Pill Cutter Mechanism Prototype

For our prototype we focused our attention on what we felt were the most crucial aspects of the device, specifically its ability to both properly dispense a single pill and cut it without manual intervention. This proved to be a very daunting task. The pill cutting and alignment mechanism was a very difficult thing to design. In the end, we were successful at dispensing a single pill from a container as well as correctly aligning and cutting the pill.

|The dispensing unit is propped at a 45 degree angle to allow the pills to gradually progress down the cutting mechanism.|

Prototype Alignment and Cutting Mechanism

Figure 1: The linear door actuator solenoid is used to produce the sharp blow force used to drive the blade downwards and cleve the pill. The force is transferred into a spring-loaded plunger brass plunger with a carbon steel blade soldered onto the end. Once the cut is made, a reverse polarity current is provided to the actuator, retracting it and allowing the plunger to return to the blade-out position.

A car door lock actuator was chosen as the driving force behind the cutting action. The force generated by the linear actuator provides a quick high force blow. The plastic end of the actuator strikes the rubber bumper on top of the plunger that is conencted to the blade. The force of the blow splits the pill in two with virtually no crumbling. The cutter is placed at a standardized 45 degree angle to allow gravity to drive the pills through the mechanism.

The control of the automated pill cutter involves the use of two hobby servo motors, a linear actuator or “solenoid” pulled from a car door lock actuator mechanism, and a stepper motor. When the pill is loaded, the linear actuator presses down on the spring loaded pill cutting mechanism, dropping the blade in place so that the pill does not slide through the entire mechanism. The top servo motor is raised via a command from the microcontroller, to allow the pill centering mechanism to be raised into position. The bottom servo is connected to a cam. When actuated the rotation presses the cam into the bottom of the pill centering mechanism, tilting it into position along its track. Once the centering mechanism is in place, the blade is raised by reversing the direction of the linear actuator and allowing the spring restoring force of the cutter blade to return the cutter to the upright, disengaged position.

The centerer is then actuated and a direction and pulse train are sent to the centering mechanism driving the stepper motor towards bringing the two pill centering pins together. After 259 pulses, the device is deactivated and the pill is assumed to be in position. The top servo motor is activated, causing depression of the pill retaining mechanism, thereby locking the pill in place. The centering mechanism is then expanded by sending a reverse direction command followed by an additional 259 pulses. Once the mechanism is expanded, the bottom servo motor is returned to default position, dropping the centering mechanism out of engagement. The pull cut action is then actuated, and the linear actuator then slams down, causing the blade to sever the pill along the center line. Previous tests have shown that pre-existing score alignment is not critical to ensure proper cutting. The score establishes a weak point that will sever even if the force is not applied directly to the top of the score. The round pills used in the demonstration are the most challenging to cut, but overall, all types of pills tested were properly cut. Once the cut had been accomplished, the blade is held down and the retainer is released. The first pill half will be dropped. Once the blade is released, via the retraction of the linear actuator, the second pill half will be dropped. The mechanism is then ready to repeat the previous process.

Figure 2/3: Close up of cutting blade mechanism.

Figure 4: Close up of cutting blade mechanism integrated with electronics.

The positioning of the servos are made such that interference is minimized. The rubber bumper on top of the plunger has been slimmed down through the design modification process to reliably not be in the way of the pill retainer mechanism driven by the top servo.

Figure 5: Close up of pill alignment/centering mechanism in its disengaged (lowered) state.

Figure 6/7: Final Pill cutting setup. Electronics mounted to aluminum block

Figure 8: A PIC 16f84 microcontroller is used to produce the pulse trains necessary to drive both the stepper and servo motors in the pill cutting mechanism. Full automation is managed through a program resident in the microcontroller. For the sake of the demonstration, the individual steps required to cut a pill are mapped to pushbuttons, allowing manual activation. In the final unit, a logic pulse from the CPU triggers the pill cutting process. Once the pill is cut, a return pulse signals that the operation is complete and the unit is ready to re cycle.

