Thursday, February 19, 2015

Motor and ESC considerations for the CableCam

This Blog post is one of an entire series
Motivation and Design
Download Link for CAD drawings (updated regular)
Bill of Materials
Motor and ESC considerations
CableCam rope
Building the DSLR CableCam
CableCam controller board

Brushless versus Brushed DC motors

Without going into the details of what each is, the difference between the two options are
  • Brushless is way more powerful. The 300g motors we use have 1800Watt of power
  • Their weight is significantly less than an equally powerful brushed motor
  • Brushless motors come in various KV numbers which lets you adjust the rpms by simply exchanging it
  • They are cheaper and easier to get
  • Brushless motors have issues at startup unless they have a sensors built in
  • The Electronic Speed Control (ESC) is way more difficult to build
With the price of current ESCs we have committed ourselves to using brushless motors but they have to be sensored and the ESC has to utilize the sensor input signal.

This way we get the smoothness of the brushed motors with the advantages of the brushless motors. Brushed motors might perform a tick better at ultra slow speeds but decide yourself, I have created a video for that. It also shows the difference between a sensored motor and sensorless, by simply disconnecting the sensor cable.


Picking the right motor and some math

In the intro page of the cablecam I provided an example where a RED camera and gimbal is used. The overall weight is 12kg for the entire rig and they are using an LRP Dynamic 8 1600kV motor at 6s (=22V).

We should now approach the calculation from two sides. First the rpms at no load at all, then the power it provides.

RPM check

For the rpms the KV number is the key. The more windings then motor has, the stronger the magnetic force but at the same time, it will rotate slower. 1600kV tells us that the motor is built to provide 1600rpm per volt. Since we use a 6s battery in this example, meaning 22V, the motor speed would be 35200rpm.
That boils down to 586 revolutions per second (=35200/60).

Since we have a timing belt with 16 teeth on the motor and 84 teeth on the drive shaft, the wheel itself will rotate slower at 112 revolutions per second (=586*16/84).

Let's try to convert that into forward speed. The wheel has a diameter of about 76mm, one revolution means a distance of 238mm (=76*pi) or 0.238m.

If the wheel makes 112 revolutions per second, that is 26m/s (112*0.238) or 96km/h or 60mph.

So from this side we are safe, this combination is fast enough. The only problem might be that the motor does not have enough power. Just imagine above motor is very tiny. It can be rated to any KV number, it will never have enough power to even move the cablecam a bit.

Power check

But actually, above motor provides 1760W power at 14V. How much that might be at 22V the vendor does not tell. It could be up to 2640W if the power increases linear with the volt. Anyway, let's stick to the 1760W.

Power = Force * Speed

Okay, we have a problem here. What will the force be? The force will consist of roll resistance, wind resistance, grade resistance and inertia. So let's try a very extreme example, our cablecam should drive vertically up, what will its possible speed be?

1760W = 12kg * 9.81m/s² * speed
<==>
speed = 15m/s

That's 54km/h or 34mph driving vertically up. In other words that is more than enough power for our applications.

On the other hand, let's assume our rope is not horizontal but goes uphill at a 30° angle. Then the downhill force would be 50% of above. But we want to accelerate the cablecam quickly as well, but at the same speed we would achieve just 0.5g acceleration.


GoPro CableCam example

Motor is a 300kV motor with 1500W at 6s, but we operate it with 3s batteries only. No gear is used, it is a direct drive. Weight overall is 2.5kg.

300kV * 11V / 60 = 55 revolutions per second
55 * 76mm * pi /1000 = 13m/s or 47km/h

750W = 2.5kg * 9.81m/s² * speed
<==>
speed = 31m/s or 110km/h

Here we have an example of a motor that has more than enough power but is limited by the rpms. 

Guess what the GPS measurement showed? 45km/h!

Keep in mind, we have approached the problem from two extremes, the rpms with no load at all and the power. But in reality it is a mixture of the two. Imagine the motor would provide just half the power. For sure it would not be able to reach the rated rpms just based on the kV number. Not even close.

