Medication Dispensing Device
RERC ACCESSIBLE MEDICAL DEVICE
PRELIMINARY DESIGN 2
Team 6
Kevin Villani,
Eva-Marie Suarez,
Jacquelyn Masse
October 28, 2005
Outline
I.Introduction
- Design
- Basic Components
- Vacuum Fan Requirements
- Robotic Arm
- Servo Operating Theory and Circuits
- Pill Capture and Cutter
- Power Supply
- Microcontroller
- User Interface
- Liquid Crystal Display Module
- Keypad
- Weight Verification System
- Dispensing Assembly
- Pathway of the Pill
III.Budget
INTRODUCTION
With individuals taking a growing number of different drugs, the market has developed several ways of making medications easier to handle. Products range from simple containers to store pill dosages per day, hand held pill cutting tools, and medication reminder alarms to more expensive, complex devices. Many of these devices can be problematic for people with poor eyesight, limited fine motor skills or mobility, Parkinson’s Disease, or other physical ailments. The lack of existing products on the market to suit such clientele gives rise to the need for an automated medical device that will administer medication to the patient in an accurate, dependable manner.
The device will be cost-efficient and reliable. It must remain accessible and easy to use for individuals who lack fine motor control, are vision impaired, or are limited by unsupported vertical access. The size and portability should be suitable for residential or clinical settings. Automation will be the device’s most distinguishing feature. It will mechanically regulate medication of 1, ½, or ¼ pills of various sizes and shapes and will manage many different medications at once. Ideally, the device will have alarms to signify the time medication needs to be taken or refilled. Information regarding dosage amounts, times, and expiration dates will be internally stored and a tracking system will record a history of dosages dispensed.
DESIGN
Basic Components and Locations
Components are located within the case to allow for ease of access and ease of replacement.
Vacuum Fan Requirements
Based on the maximum pill weight and the minimum pill thickness a vacuum requirement is identified. This requirement provides the differential pressure across the medication to ensure the medication can be removed from the storage module and delivered to the rejection assembly. By using simple pressure and weight definitions a relationship for the required differential pressure is attained. By measuring several over-the counter and prescription medications a minimum thickness was found to be 4 mm, while the maximum mass was approximately 1000mg. Substituting these values into the equation below a net pressure difference required was found to be 6.4kPa. In more traditional units of vacuum this corresponds to 1.94 in Hg. Keeping in mind that traditional vacuum systems draw a vacuum of 27 to 29 in Hg. Centrifugal fans systems are evaluated in meters of water, the range of operating head is 0.5 to 1.5 meters of water. The corresponding required differential pressure is 0.0622 meters of water, well within expected operating head.
P = F/A
F =mg
A= πr2
Therefore,
P= mg/ πr2
To satisfy this requirement we have selected a fan impeller from a dirt devil 6000 series battery operated portable vacuum. The vacuum sells on-line for approximately $55 however the impeller is also available for sell for $4.95. The required prime mover is a 400 series electric motor. Electrical requirements for the motor are 7.2V at 7 amps at full loaded capacity, 15,400 RPM continuous operation. This electric motor can be purchased from a craft hobby store for about $8.90.
Using a centrifugal fan impeller has several advantages over other vacuum generating options. First the price centrifugal fans are lower. Secondly, a centrifugal fan reaches shut off head. This shutoff head prevents drawing too high of a vacuum while still maintaining the fan in operation. This means that for our application there is a requirement that a pill be held by the vacuum tube and moved. By reaching shutoff head the pump will continue to spill however moving no air unless air slips into the system either from leakage by the pill or across the pump. Third this pump weighs less and is far easier to construct than other vacuum assemblies.
The vacuum fan assembly will be driven by 7.2 V power source through a low tech motor controller. Control over motor speed is essential for proper calibration of the fan assembly. In this application increasing the speed of the prime mover increases the vacuum drawn. The controller will be a simple parallel resistance network. Having the motor load in parallel with a variable resistor will provide linear control over the current provided to the motor. The equation used for this relationship is IL = Is(Rvar/Rt). By increasing the resistance in the variable resistor a lower current is provided to the motor reducing its speed.
Fan and system characteristic curves are used to determine the flow rates and pressures within closed systems. By measuring the differential pressure across the fan at various speeds and volumetric flow rates, a graphical representation of the fan operating characteristics can be achieved. Coupling this operating characteristic curve and a system operating curve; specific system flow rates and operating pressures can be determined by the points of intersection between the various curves, commonly referred to as “fan laws”.
