The Wemos D1 Mini is a WiFi equipped microcontroller with 11 digital I/O pins, 1 analog input, and equipped with a microUSB connector for both programming/power.
Stepper motors are DC motors that move in discrete steps. They have multiple coils that are organized in groups called "phases". By energizing each phase in sequence, the motor will rotate, one step at a time.
When evaluating the differences between stepper motors however, you need to factor in the nominal voltage they operate in and how much torque they can produce to perform work.
Friction torque is also referred to as pull out torque.
Pull-out torque is the maximum torque that can be delivered without losing steps. It reaches its maximum at the lowest frequency or speed, and decreases as frequency increases. If the load on the stepping motor during rotation increases beyond the pull-out torque, the motor will fall out of step and accurate operation will not be possible. Pull-out torque is typically the most emphasized of the four characteristics and is represented by the pull-out torque curve or slew rate showing torque vs speed (frequency).
Pull-in torque is the maximum torque at which a motor can start rotating at a given frequency. The stepper cannot start rotation with the load torque exceeding the pull-in torque. The pull-in torque also decreases as the frequency increases and is represented by the pull-in torque curve showing torque vs speed (frequency)
The 28BYJ-48 stepper motor is a miniture stepper motor with an operating voltage of 5VDC. It has a step angle of 5.625 degrees, with 64 steps making a complete 360° rotation.
The 28BYJ-48's friction torque and pull in torque is 1200 gf.cm and 300gc.cm. While the friction torque is 1.2kg.cm so the stepper doesn't lose steps, the pull in torque is quite low at 300gc.cm. This means that it cannot start moving if the load is greater than 300 grams.
The 28BYJ-48 stepper motor is generally arranged in a unipolar configuration with 5 pins. The ULN2003 unipolar stepper driver is also generally supplied with the 28BYJ-48 stepper motor. The red lead is connected to both poles of the stepper motor.
A modification of the 28BYJ-48 stepper motor into a bipolar stepper motor involves severing the connection to the two windings of the stepper motor. This will allow the stepper motor to be powered via a bipolar stepper driver like a A4988 or DRV8825 stepper motor. According to several sources, the modification can supply approximately double the pull in torque.
Thus, 300gf.cm becomes 600gf.cm which is right in the ball park for moving a 500mL HDPE bottle of Ethanol. Ethanol has a lower density than water, and because 1L = 1KG than 500mL of water is 500 grams. The measured weight of Ethanol in a 500mL HDPE bottle is 470 grams.
Unipolar ULN2003 Stepper driver while moving:
5VDC @ 177mA
Unipolar ULN2003 Stepper driver while holding position/idle:
5VDC @ 170mA
Since 5VDC * 170mA = 0.85 Watts, the stepper motor does use quite a bit of power. Nearly 3x to 4x the amount of power of the ESP8266 when idle at 0.23 Watts.
Therefore, to preserve available power the stepper motor would need to be disabled after performing it's task.
As indicated, the 28BYJ-48 stepper motor can be modified to produce twice the amount of torque. This involves cutting a trace on the circuit board of the stepper motor itself and converting it from a unipolar stepper motor into a bipolar stepper motor.
Note that you will also need to calculate and adjust the current on the A4988 stepper motor driver as well. This doesn't just apply to the 28BYJ-48 stepper motor, but any bipolar stepper motor such as the NEMA 17.
Here is an instructional tutorial on performing that: Adjusting stepper motor current
Initial testing of the 28BYJ-48 stepper motor and ULN2003 stepper motor driver have indicated that it isn't up to the task. This is even after the bipolar modification to the stepper motors to increase the available torque and driving the stepper motor with an A4988 bipolar stepper motor driver.
As a result, both the 28BYJ-48 stepper motor and the ULN2003 stepper motor driver will be put aside.
An alternative stepper motor option is to use a NEMA 17 stepper motor. The NEMA 17 stepper motor isn't a specific model or brand as it is a specification that applies to a large number of stepper motors that adhere to the NEMA 17 specification.
The NEMA 17 has quite a bit more torque and is generally used in 3D printers and other applications. As a result, they are also an inexpensive option. However, they are not quite as affordable as the 28BYJ-48 stepper motors. A NEMA 17 stepper motor generally retails for around $25 to $35 CAD.
The NEMA 17 rotates at 1.8° per step, with a complete rotation taking 200 steps. The NEMA 17 is also a slightly different than the 28BYJ-48 stepper motor as it also uses a permanent magnet in it's construction.
A great primer on the NEMA 17 stepper motor is available here
The first test involves testing the NEMA 17 stepper motor to determine if it can reliably lower a 500mL HDPE bottle of Ethanol 12 feet into the permafrost.
Rather performing this test in an ABS pipe that has been installed in the ground, the test will instead be performed both above ground and indoors.
This initial indoor test will determine how much power is used by the NEMA 17 stepper motor, the A4988 stepper motor driver, and the ESP8266 microcontroller controlling them during operation in ideal conditions.
While the ESP8266 itself should be capable of running nearly indefinitely on 4x AA batteries, the NEMA 17 stepper motor and A4988 stepper motor driver uses quite a bit more power.
A 12 foot 3" ABS pipe will be setup and fixed in place. At the top of the ABS pipe will be a 3D printed assembly to hold the tested stepper motor and a winch for a stainless steel bead chain that runs from the top to the bottom within the interior of ABS pipe.
The 2.4mm stainless steel bead chain has a tensile strength of approximately 25 lbs or 5kg, and is more than adequate for the load being placed on it.
And rather than using Ethanol for the tests, the 500mL HDPE bottle will be filled with 470 grams of water rather than with Ethanol. Due to the lower density of Ethanol, this is the approximate equivalent of 500mL of Ethanol.
A 3D printed holder for the 500mL will also be attached to the 2.4mm ball chain and as the stepper motors turn, the bottle and holder will be lowered down 10 feet. To simplify the recovery technique, the 2.4mm bead chain will instead be placed on a large spool.
The exact amount of power used to perform the steps to move the bottle from the very top to the very bottom will be performed to ascertain how much battery power is needed to perform the task of lowering the bottle of Ethanol into the permafrost.
The ABS pipe will then be setup outdoors with batteries to supply power seperately to both the ESP8266 and the stepper motors. The batteries for the stepper motor will need to sit idle for a prolonged period of time in temperatures below -30°C.
As indicated in the Arctic Malaise Trap Experiment page, the ESP8266 can be reliably powered for an extremely long period of time using either an 18650 Lithium Polymer battery, or with Energizer AA Ultimate Lithium batteries. The duration and longevity of the 18650 Lithium setup is unknown and the Energizer AA Ultimate Lithium batteries are confirmed to work extremely well even in temperatures below -40°C.
The outdoors test will involve setting up the ESP8266 to trigger the lowering of the 500mL HDPE bottle at a predetermined time after being left outside for a month.
The method for closing the Ethanol bottle has yet to be developed, but will likely involve the compression of a hollow silicone cork installed in the top of the 500mL HDPE bottle.
The deposition of a hollow silicone plug could be installed on a flap mechanism facilitated by a simple low cost servo, and the silicone plug itself could then be compressed using a machine screw to positively seal the sides of the plug through compression.
Provided that works, it would serve as the culmination of the primary mechanisms in use. Namely, opening and closing a valve, and lowering the Ethanol into the permafrost for long term preservation.