The pill centering mechanism allows our pill cutter to work with pills of many sizes and dimensions. Multiple attempts to manufacture a pill centering mechanism were made before the final production version was created. Initially a double rack and pinion drive mechanism similar to those used in FAX machines to center the paper feed was proposed. The complexity in manufacture of the small components needed to produce this mechanism hindered its final production. A second design intended to use an expanding parallelogram driven by a screw to drive the centering pins. This design is similar to the emergency jacks included with the spare wheels on cars. As with the first design, the complexity of manufacture hindered its production. The final design came as an offshoot of the parallelogram design. A standard screw can pull a moving plate towards a fixed plate in the similar way a shop vice works. For our design, both sides of the ‘vice’ arrangement needed to move together simultaneously so that they will always meet in the middle at the same place. Any size pill that is inserted will be automatically centered once the pins come in contact with both sides of the pill. The mechanism had to use a design that was smart enough not to crush the pill once centering has occurred.

In order to make two nuts come together on the same screw, the use of a fused left-handed and right-handed screw must be used. The production of left handed screws is limited. In order to obtain a left handed screw and nut, a turnbuckle was sacrificed. The right and left handed screws in the turnbuckle were centered together in a jig and then brazed to union. The excess brazing material was then lathed off, and a small coaxial hole in one side of the right handed screw was drilled on the lathe to accommodate the motor spindle. Tow aluminum blocks were machined to accommodate a standard right-handed nut in one and the nut-portion of the turnbuckle in the other. A third aluminum block was plunge and profile milled to provide a channel for the two ‘nut’ blocks to slide in. The ‘nut’ blocks were threaded onto the screw from either end and adjusted so that their center alignment is oriented to the middle of the block. The track was greased, and the motor spindle was inserted into the coaxial hole and the screw crimped onto the motor. The motor is then adhesive bonded onto the aluminum block. The two stainless steel alignment pins are pressed into the aluminum ‘nut’ blocks via two pre-drilled undersized holes.

Figure 9: Stepper motor phasing control is managed by a MC3479P microchip. The direction and pulse rate are controlled via the PIC microcontroller. For the sake of design simplicity, a Ramsey SMD-1 Stepper motor driver was used with the internal logic generator bypassed such that the PIC controller generated all driving commands.

The motor used is a bipolar stepper motor. It is driven off of a 16V power supply delivering a nominal 250mA. The MC3479P stepper motor driver is used to directly drive the stepper motor from the logic pulses generated by the PIC 16f84 microcontroller. The limited current to the motor allows slipping to occur when the pill is in position. Per the screw pitch, 259 pulses corresponds to full travel. Once the pill is encountered in travel, the stepper motor continues to actuate until a total of 259 pulses is sent. To disengage, another 259 pulses are sent. Once full open is achieved, the stepper motor will slip until all pulses are sent.

Pill Alignment and Cutting Demonstration Video

For the demonstration, all automated actions were driven via a manual interface. Since no feedback loops exist internally to the mechanism, some of the actions are executed multiple times to ensure proper desired operations. The cutting blade is slammed down twice to confirm that a complete cut has been executed.

For the sake of the demonstration, all actions are triggered manually and performed automatically with exception of the return of the blade cutter. The trivial motion reversal of the cutter is performed by hand as changing alligators clips would is necessary to perform this motion automatically. All motions are demonstrated in the videos. All pieces of the mechanism are shown in he photographs.

The servo controller and unit controller circuit is shown installed on the breadboard.

The pill alignment and cutting demonstration video can be found here

LED Display

For visual feedback our design implemented an LED display. This display was purchased from allelectronics.com. Using a datasheet provided at¹ we were able to get the LED display working nicely. The LED is updated 200 times each second, with one fifth of the display active at any one moment.

Figure 10: The Arduino microcontroller is capable of both the control of electrical components and user interfacing as demonstrated by the output displayed on highly visible alphanumeric LED array.

¹ RobotRoom Digital Electonic Blog