Robotic Arm
The robotic arm was chosen not only for the “bells and whistle” quality it possesses but also because it has a wide margin of versatility, one to one control over medication through process, allows for verification of medication at point of acceptance and retrieval. This particular robotic arm is made of plexiglass, once again for asthentics and strength. The arm serves to acceot medication fro the user, transport for storage, retrieval from storage and transport to pill capture and cutter and finally for transport to dispensing tray. The arm is modeled around a lynxmotion 6 axis arm. The major change being the addition of vacuum assembly and linear gripper to the arm. This arm contains five servo motors for movement in all directions. For assembly of this parallel plate robotic arm changes to the inter-dimensions between servo motors allows for a smaller over all design. To the left is the lynx 6 axis arm and to the right is the proposed changes.
Gripper assembly-while part of the robotic arm its primary function is the acceptance and retrieval of medication from or to the user. The gripper contained two piezo electric sensors to determine the pressure exerted on the pill vail. The action provided by the gripper is in one dimension to either open or close the unit. To operate the gripper assembly a stepper motor coupled to a screw gear rotates, the moveable arm is coupled to the screw and is moved linearly outward. (Action is picture above)
Vacuum Retrieval Assembly-This assembly is tasked with the retrieval of pills from the storage module for delivery to the pill capture and cutting station at dispensing set points. The assembly is made of four components; pill retrieval vacuum tubing, supply tubing, servo motor and the proportioning valve. A vacuum is drawn on unit through the proportioning valve setting up a pressure differential across the medication. This pressure differential holds the medication in place for transport to the pill capture and cutting station. Pressure is passed back through the proportioning valve allowing a discharge pressure to release the medication from the pick up tube. The type of device was chosen to reduce the possibility of drawing multiple medications in a single pass. This single event can then be related more accurately to pill retrieval numbers (assembly pictured below).
From the discussion of vacuum fan requirements the area of the pill pickup tube is approximately 1.26e-5 m2 or a circle with a 4mm diameter. The pill pick up tube is located to the left in the picture on previous page. The proportioning valves are located at the center of the device and control flow into an out of the pill pick up tubing and through orifice plate. The entire vacuum assembly is rotated into position to pickup by a servo motor attached to the assembly.
The proportion valve is a Burkett (6012) 3-way valve operating with the following technical requirements. When deenergized the valve is positioned to relieve pressure to atmosphere. When energized to valve positions to permit vacuum pressure to be felt in pick up tube. The required operating voltage is 24V DC±10% or 24V AC. Maximum operating temperature is 130F at a maximum viscosity of 21 cSt. The response times for this valve are opening 7-10 ms and closing 7-12 ms (DC).
The robotic arm as a unit is required to carry out many functions as described above those acts can be seen in the following pictures. Such as pill retrieval and medication acceptance
Figure 8
Figure 9
Servo Operating Theory and Circuits
A servo is a geared microprocessor controlled DC motor. Generally servos sweep over a 90 degree arc. However, removing the stops within the servo and changing the programming allows for operation at arcs greater than 90 degrees. The servo controls of the vacuum retrieval assembly require only 90 degrees of rotation. A probable candidate for the servo required is pictured to the left. This required a 6V power supply. The operating circuit for this servo is pictured below and is capable of expansion to power additional servo motors.
Basic servo theory- is centered on changing the pulse width of the control signal. This change in pulse width corresponds to a position of the servo motor. A 1.0 ms pulse rotates the shaft all the way counter-clockwise. A 1.5 ms pulse puts the rotor at neutral (0 degrees), and a 2.0 ms pulse will position the shaft all the way clockwise. The pulse is sent to the servo at a frequency of approximately 50 Hz. The relationship between the pulse width and the rotor position can be seen in the picture above. .
Programming the microcontroller will be modeled around the following program. Where in this case 100 is extreme left and 200 is extreme right, this circuit is limited to oscillation from left to right and required modification.
Pill Capture and Cutter
Design limitations of design 1 are directly answered with the secondary design of the pill capture and cutting device. This design removes the need for the user to load pill manually while still retaining as high accuracy is segmenting the medication dosage. As seen in research pills and tablets do not need to be cut along a score line to remain consistently segmented correctly. If fact personal research was conducted to cut tablets lengthwise with high accuracy. With this in mind the secondary design approach to pill capture and cutting is centered about geometry. Assuming all pills or tablets that shall be cut have a point of symmetry about them the capture swing arm shaped as an arc is capable of placing the point of contact between the pill and the swing arm about this center of symmetry. In the same motion align the cutter along this center of symmetry. This orientation allows a pill to be segmented in half, by repeating the process ¼ segments are achieved.
