours. To establish a fair comparison of medical beds is quite complicated, nonetheless, in three models of different competitive manufacturers are presented including their estimated prices, which at the end of the paper will allow us make conclusions about the affordability of our proposal. It is important to mention that all of these beds have only nine positions and none of them have a system to sense the patient's position on the bed.

Table 1.

ModelPrice (US$)Image Hill-Rom Advanta 20,000 Stryker-InTouch 22,000 Multicare-LE 19,000 Open in a separate window

In this work, the design approach for the hospital bed goes beyond, and it is based on patients', nurses' and stretcher-bearers' specific requirements. Hospital human resources are the people in daily contact with real situations and needs. For this reason, their feedback is essential to producing a useful hospital bed. This design also forms the basis for considering a functional set of positions demanded by real bed needs. Then for each required position a mechanism synthesis stage creates a solution for the motion of each required tool. Finally, using the tools of mechanical engineering, the complete design can be developed. It is important to mention that the bed construction involves design and manufacture in various areas (mechanical, electronic, industrial and graphic design). This integration produces a functional device in combination with an intelligent system.¹¹

Special features and functions

Traditionally, special functions are offered by manufacturers as specific and expensive extensions of standard models of hospital beds. Nevertheless, our approach is to satisfy the requirements of the market, that is, the users of the bed. From visits to HJM, our study obtained results on the analysis of the dynamics of running a hospital room for the different usage scenarios and the description of the users involved in the use and maintenance of a hospital bed. From these results, we defined a list of general requirements which must be fulfilled in order to achieve an appropriate working relationship and successful use of this specialized medical device. In addition, to satisfy this feature set, additional considerations complying with IEC-60601-52 and UNE-EN 1970 standards were taken into account. The final set is listed below.

• Ensure the stability of the device in any of its positions.

• For security, no user should have contact with mechanical parts.

• The railings must have free movement in any position.

• Access controls should be comfortable and live (even without electric energy).

• Access medical peripherals and accessories must be free and comfortable in any position.

• The position of the device should not limit the use of peripherals.

• It must attend the medical user to find the right position for the patient in different circumstances given by the condition of the patient.

• The rails and foot-board should allow visibility of the medical staff at any time and should not obstruct patient monitoring.

• It should allow access to perform common toilet tasks.

• Ensure stability in patient transfer conditions, even with two people on it.

• The device should provide a good service (maintenance) to the user during their stay in hospital.

• It must make the patient's stay comfortable taking into account as far as possible the emotional aspect of it (e.g. sense of stability and safety during movements).

• It should avoid, prevent and/or minimize any risk, both use and health, for all users, especially for the patient

• A safety loading of 3000 N, must be resisted by the mechanical structure.

• A minimum and maximum height of 47 cm and 90 cm respectively must be provided by the bed. It will be useful for the “help to stand up” position.

Positions

Several hospital bed manufacturers provide a wide range of models that are suitable either for intensive therapy or for hospitalization. Depending on specific requirements, some positions are provided by each bed model. The most common positions are orthopedic, cardiac, Fowler and Trendelenburg. Nevertheless, other useful positions are foot elevation, panning or tilting, and sit.¹² From the universe of possible positions, the results of our study indicate that there are 12 required positions, depicted in. It is important to note that the home position includes vertical motion, which provides the adjustable height of the hospital bed.

Open in a separate window

In order to achieve the desired positions, different mechanisms were synthesized to provide the desired ranges of motion.¹³ Such ranges were also obtained by an ergonomic study which was carried out at the Center of Investigation in Industrial Design in Mexico City.¹⁴

Mechatronic design approach

One mechatronic design key is to develop tasks of mechanical, electronic, control and industrial design in a concurrent fashion at the same time that full and harmonic integration of all the components is achieved. This is in fact an actual problem from the mechatronic design point of view.¹⁵ A lot of information from several areas must be processed in order to fulfill all criteria for the design. Then, during the development of the design tasks a high degree of coordination must be achieved. The following section describes the positions and special feature requirements obtained by a serious study at HJM over two months of applying our diagnostic methodology.³ About 300 medical experts were asked to define these requirements. As a benchmark for the requirements, Latin-American patients' heights were considered an essential part of the design process.¹⁶ In the following sections, the tasks for the mechanical design, electrical design, control system design, intelligent system and industrial design are described. It is fundamental to remark that all the tasks were carried out in a concurrent fashion as established by the mechatronic approach.

Electronic design

This section is concerned with the development of the electronic design. Electronics aims to feed the sensor assembly carrying the bed (tilt sensor, sensors lifting, weight sensors) and it is also responsible for providing power to all output devices (LEDs, touchscreen, motors) logically programmed via a central control device. Once the control part makes a logical or intelligent decision, the power stage is responsible for providing power to all output devices such as motors and lights. The electronics design was split into four stages: (1) selection of major electronic devices; (2) design and simulation of the electronic circuit schematic, (3) design of printed circuit boards (PCBs), and (4) design of a dedicated control system. To perform some of these tasks Altium Designer (a computer package for electronic design as well as free software and programming in C language, HTML5 and PHP programming on the Linux platform was used. depicts the electronic and control signals flow for the robotic bed.

