Wax On, Wax Off: An Assistive Dispensing Device for Workers with Cognitive Limitations

Developers:  Matt Udelhofen, Yasha Saxena, Yash Bhatnagar

Advisors: Tara Moore

Supervising Professor:  Kevin Caves


This report covers the development of an assistive, automated wax dispensing device for a vocational rehabilitation center, Durham Exchange Club Industries (DECI). Workers at DECI are tasked with weighing out and packaging exactly 10.08 or 1.02 lbs of candle wax, but their cognitive disabilities make it difficult to read and interpret a scale. The purpose of the device is to aid clients in reaching these precise target weights without the assistance of a staff member. The final device fits into the existing workflow and work space at DECI, is easy to use, and enables clients to independently fill 10.08 and 1.02 lbs bags for the first time. It achieved 100% satisfaction from DECI staff members.


DECI is a vocational rehabilitation program that provides job training and placement for adults with disabilities.1 Carl and Kelly work within this program, which requires precise measuring and packaging of shaved wax for candle-making kits. While they possess the motor skills required for the job, our clients have mild to moderate cognitive limitations that make reading and interpreting numbers on a scale difficult.2 As a result, they are unable to gauge exactly how much wax must be added or removed to reach a specific target weight. Instead, our clients fill bags with an approximate amount of wax, then transfer the bags to a staff member who adjusts to precise target weights: 1.02 or 10.08 lbs.

The clients are attentive, and understand and follow simple verbal and visual clues. Through continued verbal feedback (e.g. “add/remove more”, “add/remove a little,” “add/remove a pinch”) one of the clients is able to reach the target weight exactly. However, the fast-paced environment of the vocational program makes the current process most efficient with a staff member performing the final adjustments, thus limiting the independence of our clients.

Currently, simple read-aloud scales exist, but cannot quantify how much wax needs to be added or removed to reach the target weight.3 Additionally, automated dispensing systems are commonplace in industrial settings, but have tremendous cost, focus on powders and grains, and are counterintuitive to the goal of the vocational program: providing job training and placement for adults with disabilities.4,5

Project Goals

The goal of this project is to create a device that will interpret the quantity of wax required to reach the precise target weight and aid our clients in hitting that amount without staff intervention.

The device must operate at precision of 0.02 lbs and be versatile for target weights of 10.08 and 1.02 lbs of wax. In addition, the device must be resistant to wax buildup and easy to clean. The device must account for variability in wax density. It must also remain powered throughout the entire work day.

Design and Development

Our device is split into 2 stations to optimize workflow: a manual filling station that undershoots the target weight, and an automated dispensing system for precise final adjustments. The first station is a standardized filling container. The automated dispensing system consists of four components as shown in Figure 1: the scale, the microprocessor, the motor drive, and the wax hopper. The client begins at the manual filling station, scooping wax into a standardized bin that undershoots the target weight by 0.5 – 1 lbs. Then, the client transfers the bag onto the scale at the automated dispensing system. The client presses a button to initiate processing and dispensing. The microprocessor reads the current weight from the scale, and enters a feedback loop that spins an auger within the hopper to dispense wax while monitoring weight. When the target weight is reached, the client receives confirmation that the bag is full and the system resets.

I. Manual Filling Station

The first filling station preserves current procedure at DECI, with one added component. Clients place the empty bag into a standardized container of 9.25±0.25 lbs. Then, clients manually scoop wax to the top of the container, then transfer the bag to the scale. This purposely undershoots the target weight to avoid removing wax from the bag, but also leaves room for variability due to wax density. Thus, the automated dispensing system only serves as a precise top-off.

II. Automated Dispensing System

A. Scale and RS232 Output
To save cost, the automated dispensing system utilizes the scale currently used at DECI (Figure 2). It is an Ohaus T31P, precise to 0.02 lbs and capable of RS232 serial communication. The scale is configured to transmit its weight once per second. With these settings, the client simply places the bag on the scale and the weight is automatically sent to the microprocessor.

B. Arduino Microprocessor
The Arduino Uno is integral in interpreting the quantity of wax required to reach the precise target weight, and then driving a motor to dispense wax to that weight. The client initiates processing with a pushbutton, which then locks out the user until the task is complete. The Uno is equipped with a LinkSprite RS232 Shield V2 that connects to the DB9 port of the scale. The Uno receives the scale’s weight and calculates the difference between the current and target weights, dictating the speed at which wax dispensing will occur. An Arduino Motor Shield allows the Uno to drive a motor at high power. A continuous feedback loop ensures that the weight of the scale is checked while the motor is slowed as the target weight is approached. When the target weight is reached and stable, the motor is stopped to signal completion.

