IMU stands for “Intertial Measurement Unit”. It measures accerations, angular velpcity and magnetic field. From these measurements and with some clever Mathematics, we can calculate the orientation of a device in a Gravitational field (Earth) and the direction it is pointing in (using the Magnetometer).

Why do we want an IMU?

It is important to know what state the rocket is in:-

• Immediately before launch – i.e. is its movement within prescribed safety limits?
• During launch  – i.e. what is the acceleration that it is experiencing, in what direction is it accelerating?
• And after launch – i.e. how long is it in freefall? Has the parachute deployed?

This IMU will give us a very good idea of how it is behaving. The groundstation will receive all raw IMU data as well as pitch, roll and yaw and will save this to the local database. We have created several alerts that feed on this data to advise us of particular states. e.g. excessive rotation,  acceleration milestones, freefall detection.

We will also have a 3-D representation of the rocket, so we can quickly picture its orientation and motion. Below is a Youtube clip of us testing out an initial release of the code. It works, but there is much to be done.

The IMU is not mounted in the normal sense, with Z up, X and Y in the horizontal plane. We have it mounted with positive X axis up instead. We expect the rocket to be predominantly in X axis. So we are low risk of Gimbal Lock. As it goes up we expect certain amount of swaying from this position. As it is accelerated, (for that few seconds), the acceleration will far exceed the gravitational acceleration, so the routines for measuring orientation will not be of much use. While the rocket is coasting, the IMU will detect ‘freefall’ state. We won’t be able to sense its orientation too well at this point. Once the drogue parachute is deployed we should get some good data on orientation…because the rocket/parachute will more or less be falling at a constant velocity.

I have recently received the PCB sensor board from the supplier and have started soldering components. A few pictures of this are shown below.

Five boards were received in total. (These are “prototype” boards and the minimum order quantity is five). As you can see, the quality of these is exceptional….much better than anything I could do at home.

Above is the GPS module (capable of operation up to 50km) and a Level converter. The Level converter is used by the GPS module and the IMU module. These two modules use 3.3 volts. The Arduino EtherMega operates at 5 volts.

We put this on the bottom for space reasons. This just misses a few pins on the EtherMega by a few mm. So things are tight!

The REAL work beings soon…programming all these sensors. I expect the IMU to be particularly difficult.

We will start using this launch system for small Estee engines/igniters. So the first thing I wanted to do was model an Estes Igniter to find out how the battery would perform.

Below is a picture of a typical igniter.

The resistance was measured to be approximately 3.5 ohms.

If we assume resistance of battery and wires combined is approximately 0.5 ohms, we are looking at a total resistance of ~4 ohms. With an igniter battry of 12 volts, we are then looking at a current of approximately 3 Amps. This is well below the maxiumum current the relays can take (5Amps) and the wiress, connectors should be able to handle this current.

NOTE: When we get the relay board made to replace the current prototype, we will get 2Oz Copper board produced…which should double the current it can handle.

We really wanted to measure the resistance of the battery and the power dissapated by the battery. We do this because want to be sure the battery is not under too much ‘stress’. We know it _should_ be able to handle this current draw. It is rated as 10c (8Amp) continuous dis-charge and Max Burst ate of 20C (16Amp) so it should have absolutely no problems.

Nethertheless, we wish to understand its operation completely so that we are sure we don’t ignite the battery instead!

We decided to perform a small experiment to calculate the internal resistance when a current of 3Amps flows.  Below is a picture of the test load.

Each resistor is a 10Watt resistor with a restance of 4 Ohms.

This means the total resistance of the load is 4 Ohms and it can dissapate a maximum of 40Watts (without exceeding specification of resistors).

We measure the open-circuit resistance of the battery and the closed-circuit resistance of the battery (when connected to the load). The values we got were:-

Vo = 12.15 V

Vc = 11.67 V

We then computed the internal resistance to be roughly 0.14ohms

So the power dissipated through the battery at a rate as shown below:-

P = VI = (12.15 – 11.67) ^2 / 0.14 = 1.6Watts

The power dissipated through the igniter is:-

P = VI = 11.67 ^2 / 3.5 = 38.9 Watts.

This is what we hoped for and expected. The igniter because of its much lesser mass should heat up significantly, exceedingly faster then the battery and ignite the pyrogen.