Open in a separate window

The methodology for the electronic design was as follows. First, the selection of the electronic components to perform the different tasks of the robotic bed was carried out. These components are sensors, components for signal conditioning and power supplies. Then, the schematic design and simulation of electronic circuits was performed. Along with this document the set of.sch extension files representing each design schematic for manufacture was generated. This set of designs is made to a professional electronic standard that ensures proper operation during execution. Afterwards, the design PCB was developed. It is noteworthy that a total of six completely unique designs made especially for this application are presented. Next, a dedicated control system was designed. The way the robotic bed controls all peripherals through a detailed mainframe and how it performs each of the functions that involve electronics was described. Then, the programming of the LCD touch interface was performed, and finally the locations of all electronic components within the robotic bed were detailed.

Control system design

The main goal of the control system is to render smooth motion between the bed positions. This smooth motion is already rendered by the actuators, which are only commanded to a desired position using polynomial interpolation between each position.

The main task of the control system is to review, at each sampling period, the existence of errors in different parameters related to the bed's posture, giving priority to the bed height, horizontal and vertical tilt: if there is error in any of the aforementioned parameters a signal is sent to the actuator to increase or decrease the parameter. When errors are within an allowable range control, the control system proceeds to review the parameters backrest angle, thigh angle and foot angle.

Through the graphical interface (touchscreen in), the user (nurse or doctor) can schedule routine bed movements, specifying the date and time they want the bed to change position automatically to one of 12 presets. The way control meets the above task is divided into two blocks or functions. The first is to compare the current date and time with the programmed one, and when they are equal proceeds to call the second function involved (routine). When the control system determines that it is time to change position, it calls a routine that writes new parameters into the tables.

Open in a separate window

Intelligent system design

For the automatic motion of a robotic hospital bed, based on posture classification and identification, an intelligent monitor system was designed. This has been developed as a response to the patients who require a certain routine of motion application. The proposed intelligent system allows medical experts to program movements of the robotic bed by considering the patient's posture and time. The intelligent monitor system for body posture classification works in real time and is based on a histogram of oriented gradients (HOG) descriptor and a support vector machine (SVM) classifier.¹⁸ Moreover, the intelligent system considers the problem of human posture classification with limited information, that is, sensors. Then, by applying digital signal processing, the original data is expanded to get more significant information for the classification.

Our robotic hospital bed is able to render several positions depending on the needs of a particular patient. The main task of the intelligent system is to decide when it is feasible and safe to move from a given position to a desired one. For this, a state transition diagram has been specially designed, guaranteeing safe transitions between the bed positions; see.

Open in a separate window

shows the main stages of the Intelligent System (IS) for posture recognition. In the initial stage the images representing pressure distributions are obtained from the pressure sensor array. Then, in the second and third stages a feature extraction using HOG¹⁹ and SHIFT²⁰ descriptors are applied over the pressure distribution images. For this, they are considered as gray scale images. After, in the fourth stage a database of features is constructed, and finally in the last two stages a model for feature classification and prediction is build by comparing the results of three classifiers: SVMs, decision trees and naive Bayes networks. To simplify the posture recognition we consider three basic postures: the right lateral decubitus, supine and the left lateral decubitus positions (see), and since the prone position is almost the same as the supine position, its detection is achieved by an analysis of the pressure distribution. shows the three basic correct positions displayed as gray-scale images, obtained from simulated data of the pressure sensor array.

Open in a separate window

Open in a separate window

Industrial design

The industrial design was considered during the whole design of the robotic bed as an integral part of it, resolving from a formal geometric proposal the perceived image of the final object. Moreover, such design must communicate a modern medical image, that the user trusts during use, taking as general considerations communication, safety, comfort, and the necessary anthropometric measures for the proper use of a Mexican population based on percentiles tables.¹⁶

Safety, from the industrial design point of view, is the set of conditions that guarantee that a patient will be protected from suffering new health problems, independent of those which led him to seek medical assistance. Therefore, this device must be able to function without this representing any risk to any of the different users. The robotic bed should provide inpatient accommodation 24 hours a day. This plays an important role in the recovery of the patient, providing convenience and comfort. The comfort of the patient depends on the state of his bed, especially if he uses it for long periods. Providing for the above patients depends directly on the proper relationship between the size of the user, and the dimensions and proportions of the area of the bed, as well as the physical qualities of the mattress and the materials to be used in building the device.

From visits to HJM and analysis of the dynamics of running a room for hospitalization, different usage scenarios, maintenance of a hospital