VendArt at MakerFX

 

 

This effort aims to restore a National Vendors, Inc. Manual Cigarette Merchandiser Series 222, which was a classic mechanical cigarette vending machine that was popular between the 1960s and 1980s.  The machine that I’m helping restore is from 1975, as indicated by a stamp on the vending machine’s internal hardware. 

Date stamp found inside the merchandiser

 

Another MakerFX member owns this project, but I’m supporting the effort by restoring the mechanical vending mechanisms and integrating modern payment solutions while preserving the machine’s original and existing hardware.  When completed, this vending machine will sell local makers’ unique makes at various Central Florida maker events, like Maker Faire Orlando. 

The machine’s vending mechanisms are locked when idle, preventing the pull knobs from dispensing items from their respective magazines.  When payment is accepted, a solenoid engages the vending mechanisms to unlock the pull knobs and enable vending.  I was tasked with replacing the missing solenoid, designing and implementing a solenoid driver, and writing the code that drives the solenoid during a transaction.

 

 

Tech Stack

 

 

Mechanical

 

Left to Right: Internal vending unit front, merchandise magazines, internal vending unit side, vending lock mechanism detail

 

 

Solenoid 12V 10mm 5A Push Pull

 

Technical Specifications

 

 

XYZ Adjustable Solenoid Carriage (left) and Mount (right)

 

XYZ Adjustable Solenoid Carriage and Mount Assembly

 

 

·       Carriage and mount were designed in Fusion 360

·       Carriage and mount were 3D printed on the Bambu P1S

 

Detail of solenoid on XYZ carriage and mount, and fastened to existing machine bracket

 

 

Retrofitted XYZ Adjustable Solenoid Carriage and Mount Assembly on Existing Bracket

 

·       The solenoid carriage and mount are adjustable in the XYZ directions, so the solenoid can be precisely positioned to engage the vending lock mechanism

o   The solenoid fastens to the carriage

o   The carriage fastens to the mount

o   The mount fastens to the vending machine’s existing bracket

 

·       I designed this XYZ solenoid carriage and mount assembly to fit the existing bracket

·       This design makes it possible to position the solenoid plunger (piece that extends and retracts) with the vending machine’s mechanical lock mechanism

·       The XYZ carriage and mount positions are ‘locked’ with screws, washers, and nuts creating bolted joints

Bolted Joint

 

 

Solenoid Cycle Temperature Testing

·       The solenoid’s plunger extends by creating a magnetic field when current flows through the tightly wound coils surrounding the plunger

·       Continuous current flowing through the solenoid coils produces a lot of thermal energy (heat)

o   I reduced the solenoid’s input voltage from 12V to 6V to reduce the heat produced when current is flowing through the coils

·       Since the solenoid is mounted to a carriage and mount that was 3D printed with PLA, I want to make sure that the hear produced by the solenoid does not exceed the melting point of PLA

 

·       I created a test bed to measure the temperature of the solenoid with an infrared thermometer

o   I attempted to keep the ambient temperature constant throughout each experiment run

§  No AC running, ambient temperature measured during the experiment

o   The infrared thermometer was mounted onto a tripod to secure its distance from the solenoid through the experiment

§  The infrared thermometer’s documentation recommends measuring a temperature from 14”, so I mounted the thermometer at 14” from the solenoid’s top surface where the temperature was measured

o   The solenoid was attached to the carriage mount to observe the solenoid’s temperature effect on the PLA

o   The carriage mount was securely taped to the test bed to reduce movement between engages and disengages

 

·       I wrote code to iterate through an engage and disengage procedure that parallels the use-case

o   This makes the experiment repeatable and the data more accurate

o   Code available on github

Solenoid Cycle Testbed Setup

 

·       This experiment iterated through an engage-disengage cycle for multiple cycles, and was repeated at various locations on the solenoid

 

·       Procedure

o   Measure surface temperature at the designated location BEFORE starting engage-disengage cycles

§  This location is where the cycle temperatures will be measured for the entire experiment

o   For 130 cycles

§  Solenoid engages for 15 seconds

§  Solenoid disengages and temperature is measured at the designated location

§  Solenoid remains disengaged for 10 seconds

o   NOTE: DO NOT touch the solenoid or move the infrared thermometer during the experiment

 

Locations on solenoid where temperatures were measured

 

 

 

·       Results

o   Cycles 1-100 cycles: Capture the temperature rise toward a temperature steady-state

o   Cycles 101-130: Measure steady temperature

 

o   The results from the experiment agreed with the claim that the solenoid is hottest near the center of the coil length, as shown in my data below

o   The experiments’ maximum temperatures had a delta of approximately , with the center of the coil length hitting  and the end of the coil length hitting

 

Solenoid Cycle Temperature Rise and Steady-State Data

 

Solenoid Steady-State Temperature Data

 

 

 

·       Data Variability

o   The Average Solenoid Steady-State Temperatures had a sample variance of 5.42℃^2, or a standard deviation of 2.33℃

§  Variance represents the average of the squared distance from the mean, which conveys the distribution of the data

 