Further more, while an acceleration of 1g sounds like a lot, it actually isn't for the GoPro CableCam. You want to get to to speed quickly, adjust your speed with the object to film asap. I would not opt for a motor with significantly less power. Does work for sure, but limits your options.
That's the beauty of the brushless motors, we do not have to restrict ourselves to low powered motors as their weight and costs are the same essentially.


The proper ESC

Most mistakes are made when choosing the ESC. At first sight, any ESC supporting a brushless motor and the motor's sensor signal would work. Work in the sense of "being able to turn it".

First distinction is car versus plane/boat. An ESC meant for planes is not built for smooth startup, it might not even allow to go into reverse. So such an ESC would be completely useless for us.

For cars there are various types as well: Street cars, Rock Crawler, Truck Puller.
Usually there is not one ESC per type but one ESC either supports all or just street cars.
I would suggest to pick an ESC that supports Rock Crawlers as this comes our application the closest. Slow movements, forward/reverse etc. In particular the ESC should have the following options explicitly called out
  • Sensor support
  • Reverse 100% of Forward speed. We do not want to drive the cable cam at different max speeds depending on the direction.
  • Forward/Reverse mode without brake. We want to put the stick in one position to drive forward and in the other position for reverse. Most ESCs engage the brake when going into reverse and you have to pull the stick back in neutral and into reverse again to drive backward.
  • Drag Brake to control the amount of braking force when the cablecam is moving and suddenly the stick is in neutral. We want a rather high drag brake as stick in neutral means to stop the cablecam asap.
  • Initial Brake to hold the cablecam in position. On a horizontal rope not needed but you do not want the cablecam to roll downhill by itself.
Finally, the ESC has to match the motor. This is not much of a problem with regular inrunner brushless motors but the AlienPowerSystems 300kV motor used in the gopro cabelcam is an outrunner and most ESCs were not even able to bring it up to medium speed.

The ESCs I tend to suggest currently are the SkyRC TS120 for the gopro and the DSLR cablecam, when 3s batteries are enough. Else the TS150.


The best ESC

I have a request for you: Could you please send a suggestion to SkyRC (http://www.skyrc.com/index.php?route=information/companyinfo) to enhance their ESC supporting a Car-Governormode with a sensor motor and in forward/reverse mode?
The idea is the following:
What happens when you put an RC car on a downhill road and the throttle is in neutral? It will roll downhill. Not too fast because of the initial brake setting but it won't stop or stand still. But isn't that exactly what we want? The stick should not control the throttle (=energy) it should control the speed (=rpm). In Heli mode ESCs do have such mode to make sure the rotor speed is the same no matter of the rotor blade pitch. I'd like to have the same for the car forward/reverse mode as well.
That would have numerous advantages:

  1. Above "stick in neutral means stay where you are" behavior would be great already by itself.
  2. The initial brake would consume less energy as it is dynamic. The ESC does reduce the initial brake power in neutral until the car starts rolling, then it does increase it again. So depending on the steepness of the road, it applies more or less power to the motor winding.
  3. Super low speed. Currently you increase the power slowly to overcome the static friction, but the rolling resistance is less and hence the car picks up speed and you have to carefully dial back the throttle. Which does not work well. As a result there is a minimum speed for the car which is quite high in fact. Something around walking speed. If the ESC would consider the input as an rpm signal, it could adjust the power to any kind of speed.
  4. More natural control. With the car you want to control speed, not throttle. Moving the stick from 25% to 50% should double the speed, not double the energy and hence increase the speed by half or less (kinetic law, wind resistance).
  5. Smooth brake control. Today, when you are in the forward/reverse mode and you want to brake, you put the stick in neutral and the drag brake kicks in. The drag brake is a fixed value, so either you are braking with full force or you are not braking, just reducing the energy/throttle. But if the stick position would control the speed, then the ESC would know that you want to decrease the speed to e.g. half the current value and it can by itself activate the drag brake until the target speed is reached and then increase the power to hold that speed. Today you have to move the stick into neutral, wait until the target speed is reached and then increase the power again to hold that speed.
I am of the opinion that such a feature would be excellent for the cable cam but actually for all cars except street racers. Trucks, Rock Crawlers, Buggies,.... all would benefit from such.




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