Hi all! First post here; I read through the "How To..." so hopefully I'm not missing any needed info.
Please note that I haven't started programming/building of my project yet; rather, I am in the conceptual design stage and would like to be better informed before the buying begins $$ Also, this is a bit of a lengthy post so to cut to the chase I'm trying to decide between 1) stepper motors and 2) brushless motors w/ encoders for my application (but I come to that question at the end as well).
I am attempting to make a system similar to NASA's ARGOS (Active Response Gravity Offload System). Googling "NASA ARGOS" will bring you here: NASA - JSC Engineering - Active Response Gravity Offload System.
It's essentially a large overhead crane that tracks the position of the payload (in ARGOS's case, a person) and applies a constant tension to the cable attached to their harness perpendicular to the X/Y plane (which is anchored approximately around their CG), reducing their downward acceleration and mimicking a micro-gravity environment.
From the website, ARGOS's capabilities are below:
- Working volume: 41’ x 24’ x 25’ (LxWxH)
- Z-axis speeds of 10ft/s w/ 300 lb payload; 4ft/s w/ 750 lb payload
- X/Y-axis speeds of 10ft/s w/ any weight
I hope to build a system with the following capabilities (differences):
- Working volume: 12’ x 8’ x 5’ (LxWxH)
- Z-axis speeds of 10ft/s w/ 2 lb payload
- X/Y-axis speeds of 10ft/s w/ 2 lb payload
My current concern is with the type of motor to use for the X/Y axis. Knowing the position/velocity/acceleration of the payload, I need to be able to move the X/Y motors to track that position, up to 10ft/s (a hopeful ballpark acceleration is 16 ft/s^2 or about 0.5G).
Attached is a rough layout sketch (Gravity_System_Layout.jpg) with what I'm thinking looking at the system top-down, including color-coded components. My payload will be a small RC car which weighs about 4 lbs. When I say 2 lbs payload above, what I really mean is 2 lbs offloaded; this is independent of the attached payload weight, since the tension in the cable attached to the RC car will ~2 lbs at all times.
Some quick details about the image:
- Black - system structure/frame
- Orange - rails for y-axis gantry movement
- Red - gantry AND rails for x-axis offload rig movement
- Green - offload rig (containing offload motor not shown, topic for later discussion)
Motor type will depend on a) needed rpm/torque (tradeoff game) and b) positioning accuracy.
a)
x-axis force math
To simplify the math, I'll be making a bunch of shortcuts. The offload weight is 2 lbs, but that is the force acting down through the offload rig. If we assume the offload rig weighs 1.5 lbs, and the coefficient of friction is u=0.3 (assume kinetic), our normal force of 3.5 lbs would give us a frictional force of 3.5[lbs] * 0.3 = 1.05[lbs] in the x direction. So, the x-axis motor would need to meet/exceed the velocity and acceleration requirements moving 1.5 lbs of mass and overcoming ~1 lbs of frictional force.
y-axis force math
With the same frictional constant and a gantry weight 3 lbs, we get the following:
gantry weight + offload rig weight = 3[lbs] + 1.5[lbs] = 4.5[lbs]
normal force = 4.5[lbs] + 2[lbs] = 6.5[lbs]
frictional force = 6.5[lbs] * 0.3 = 1.95[lbs]
So, the y-axis motors (collectively because there are 2) would need to meet/exceed the velocity and acceleration requirements moving 4.5 lbs of mass and overcoming ~2 lbs of frictional force.
b) Ideally, the positioning would have an accuracy of about 1/16" or less, but the ability to control the gantry within about 1/8" might be good enough.
The first thought I has was to use stepper motors because they are easy to control and have high holding torque. A setup I envisioned included the following components:
Nema 17 Bipolar Stepper
- 42mm x 48 mm (Width x Depth)
- 83.6 ozin (5.2 lbin) holding torque
- 200 step (1.8 deg)
- 2.0 A rated current
- (datasheet attached)
OR Nema 23 Bipolar Stepper
- 57mm x 56 mm (Width x Depth)
- 178.5 ozin (11.2 lbin, ~1.3 Nm) holding torque
- 200 step (1.8 deg)
- 2.8 A rated current
- (datasheet attached)
Arduino Uno CNC Shield V3 w/ A4988 Drivers
- 12-36 V board operating voltage
- Driver step modes: full, 1/2, 1/4, 1/8, 1/16
- 2 A driver rated current
- 35 V driver rated supply voltage
- (Driver datasheet attached)
- (CNC Shield image attached)
- I apologize, I can't find anymore specific datasheets for the board
The thought was to use the NEMA 17 for the x-axis and two NEMA 23's for the y-axis, using 6mm wide GT2 timing belts and synchronous wheels. The main problem I'm facing is that stepper motors don't have a very high rpm limit compared to brushless motors, so their synchronous wheels would need to be pretty big to come close to the 10 ft/s velocity needed. To give an example, attached is a torque curve of NEMA 23, similar to the one I've described (the product number is: 23H2065-300-4A, NEMA 23 Rotary — DINGS' Motion USA). The NEMA 23 in the torque vs rpm plot was run at the following values:
- 1.7 N*m holding torque
- 3.0 A
- 1.8 deg step angle
And the motor I have shown above has 1.3 N*m holding torque and would only be able to run at 2 A.
These steppers and the CNC Shield are an attractive option because they are super easy to interface with the Arduino Uno Rev 3, but do you think I'm going down a rabbit hole with trying to make stepper motors work to fit my needs? Are these components just not powerful enough? And should I look into brushless (w/ encoder and FOC controls) or other options?
I'd like to keep the total cost of motors and related components under $250 if possible which is why I was pushing for stepper motors. Maybe I should go for a CNC board/drivers rated for more amps, more powerful stepper (even larger NEMA 23) that is geared up and lower my requirement standards? (yes I'd like to meet the current ones but can put into easier reach if necessary)
Again, sorry for the long post. I didn't want to leave my project under-constrained for the reader's understanding!
NEMA_23_Datasheet.pdf (758 KB)
A4988_Datasheet.pdf (1.04 MB)