 

o   This data has a low variance, which means that the data is closer to the average

o   Low variance is an indication of consistent measurements during data acquisition.  Conversely, high variance indicates unusual or inconsistent data

 

·       Conclusion

o   The Average Solenoid Steady-State Temperatures had a sample variance of 5.42℃^2, or a standard deviation of 2.33℃

o     PLA begins to soften at  and melts at

o   This cycle test was more aggressive than the cycles that the fielded solenoid will experience and was still  under the softening temperature

o   The heat produced from the active solenoid should not affect the PLA carriage mount

o   As an extra experiment, I allowed the solenoid to cycle through engage and disengage for 200 cycles with no impact to the solenoid’s performance or the PLA carriage mount

§  No data collected – Just observation for any impacts to the system at high cycle counts

 

·       How to Improve Future Experiment

o   Automate temperature readings with software instead of manually pressing the thermometer trigger

§  Reduces human error by with more consistent timing between measurements

§  Eliminates all shaking of the thermometer when user presses the thermometer trigger

§  Collect more frequent temperature data throughout the experiment to create a more complete temperature profile of the solenoid

o   Perform experiment in a more controlled environment

§  Thermal chamber

 

 

Electronics

Raspberry Pi Pico W

 

 

MP1584EN DC-DC Buck Converter (Step-Down Converter)

 

·       I wanted the solenoid and microcontroller to share one 12VDC power source, but both components have different electrical requirements:

o   Solenoid: 12VDC, 1.7A

o   Raspberry Pi Pico W: 5V, 95mA

·       A buck converter, AKA step-down converter, efficiently reduces a higher input DC voltage to a lower output DC voltage

o   This protects the lower-voltage components that are connected to a higher DC source voltage

·       An inductor stores and releases electromagnetic field and smooths the current by resisting rapid changes

·       A switch creates a duty cycle that outputs a desired voltage lower than the source

o   The lower the duty cycle, the lower the store in the inductor

o   Think Pulse Width Modulation (PWM)

o   The switch is usually a transistor (MOSFET)

·       A diode creates a path for the current when the switch is turned off

·       A capacitor smooths the voltage rippled

·       The buck converter unit uses a control (potentiometer) to control the output voltage

 

 

IRLZ44NPBF Logic Level MOSFET

IRLZ44N Logic-Level MOSFET

 

·       A MOSFET is an electronic switch that controls current flowing through a semiconductor channel using an electric field

o   MOSFET Terminals

§  Gate (G): Control Input

§  Drain (D): Where current flows out

§  Source (S): Where current flows in

 

o   N-Channel:

§  Turns on when the gate is more positive than the source

§  Current flows from drain to source

§  Efficient and low heat

§  Motor drivers, solenoids, power supplies, high-current applications

 

o   P-Channel

§  Turns on when the gate is more negative than the source

§  Current flows from source to drain

§  Simpler but less efficient at high currents (higher )

§  Battery-powered devices

 

·       The IRLZ44N is a logic-level N-channel MOSFET, which allows us to engage and disengage the solenoid by passing a boolean to the MOSFET gate with the solenoid driver code

·       2 main parameters for selecting the appropriate MOSFET: Voltage (V) and Current (I)

 

 

·       MOSFET Spec Sheet

o   Spec sheet assumes proper cooling, so operating the MOSFET at the rated specs without proper cooling (fans, heat sinks, etc.) can overheat or melt the unit

o   The Current spec only says that the MOSFET will not melt when operating at 47A when properly cooled

§  While the Current and Voltage are used to determine how a system will operate, the MOSFET’s internal resistance, , sets the threshold

·        tells how much power needs to dissipate from the MOSFET at a constant load

o   Constant Load: >10 seconds without a heat sink

IRLZ44N MOSFET Spec Sheet

·       Thermal Power Generated

o   Recall – the internal resistance, , is the spec that we will use to threshold our MOSFET selection calculations

o   Since there isn’t perfect control of the load on the MOSFET, we degrade the  by 20-30%

§       Increase  by 20%: 

 

o   Now calculate the thermal power generated, , by the system given the load’s specs

§  Since we are using the MOSFET to drive the solenoid, we are using the solenoid specs defined above as the load

 

 

·       Thermal Power Lost

o   These thermal losses need to somehow escape the MOSFET, and that can be determined with the thermal resistance,

§  This tells you how much the MOSFET will heat up for each watt of thermal loss,

 

o      Now that we’ve solved for , we can calculate the thermal losses to confirm that they are within the MOSFET’s specifications

§  The MOSFET spec sheet indicates that the Thermal Resistance, , for a MOSFET without a heatsink (Junction-to-Ambient) is

·       Use the heatsink spec sheet if a heatsink is screwed onto your MOSFET

·       We assume that ambient is  when unknown or unspecified

§  We calculate the thermal loss by multiplying the thermal power generated, , and the thermal resistance,

 

o   The calculated thermal loss can be compared to the Operating Junction  and Storage  Temperature Range indicated on the spec sheet:

 

o   Since the calculated temperature from thermal losses falls within this spec, we determine that the MOSFET can safely operate under the solenoid’s load

 

 

