Options for Brushless Motor Control?

[r691175002]

Ether is correct that I will need some kind of feedback in this application. Most of the motion will be at very slow speeds or completely stopped and there will be a lot of reversals. The motors will also be fighting a lot of torque. Unfortunately most hobby controllers won't even let you reverse, so accurate positioning is out of the question.

There are a few reasons that I want to avoid off-the shelf sensored motors:
Because of their applications, they tend to be more expensive, heavier and harder to find than hobby motors. They are also mostly inrunners and I really want to avoid having to gear down 60k rpm four times.

I have no problem attaching an encoder to the back of a sensorless motor to get feedback.

I still find it weird that it is so hard to find something that will work. Closed loop brushed controllers practically grow on trees - brushless motors aren't that unusual...

[BornaE]

How about we go at this the right way.

Define your functional requirements,
Come up with ideas that meet those requirements,
Rank and select the best one.

My main question is why brushless when brushed motors are so easy to obtain with a very reasonable price?

-Borna

Edit:

-Requirements-

Robot pushing force:
Maximum robot Speed:
Voltage:

[Ether]

Originally Posted by BornaE

How about we go at this the right way.

Define your functional requirements,
Come up with ideas that meet those requirements,
Rank and select the best one.

You're assuming he hasn't already done this. Perhaps you're right... perhaps you're wrong. It's always good to ask though.

[r691175002]

Robot must carry ~15LB between two waypoints several kilometers apart without GPS. Terrain is generic grass/rocks/dirt/sand whatever is in the way. The lighter the robot is the better.

I don't consider brushed motors an option. For example, this 3000W motor from NPC:
http://www.npcrobotics.com/products/...at=20&mode=gfx
weighs 15 pounds. A brushless motor of the same power would weigh around a pound.


We still don't know exactly what the robot will look like and not much planning has been done yet, I'm just looking at some options. I'd like to avoid ackerman steering because some of the manoeuvres will need to be precise but it depends on what the software guys want. Crab drive/meccanum wheels and skid steering are also all on the table.

Given how light brushless motors can be it is my opinion that it is easier and lighter to have a separate motor for each powered wheel since there will be some form of suspension.

Hoping for a weight of ~40KG and a max speed of 4m/s.

[Ether]

Originally Posted by r691175002

Robot must carry ~15LB between two waypoints several kilometers apart without GPS. Terrain is generic grass/rocks/dirt/sand whatever is in the way.

Are you saying you want the robot to travel several kilometers over rocks and sand autonomously, with nothing but dead reckoning to keep it on track?

I don't consider brushed motors an option. For example, this 3000W motor from NPC:
http://www.npcrobotics.com/products/...at=20&mode=gfx
weighs 15 pounds. A brushless motor of the same power would weigh around a pound.

With a similar torque vs speed curve? Can you post a link please...

I'd like to avoid ackerman steering because some of the manoeuvres will need to be precise but it depends on what the software guys want. Crab drive/meccanum wheels and skid steering are also all on the table.

You're not going to get precise maneuvers on rocks and sand with mecanum wheels.

[r691175002]

A satellite image will be provided. The software guys would know more specifics but the current plan is some kind of SLAM combining data from cameras, encoders, LIDAR and an IMU.

My weight comparsion was perhaps an exaggeration but not by that much.

The NPC motor has a peak power of 2800W. This AXI motor doesn't list any curves but the peak current draw is 4000W and if I do the math with their minimum efficiency it will be ~2700W.
http://www.modelmotors.cz/index.php?...14&line= GOLD
The AXI motor weights 2.2lb. The brushed motor also runs at ~60% efficiency while the AXI can maintain 85%+. Regardless of the exact numbers, the difference is huge. AXI is one of the most expensive brands, but similar motors can be had for <100$ if so desired.

I've thought a bit about my options and the chances are we will try to roll our own and fall back to the roboteq controller if it doesn't work out. I'll skip rotary encoders and just add hall sensors to the AXI motor since we don't need high resolution and it makes the motors compatible with any sensored controller.

[Ether]

Originally Posted by r691175002

This AXI motor doesn't list any curves but the peak current draw is 4000W and if I do the math with their minimum efficiency it will be ~2700W.
http://www.modelmotors.cz/index.php?...14&line= GOLD

They give you Kv and R, which is all you need to approximate the torque vs speed curve.

R=0.027 ohms, Kv=225rpm/volt = 23.56 (rad/sec)/volt

So at V=25 volts:

stall current Istall = V/R = 926 amps (WAY beyond the operating envelope)

stall torque Tstall = V/(R*Kv) = 39.3 Nm

no load speed FreeSpeed = V*Kv = 589 rad/sec

There's your torque vs speed curve (approx).


The spec says this motor can tolerate 110 amps for 20 seconds.

So, at I=110 amps:

torque = (I/Istall)*Tstall = 4.67 Nm

speed = (1-I/Istall)*FreeSpeed = 519 rad/sec

power = torque*speed = 2424 watts (for 20 seconds, before it burns up)

The brushed motor also runs at ~60% efficiency while the AXI can maintain 85%+.

At 110 amps and 25 volts, the AXI is drawing 110*25 = 2750 watts and putting out 2424 watts. That's an efficiency of 2424/2750 = 88%. And it can only do that for 20 seconds, according to the spec. At 85%, it will be less than 20 seconds.

[r691175002]

The axi motor I listed is a larger model that is run on 10 cells (LiPO so 3.7-4.2V a cell). I did 37V*84A*0.85% = 2641W. It will peak higher.

[Ether]

Originally Posted by r691175002

The axi motor I listed is a larger model that is run on 10 cells (LiPO so 3.7-4.2V a cell). I did 37V*84A*0.85% = 2641W. It will peak higher.

OK, but now you're comparing apples to oranges: a brushless motor at 37 volts to a brushed motor at 24 volts.

FWIW, the heavier brushed motor will likely have a much wider operating envelope.

[BornaE]

Ok. if you want your maximum speed to be 4 m/s (lets say that is at 90% of your free speed). it means, your robot will have maximum power at a speed of

(4m/s)/0.9/2 = 2.2 m/s

if your robot has 4 x 3000W motors, your total power is 12KW.

Since power = Force x Velocity, at 2.2 m/s, your robot can pull 5.45 KN which means it can lift~ 545 kilograms vertically.

Here is where the issue is. The maximum coefficient of friction/traction that you are can dream of getting is lets say 1.5 Which means you are only going to be able to pull 60kg and the rest of the power is wasted.

Thus your usable power is: 60 * 10 * 2.2 = 1320 watts. or 330 watts per motor.

I am guessing you have not been involved in FRC. a CIM motor with a Jaguar speed controller on each wheel will give you the necessary power to accomplish your task.

motor controller: http://parts.digikey.com/1/parts/180...mdl-bdc24.html

motor : http://www.andymark.com/product-p/am-0255.htm

you also need a gearbox for that. with 15cm diameter wheels, you would need an 8:1 transmission found here:
http://www.andymark.com/product-p/am-0145.htm
modified with a 19tooth and a 45 tooth gear.


Hopefully I was able to help.

Also here is a robot we build in 2010. I am sure it would be able to accomplish your task. with some minor modifications

http://www.youtube.com/watch?v=LVtHI6fpeGM