C. DC Motor Drive and Auger Bit
A variable speed DC motor with external gearing is driven by the Uno. In turn, the motor spins an auger (Figure 3), which is a helical bit used to move material from one end to another.6 The motor’s external gearing allows it to churn through wax clumps without drawing too much power. A hex shank keyless chuck couples the motor shaft to the auger bit, but can be unscrewed by hand to remove the bit for cleaning. The motor drive and its housing are suspended above the wax hopper, secured with nuts and bolts that can be removed should maintenance be required.

D. Wax Runway and Dispensing Cone
A “wax runway” allows clients to safely and easily push wax into the dispensing cone (Figure 4). The base of the runway is constructed of Plexiglas to reduce friction between the pushing tool and to minimize wax buildup. An additional Plexiglas sheet is secured between the user and the spinning auger to act as a safety mechanism. A 1.5” x 3.5” opening in the Plexiglas shield allows for wax to be pushed into the dispensing cone.

The bottom of the cone extends into a 2” diameter pipe that houses the auger bit, to protect from the end of the auger. This 2” diameter matches the size of the auger to prevent wax from dispensing when the motor is not spinning. The pipe drops wax above the plate of the scale, providing an easy dispensing mouth that the client can situate the bag of wax around. The 75˚ angle of the cone wall optimally guides wax down towards the auger bit—not too steep to clump wax at the bottom due to its own weight, and not too shallow for wax to stick on the walls.

III. Summary

Together, these components produce a device that meets design constraints and criteria. It fits into the current workspace at DECI and is comprised of durable materials that are resistant to wax buildup. The end device streamlines process workflow and is simple to use. It allows the clients to perform their job more independently by interpreting the exact quantity of wax required to reach the target weight, and dispenses wax to that amount.


The automated wax dispensing system was evaluated against design specifications and performance criteria. This consisted of verification testing by the developers and validation with clients and DECI staff. Lab and client testing revealed the device meets the tolerance required at DECI of the target weight ± 0.02 lbs. The device consistently operates with this degree of precision and has been adequately designed to prevent false positives. All electrical components were selected and housed to be safe and fulfill the demands of the job—current and voltage measurements confirmed that the motor is adequately powered. The device functions on a standard wall outlet, making setup easy for staff members. The device fits well into DECI’s current workspace and workflow. It also is appropriately sized for the clients’ use. All materials intended to be in contact with wax were selected to minimize sticking and aggregation of wax. The materials are also suitable for easy cleaning. Clients were trained to use the device, while the DECI staff was trained to clean and maintain it. We observed that the clients were able to use the device to weigh out exactly 10.02 or 1.08 lbs of shaved candle wax. A survey with a 5-point Likert scale was developed and administered to the staff in order to assess the usability, functionality, and safety of the device. Results from surveys showed complete satisfaction, as all questions received a mean score of 5.0. See Appendix C for detailed device evaluation.


The 2 station workflow—a manual filling station that undershoots the target weight, and an automated dispensing system for precise final adjustments—comes together to produce a device that allows our clients to independently fill 10.08 and 1.02 lbs of wax. The design eliminates the need to read and interpret a scale, functions within the precision tolerance at DECI, is durable, and simple to clean. Client testing confirmed that the device is easy for clients to use and fits into the existing workspace and workflow at DECI. The final price of the device is $205, which is thousands less than an industrial-scale automated dispensing system. Most importantly, the assistive, automated wax dispensing device enables clients to complete their entire job for the first time without intervention of a staff member.


We would like to thank the following people for their patience and guidance throughout the conception of our device: Kevin Caves, Paul Thompson, Tara Moore, Matt Brown, Steve Earp, our Clients, the DECI Staff, and the Duke University Biomedical Engineering Department.

1. “About Us.” DECI. Durham Exchange Club Industries. Web. 6th, March 2017.
2. “Moderate Cognitive Disability.” Medical/Disability Information for Classroom Teachers. LearnAlberta.ca. Web. 5th, March 2017.
3. “Talking Scale.” AbleData. 14th, July 2013. Web. 5th, March 2017.
4. “Products.” Coogar Products, LLC. Web. 5th March, 2017.
5. “Flexible Screw Conveyors.” Flexicon. Flexicon Corporation. Web. 6th, March 2017.
6. “Auger.” Dictionary.com. Dictionary.com, 2017. Web. 4th, April 2017.

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