·       MOSFET Drive Levels

 

o   Since this MOSFET is a logic-level MOSFET, we need the voltage from the logic signal that the microcontroller produces

§  The solenoid will be driven by the Raspberry Pi Pico W microcontroller, which outputs 3.3V

o   The MOSFET spec to track here is the Static Drain-to-Source On-Resistance,

o   Notice that the resistance values on this spec are more in line with the resistance we calculated losses with at the 20% margin

§  Resistance goes up as the voltage goes down, which is the relationship indicated by

 

·       Confirm MOSFET selection

o   The spec sheet doesn’t indicate Static Drain-to-Source On-Resistance, , but I still want to confirm that my system won’t exceed the MOSFET’s thermal constraints

o      I’m using the indicated  for 5V and 4V to linearly extrapolate  for the microcontroller’s 3.3V

 

 

o      Now that  is calculated for my system, I can revisit the calculations and use my estimated  to more accurately determine the thermal effects on the MOSFET

 

 

 

o     At , I’m still well within the Operating Junction  and Storage  Temperature Range indicated on the spec sheet, so my solenoid driver will not thermally compromise the MOSFET

o   This MOSFET is excessive for my solenoid driver’s electrical parameters, but its large voltage and current margin will be reliable and usable

 

 

Solenoid Driver Wiring Schematic, designed in TinkerCAD Electrical

 

 

Solenoid Driver Breadboard

Breadboard for solenoid driver development

 

 

Soldered Perfboards

Left to Right (per image): Solenoid, Solenoid Driver, Buck Converter, Raspberry Pi Pico Microcontroller, Redundancy Engage and Disengage Buttons

 

 

Solenoid Driver/Buck Converter Stack

Assembled solenoid driver unit on soldered perfboard

 

 

Software

Vending Flow of Execution

 

·       Point of Sale (POS) Solenoid Engage

o   The trigger that commands the solenoid to engage and unlock the vending lock mechanism is an initiated transaction

o   Once initiated, the solenoid will remain engaged until an item is dispensed or the defined engage time expires

§  Solenoids engage by running a continuous current, so they get very hot when engaged

§  A timed disengage is a safety parameter that protects the solenoid from overheating in its engaged state

o   The point of sale is performed via digital payment platform (DPP)

§  Square, Stripe, Venmo, Zelle, etc.

o   When a transaction is initiated, a webhook is posted to the defined endpoint from the DPS

o   Receipt of the webhook then commands the solenoid driver to engage and unlock the vending lock mechanism

 

 

·       Webhook Implementation

Production Webhook Implementation, Raspberry Pi Zero 2W

 

o   I decided to use a webhook instead of an API to keep the design efficient

§  API: System (client) pulls data or an event from another system (host) when the client system sends a request

·       Host: Data Source

·       Client: Data Requester/Recipient from an endpoint

§  Webhook: System (client) automatically pushes data to another system (host) when the client system detects an event

·       Host: Data Recipient

·       Client: Data Source that sends a POST to a URL endpoint

 

o   Instead of polling the endpoint with an API, a webhook is sent to a server when the endpoint detects an event

§  The host (receives the webhook) is ‘listening’ for a POST request from the client (sends the webhook)

§  The API for the service I’m using limits the number of calls to the host, but not webhooks as long as data receipt is confirmed

 

o   The server to which the webhook posts is hosted locally on the Raspberry Pi, but the server needs to be publicly accessible so that the external DPP can post the webhook

o   A secure tunnel creates an encrypted pathway for data to travel between two endpoints over HTTP protocol

o   The secure tunnel securely exposes the local server on the Raspberry Pi (host) to the internet, enabling the external DPP (client) to post to it

§  The Raspberry Pi (host) server ‘listens’ for a webhook post from the external client

§  When payment is accepted by the DPP, the DPP (client) sends a webhook to the server hosted on the Raspberry Pi (host) via a secure tunnel to inform of a received payment

§  The local host must send a receipt confirmation to the client when the webhook is received, typically within approximately10 seconds of receipt

·       Confirmation: 200 code

 

 

Dev Implementation

 

Development Webhook Implementation, Raspberry Pi Pico W

 

o   My dev environment uses a Raspberry Pi Pico W, which is a microcontroller, so it can’t run a secure tunnel

o   I’m using my laptop to create a local server and implement a secure tunnel that receives webhooks from the DPP endpoint

o   The laptop sends a Boolean to the Raspberry Pi Pico W’s IP on a common network when a webhook is received, commanding the solenoid driver to engage the solenoid in the vending lock mechanism

o   The deliverable will consolidate the Raspberry Pi Pico W and the laptop with a Raspberry Pi Zero 2W, which can host a local server and implement a secure tunnel, following the first diagram in this section

 

 

·       All code implemented in this notebook is publicly available on my github!

o   Solenoid Driver Demo

o   VendArt - RasPi Pico W

o   VendArt – RasPi Zero 2W

Helpful Links

·       MOSFET Explained - How MOSFET Works

·       MOSFET Selection Calculations

·       How to Use N-channel MOSFETs as Switches

·       Secure Tunnel Implementation with ngrok