To examine the operation of this device a closer look into the geometry of the argument is needed. By looking at the complete circle and taking into account the changes in the x direction of the circle, a cosine wave form can be produced. The portion of this waveform of interest is from 0-90 degrees. Where 90 degrees is the minimum pill size relates to the maximum negative adjustment of the cutter assembly. Conversely, 0 degrees relates to the maximum pill size and zero cutter adjustment. By mechanically and continuously sampling the x position of the arc a direct connection is made to the adjustment of the cutter assembly.
In this device the arc and cutter assembly are not linked but rather in contact with one another. The set up of the cutter assembly places the blade of the cutter at the center point of the arc and the “feeler” side of the cutter in contact with the arc. A plumb line is drawn from the innermost portion of the contact of the arc and cutter assembly. This plumb line extends a length slightly larger then the magnitude of the cutter adjustment. With each smaller pill the arc is rotated by torsion spring tension against a slight axial spin tension thereby positioning the pill to the center point. Concurrently the underside to the arc acted against the cutter “feeler” and spring tension (of cutter assembly) to reposition the cutter assembly. Note from previous discussion the changes in the x-direction of the arc are translated one for one to the cutter assembly. With this set up any symmetrical medication can be centered across the cutter blade providing accurate non-approximated center points.
While the positioning capability of this sub unit are key, this sub-unit contains or supports the major differentiating function of the overall device as well as provides a far superior design to design one. The functions of this assembly are to accept, capture, cut and deliver medication. Capture and cutting have already been covered in a preliminary seems. To evaluate the function of this sub assembly a complete sequence of operation should by addressed to understand the interconnection of devices. The assembly consists of a stepper motor, three ball bearing linear tracks, worm gear and pinion, torsion spring, two axial springs (one for the swing arm and one for the cutter assembly), base, cutter arm, stage, and sliding secondary stage.
The base unit carries the weight of all components. The three ball bearing linear tracks are mounted parallel across the base perpendicular to the cutter arm. The main stage is mounted to the top two ball bearing linear tracks and is coupled to the worm gear shaft via treaded couplings. This permits the stage to project out from the base is the direction of the linear tracks by action of the pinion gears coupled to the stepper motor. Noting the accuracy of the linear movement of the stage is not associated with the accuracy of the cuts. Rather the gearing is designed for higher rates of projection and retraction. Most important to this actuator is the ability to satisfy cutting conditions. Those conditions are that the stage is fully retracted and the arc is in contact with the cutter “feeler”. The base is also used to attach the cutter arm as well as routing power lines.
The main stage contains the swing arm and secondary stage. The action of the secondary stage is to slide open perpendicular to the motion of the main stage providing a means for removal of segmented medication. The action is provided through the main worm gear energy by coupling that energy to a slotted groove located at a hyper-extended region of the main stage. The hyper-extended range is at a distance equal to the maximum opening range of the secondary stage past to point of acceptance of medication.
The most crucial component in the design is the swing arm. Balancing the load of three springs allows for the alignment of the cutter. To begin in the closed position the swing arm is at approximately 90 degrees (maximum negative adjustment). This position reduces the opening axial spring deformation while loading the cutter spring. The main stage is projected outward to the medication acceptance point where the opening spring force builds (assisted by the cutter spring) to open the swing arm against the torsion spring tension. At the medication acceptance located the swing arm is in the minimum adjustment position, providing the maximum area for deposition of the medication. After deposition of the medication the main stage is retracted. Now the torsion spring overcomes the opening spring and cutter spring tension due to the relaxed nature of the opening spring. This positions the medication and forces the cutter into position. At the fully retracted position the cutter is engaged after conditions met. At this time the main stage is again projected past the acceptance region to the dispensing region. To prevent damage to the opening spring a secondary spring with a spring constant lowered than the elastic region of the primary opening spring begins to give way. At this point the swing arm is balanced in the open position allowing the medication to be forced against the swing arm and off the secondary stage. The pills fall into an intermediate storage bin which rotates to a position outside the stage assembly. This rotations position another identical storage bin below the secondary stage. This operation is preformed to prevent confusion of segment sizes during subsequent segmentation (¼ segmentation) as well as facilitation for transport to the rejection assembly. Below is a figure depicting the action of the assembly, noting there is no tray rotation device or secondary opening spring for simplification of the figure.) Appendix I contains higher quality component drawing as well as tablet configurations. The majority of components in this assembly will be constructed using PVC clear plastic for aesthetic quality rather than longevity, which run about $15 per sheet. All springs are extremely in expensive while the stepper motor runs about $10. The tracks cost about $5 each and the worm gear from boston gear may cost as much as $40. This sub-assembly cost is about 80-90 dollars.