Tuesday, August 28, 2012

More FFv1.2s Testing / The Short-Lived FFv1.3ss

After successfully testing one FFv1.2s motor controller on the Talon quad, I got enough parts to complete a full set of six.

Unlike the first prototype, I built these in the "minimum thickness" configuration, meaning no XBee or XBee headers. They can only be programmed with an FTDI breakout board. They also use lower-profile pushbuttons and inductors. Here's a comparison to the "minimum thickness" configuration of FFv1.1:

The overall dimensions are 2.00in x 1.50in x 7mm and 11.3g for the FFv1.2s board alone. (FFv1.1 board-only dimensions are 3.00in x 1.50in x 12mm and 28.4g.) With 16AWG wiring, an external DC bus capacitor, and connectors, the FFv1.2s controller weighs in at 35.1g. This makes it a much better fit for the Talon quad than the 66.0g FFv1.1 with giant D2Pak-7 FETs at 14AWG wiring.

With a cameo by Twitch.
I also switched back to the KK2.0 flight controller to make the Talon quad as light as possible. (You will see why in a bit...) I did a bit of firmware hacking on the KK2.0 to enable the fast angle estimate, which I describe in the previous post. The result is quite amazing: it now has very precise and fast angle control. After the first indoor test from the previous post, I have been further tuning it and flying outside. It's wonderfully stable and easy to fly now, even in some wind.

The controllers are all running closed-loop speed control, so the signal received is directly commanding an RPM. So far this has been working well. Here's a data capture from some outdoor testing with the KK2.0 + FFv1.2s ESC combination:

It shows several seconds of rapid back-and-forth roll inputs, followed by a few seconds of hovering, then a few seconds of climb. The RPM tracks well, although there are some small and rapid oscillations similar to what can be sort-of seen in the indoor test video. The oscillations are especially apparent in the current data. This is, I suspect, because the inner rate loop gains were slightly too high. Testing in the wind with lower rate P gain yielded much smoother flight (still with wonderful fast self-leveling).

The reason for reverting to the lightest possible configuration for the Talon wasn't to go easy on the new controllers, though. It was to try an ambitious payload. This quad normally carries a GoPro camera, which weighs about 200g with its impact-resistant case. The heaviest thing it's carried is my handheld video camera, at 300g. But with the new motors, I suspect it could carry a lot more. Enter TOBL2.

Photo credit: ycraf.blogspot.com.
TOBL2 is the second generation of Max's multifaceted iPhone-controlled robot, probably best known for it's wall-flip ability. It will be making an appearance at Marker Faire NY in September and we thought it would be cool to try to skycrane it with a quadrotor. At about 1lb (450g), though, the first step was to see if the Talon quad could even lift it...

Well that was relatively successful. Other than the swaying, which can probably be minimized with a better rope configuration, the payload seems to be fine. Now we just have to see if TOBL can lower itself down somehow...

The Short-Lived FFv1.3ss

I've been watching Texas Instruments' line of motor drive chips for a while now. In addition to the DRV8301, which has performed flawlessly as a gate driver / current sense amplifier / buck converter controller on FFv1.1 and FFv1.2s, I've been wanting to try out the DRV8332. The DRV8332 is a fully integrated three-phase driver, meaning logic-in / FET half bridge-out. It goes up to 50V and the heatsinkable version (8332) has a phase current rating of 8A continuous, 13A peak. 

And it's tiny. Perfect for FFv1.3ss (super-small?), I thought, so I started a layout for a 1.00in x 1.00in controller based on the DRV8332. To show just how small it is, here's a comparison to the IXYS mini-brick modules I use on the 3ph line of motor controllers:

The 1.00in x 1.00in board is roughly half the board area as the IXYS module alone. The blue chip on the back of the board is the DRV8332 itself. Unlike the DRV8301, it doesn't include the nice buck converter controller or current shunt amplifier, so there was a good deal of work to do adding tiny versions of those in:

The MSOP-8 on the left side of the board is a dual op-amp, configured for low-side current sense differential inputs using two resistor networks. I really liked this layout. The cluster of components on the right side of the board are two switching power supplies. The first, a wide-input buck converter based on the  LT3991-5, would allow me to take full advantage of the 50V input range of the DRV8332 while still supplying an efficient 5V/500mA BEC for logic and receiver power. The second, a tiny boost converter based on the LT3460, would supply 12V/100mA for the gate drive.

Sadly, that's as far as I got before shelving this design. It has a fundamental flaw that I hadn't considered until this point: the FETs in the DRV8332 are not that great. Even though its current rating says it would be a viable controller for something the size of the Talon quad, the FET losses make it not worth it. Even at a relatively modest phase current amplitude of 5A, the 80mΩ per-phase resistance would add 3W of total dissipation to the controller (with sinusoidal commutation). Compare this to the minuscule 0.075W that FFv1.2s would dissipate with with its Super SO-8 FETs. 

At 150W/kg, the rough power-to-weight ratio for the Talon quad, the extra 2.9W of dissipation would be equivalent to 19.3g of weight. So, to justify the tradeoff, the FFv1.3ss board would have to weigh 19.3g (or more) less than the FFv1.2s. (Assuming the wiring and capacitors weights are fixed, as they would be for a given current.) Since the FFv1.2s boards are only 11.3g, excluding wiring and caps, there is no way the extra dissipation of the FFv1.3ss is justified. 

Factoring in the weight of the necessary heat sink for the DRV8332, I'm not sure there is a likely scenario where it would be justified based on a power-to-weight analysis. If volume is weighted more highly than weight, as it might be for some other applications like a mini robot, the DRV8332 might win out. It might also be justified on size alone at very low current where the efficiency is less of an issue. But in that case, there are things like the Toshiba TB6588FG that I used on 4pcb which could probably do the job.

It's too bad, because I was really starting to like this layout. But it seems like it wasn't meant to be, as far as flying things are concerned.

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