Andrew B. Wright, Ph. D., SM ’88
I am ambitious in my mechanical design efforts at this point. I have a 3D printer. I have Adafruit’s and Pololu’s electronic modules for small robotics. So, I’m going all in on designing a battery pack that does all the things I want a battery pack to do.
Because I can.
- supply regulated power to the motors (x4), the controller, and the sensors
- provide protection against excessive current draw
- allow for easy, flexible recharging
- provide the user with a method of instantly dropping power
- “look marvelous”
I went through many iterations, and I’m probably not done.
One of the first iterations is located on thingiverse.com. I plan to include all the final design information for this battery pack on that site once I’ve completed this project.
There are two stages in this design, the battery pack for the Beaglebone alone and the battery packs for the motor drivers. I decided to split the task because the power requirements for the motors is substantially bigger than for the Beaglebone. If the motors were to exceed the current draw, it might reset the Beaglebone during operation.
Further, by separating the design into three pieces, the complexity of each piece is dramatically reduced. The motor battery packs are duplicates of each other, so, one design can be replicated.
Battery Pack Design Process
I started the design process for a battery pack in my Elements of Mechanical Design class in Spring 2019. I have a set of parts in Solidworks to allow custom number of batteries to be used. I’m nearly ready to put these designs on this site. The last steps involve making a connection so that you can slide the battery pack in and out. This is a task that cordless power tools have been doing for years, so I’ve been reverse engineering their work. This grows out of the standard NEMA connection that you plug in all your AC appliances.
I want to use as much off-the-shelf hardware as I can get my hands on. Making custom connections, while not difficult, adds steps and complexity that will slow the process down.
The closest thing I’ve found that seems well adapted to my task is automotive blade fuses. The terminals for these fuses is 0.65 mm thick tin-plated zinc.
Motor Controller Design Process
Power is the heart of any mechanical device. In mobile devices, this power is supplied by batteries.
I have used four of the Vexrobotics 393 motors, which can continuously draw 3 A at 7.2 V, for a power of 4*7.2*3 = 86.4 W.
Using 8 NIMH rechargeable batteries in series would give a supply voltage of 8 * 1.2 V = 9.6 V. If the batteries are rated at 2 Ah, they could produce 9.6 * 2 = 19.2 W for an hour. Not a good match.
This robot could run for only a few minutes at maximum continuous power, and it would pound the batteries into oblivion in a few cycles.
Increasing the batteries to 12 and wiring them in banks of six series and two parallel gives a voltage of 7.2 V and a capacity of 4 Ah. The continuous power is now 28.8 W for an hour.
Splitting the job into two sides allows the design to be replicated on each side, and drops the current through the power system in half.
Each battery weighs about 30 g, for a total weight of 24 * .03 kg = 0.72 kg. Not bad at all. The continuous current of six amps (3 A per motor) is a little higher than the desired maximum of 5 A for the fuse. But, it can be made to work.
What can you do with 100 W?
100 kg in a standard gravity yields about 1000 N. Lifting this mass through a height of 10 m requires 10000 J. If this is done in 10000/100 = 100 s, the power required would be 100 W. In other words, this little power pack could lift a 200 lb person up 3 stories in about 2 minutes.
|‘AA’ Male Battery Contact||Keystone Electronics Corp.||5221||1||Mouser|
|‘AA’ Female Battery Contact||Keystone Electronics Corp.||5222||Mouser|
|‘AA’ Dual M/F Battery Contact||Keystone Electronics Corp.||5212||Mouser|
|‘AA’ Rechargeable Batteries (nimh)||various||3||various|
|Low Profile Universal PC Auto Fuse Clip||Keystone Electronics Corp.||3557||2||Mouser|
|1200 uF capacitors||various||2||various|
|Voltage regulator (low power)||Adafruit||Powerboost 1000 basic||1||Adafruit|
|Voltage regulator (high power)||Texas Instruments||PTN78020WAH||2||Mouser|
|Power switch (low power)||Mountain Switch||107-1261B||1||Mouser|
|Power switch (high power)||Schurter||1241.6831.1120000||2||Mouser|
|Power connector||uncertain||2.1 mm barrel connector (3310)||1||Adafruit|
|JST power connector||uncertain||1||Adafruit|
|Terminal block||uncertain||2 pin terminal block (724)||1||Adafruit|
|Blade fuse||Bussman||amperage TBD||2||Mouser|
|Wire wrap tool||Jonard||JIC-22681||1||Mouser|
Notes on my process so far …
There has been a revolution in battery powered tools. These tools use interesting configurations to allow their battery packs to be inserted, removed, and recharged. As a first pass, I want to use some of these features, without going all the way to the full bug.
Makita and Milwaukee provide options to use their battery packs with custom tools. In this case, you would need to buy the battery pack and charger, but could either buy the replacement parts for the power tool side of the package or an adapter.
Toolpartsdirect.com has schematics for many of the Makita power tools. Looking at the schematic for the drill, for instance, 650733-3 is the part number for the drill side and 643860-3 looks like the mechanical parts. So, for about $30, you can get the bits that allow you to hook an 18v LXT Makita power pack into the drill.
This is more power than a Beaglebone project needs; however, for bigger robots, this might do the trick. I’m planning on using this concept in my Mechatronics class in the Fall 2019 semester.
I used to use vexrobotics.com Victor 884 speed controllers. These operate on 12 v rather than 18 v. However, the Vex pro product line is introducing the Victor SPX and Talon SRX. The Talon SRX will allow you to go up to 28 V.
The other option in this case is to use the Makita 12 V battery packs along with the Victor SPX. This is almost certainly a less expensive option; however, it looks like power tools are headed towards 40 V, and the 12 V options may become obsolete before too much longer. Bigger voltage means lower current for the same power. All things are better with lower current.
Another feature of power tools is the switch to brushless motors. Brushless motors require a circuit to perform the pole reversal that brushed motors have built in; however, brushless motors are more efficient and wear less. Electronics have become very cheap, so it is a matter of not much time before brushless motors replace brushed motors.
FIRST robotics was a pioneer at using drill motors as the core motor for medium sized robots. Surprisingly, drill motors are compact and powerful. Over time, they began to use custom designed motors. However, the revolution in battery powered power tools may give the hobbyist or educational roboteer an option to get cheap, powerful motors that are adapted to a similar application to robotics.
In order to use these motors with a Beaglebone, the missing link is a high power motor driver, which is what the Talon and Victor provide. There is a straightforward design of a MOSFET based h-bridge that one of my former students developed that may make it a simple job to hook up a powerful h-bridge to the Beaglebone. I may put this forward to my Mechatronics students in the coming semester.