We replaced the broken MPU-6050 with the replacements we got and Crazyflie is now flying again! Until now we did not really know if the new architecture with digital sensors was working and able to fly. Now we know it can, and the performance of the copter seems really very good!

Replacing QFN components are not that easy if you don’t have access to good tools. Luckily they have a hot air station at work which makes the job a lot easier. We took a photo of the replaced MPU-6050 and it is actually hard to distinguish between the original re-flow soldered one, except maybe for the crocked capacitors. Ohh and what looks as a short on two of the pins are supposed to be there ;-)

We think the stability is a bit better than with the IDG500/ISZ500 gyros. Judge for your self ;-)

After investigating the problem with the MPU6050 from last week we found out that all our prototypes have defect MPU6050 sensors :-(. The bias offset values are way out of spec and several of the accelerometer axis is locked to their min or max value. The manufacturer must have dropped the hole batch in the floor or something because we would have expected at least one out of the six prototypes to have a working sensor. Without working sensors it is hard to make a maiden flight, which we are very eager to do. We will have to order new sensors and hopefully we can replace the new sensors without damaging them.

We also finished to patch all our test copters so that they will now be able to fly when we change the  sensor:

We have found a few workarounds for the JTAG reset problem but finally none of them are needed since we unfortunately have to make a new revision anyway and thus we can properly fix the probem. After spending hours debugging the PWM for the motors we finally opened up the errata and found a serious problem with our new version. Like we mentioned before we moved the PWM for two of the motors so we could have one dedicated SPI for the expansion header and not shared with the radio. This was done by moving one of the motors to PB5 (alternative function TIM3_CH2) while also switching to new sensors that use I2C. According to the errata it’s not possible to clock I2C1 (where the MPU6050 is connected) and TIM3 while using TIM3_CH2 remapped and as output (to drive the PWM for the motor).

On a happier note we did some range-testing of the radio since we have now changed to 0402 components in the radio filter on the USB radio dongle and the quadcopter. The measurements shows that we get up to ~65 meters before we start loosing packets and up to ~80 meters before we loose communication completely. The test was done outside and using the 250Kbps mode.

A large part of this Monday was spent assembling and testing the new prototypes with the digital sensors which arrived today. Eager to try them out we pretty quickly got stuck when we tried to program them with the JTAG. The copter wouldn’t respond to any JTAG commands… After a long time of testing we found our rookie mistake: We have been moving around some of the signals now when we have digital sensors to make the expansion port use a dedicated SPI port and not share it with the radio, which is a great improvement. With this moving around we managed to put one of the motor outputs on the optional JTAG reset pin, NJTRST. The motors are pulled low so they don’t start unintentionally which then makes the JTAG go in constant reset :o This can be walked around by temporarily holding that pin low during the first programming of the boot loader which will then immediately remap the NJTRST pin to be unused by the JTAG. If the bootloader is never removed the problem is gone but if the boot loader is unintentionally overwritten then one have to do the “pull low” trick again. So now we have to decide whether to live with this flaw or do yet another board re-spin… :-( There are still plenty of sensor testing left to do that maybe need another re-spin.

Before we have any maiden flight video the show below is a photo of the new Rev.D PCB. There are more in this album.

Crazyflie Rev.D PCB

We are still working hard on the Crazyflie code while we are waiting for the new prototypes. We are also working on finalizing the Crazyradio, the radio dongle we are making to communicate with Crazyflie.

In order for us to test the radio hardware performance we brought a RFExplorer:

The radio chip (nRF24U1) is put in continuous carrier mode, which makes it emit constantly at a single frequency. Below is a screenshot of the measured frequency and power from the radio dongle:

This measurement is not that useful as an absolute value (for one we do not have a RF test chamber) but it will give us the opportunity to compare the next prototype with this one. Our next radio prototype uses smaller SMD component for the RF parts which is supposed to give better performance. We already compared it with another dev board and our radio seems to have similar performance :).

Due to the IDG/ISZ-500 gyros EOL problem we are removing the  IDG/ISZ-500/BMA145 and going for the MPU-6050 instead. We where pretty certain the gyro part of the MPU-6050 would work but not so sure about the accelerometer.

To minimize the risk we wanted to try it out with our design before pushing the order button. We looked around for small IMUs with this chip and found that the FreeIMU uses the sensors that we are interested in testing. So we bought one and attached to the Crazyflie using the expansion connector. Here’s a image of what it looks like:

Since we now free up some space by replacing three sensor ICs with one we added a HMC5883L magnetometer and MS5611 pressure sensor which are the most common IMU sensors right now. If they will be mounted or not in the final version depends on cost and possible performance increase. If we don’t mount them there is always the possibility to do this yourself. Actually, as of this writing, we just made a virgin flight using the MPU-6050 data from the FreeIMU with good results.

So as we were putting the finishing touches on what we hoped would be our last prototype before the final production version we noticed something rather serious on the InvenSense webpage. The IDG500/650 (and the ISZ-500/650) that we are using are EOL (and the LTB has passed). So this puts us in a rather tight spot where we have a couple of alternatives:

Stay with the IDG/ISZ 500/650 – We could stay with the sensors we have and try to source as many as possible but this would leave us in an awkward position if we get more demand than we can source gyros.

Analogue replacement – We could find an analogue replacement that would replace the X/Y and Z gyros we have today (that are analogue). The best candidate for this is currently the new ST gyro L3G462A but it is still under Evaluation and we don’t know if and when it will be available. It’s easy to put in our current design but we are unsure about the performance and immunity to vibrations.

Going digital – The most attractive option but the option that requires most work is switching from analogue to digital sensors. This is a step that we wanted to take eventually but taking it now delays everything but we do get a chance to get a bit more up to date by putting in a MPU-6050 and maybe a pressure sensor. But we are not sure how the sensors will respond to the vibrations and ripples on the supply voltage.

Our current plan is to drop the old analogue sensors from InvenSense and start on the digital design using the MPU-6050. We will keep the analogue ST gyro as a backup plan just in case we hit into any problems with the digital version.

Do you have any other ideas for sensors or comments about the performance of the MPU-6050 or L3G462A (if you managed to get a sample)?