Overview

This flight computer (FC) was designed primarily to provide timing for parachute deployment and staging of multistage water rockets. The FC can drive one or two standard RC servomotors that are used to release latches on parachute deployment systems. The timing can be initiated in a variety of ways such as acceleration detection (G-switch), burnout detection (zero pressure sensor), or deceleration detection (inverted mercury switch). Each of these happens during different periods of a flight. The FC can be used on very simple rockets or more advanced ones.

Each servomotor has two configurable positions. The FC moves the servomotors to their first position when it is turned on, and moves them to their second position based on the configuration of the flight computer parameters.

 

Features

The FC has the following main features:

• Dual RC servo motor control
• 7segment LED display indicating status information
• Built in launch detect G-switch
• External launch detect / burnout / negative-G trigger input
• Buzzer for indicating status and helping to locate lost rocket in tall bushes
• EEPROM used to store settings while power is turned off
• 15 configurable control parameters
• Altimeter/auxiliary power connector

These instructions give an example of how to use the flight computer in a parachute deployment mechanism.

 

NOTE: Servo Timer II is now in stock! See here for more details Servo Timer II.

Operation

The flight computer can be switched into one of three different modes:

1. Normal - This is the normal mode for pre-launch and in-flight operation. Two of the 15 parameters can be configured in this mode.
2. Configure – In this mode all 15 parameters can be set to specific values.
3. Factory Reset – In this mode the configurable parameters are reset to their default factory settings.

Figure 1 shows the flow diagram for the three modes. Each mode will now be discussed in more detail.

Figure 1 - Operational Mode - Flow Diagram

Normal Mode

To enter normal mode, simply turn the FC on. Before delving deeper into how to configure all the FC parameters let’s have a look at a typical flight profile (Figure 2) and see how it behaves. The flight profile is broken up into a number of different phases. The 15 configuration parameters affect what happens in each of these phases. 

Figure 2 - Typical Flight Profile Operational Phases

Operational Phases
 

• Initialize – The FC sets both servomotors to their preset positions (Position 1) and reloads all the parameters to the previously stored values.

Display shows:

• Set D1_Delay (“2.”) – In this phase the user can change the D1_delay parameter by repeatedly pressing the PGM button. Any changes made are automatically stored in the EEPROM. See the Configure Mode section for explanation of the D1_Delay parameter.

Display alternates between:  <-- -->

Pressing the ARM button moves to the next phase.

• Set D2_Delay (“5.”) – In this phase the user can change the D2 Delay parameter by repeatedly pressing the PGM button. Any changes made are automatically stored in the EEPROM. See the Configure Mode section for explanation of the D2 Delay parameter.

Display alternates between: <-- -->

Pressing the ARM button again arms the rocket.

• Armed – The FC waits for the rocket to launch. Typically one would arm the rocket first and then pressurise it. While in this phase the rocket beeps once a second and toggles the display showing the values set for the D1 Delay and D2 Delay parameters. This allows for quick visual and audible check to see if the rocket is ready for launch.

Display alternates between: <-- -->
A trigger will cause the rocket to go to the next phase.

• D1 – Typically the first delay (D1) is activated at the time of launch. D1 usually operates in the ascent stage of the rocket flight. When this period expires the FC goes to the next phase.

Display shows:

• M1 On Time – At the end of D1 servomotor M1 moves from position 1 to position 2. The amount of time it spends doing that is set through the M1 On Time parameter. The servo remains in this position until power is turned off. This is the time that a parachute or the next stage is released.

Display shows:

At the completion of the servo repositioning the FC moves to the D2 phase. 

• D2 – The FC waits a second period of time before the second servomotor is activated.

Display shows:

At the conclusion of the D2 delay the FC switches to the next phase.

• M2 On Time – Servo M2 moves from position 1 to position 2. The amount of time it spends doing that is set through the M2 On Time parameter. The servo remains in this position until power is turned off.

Display shows:

When the motor finishes moving the FC switches to the lost delay phase.

• Lost Delay – If enabled, the FC waits for another period of time before starting the lost rocket alarm. Typically the rocket is found by the time the alarm sounds and power is turned off. The period can be set from 0 minutes to 31 minutes.

Display shows:

• Lost Alarm – When the Lost Delay period expires, the alarm starts sounding and continues indefinitely until either power is turned off or the batteries are run flat.

Display shows:

 

Flight Profiles

Following are a few different flight profiles that can be configured with the FC. Please refer to the examples section for more details on these: 

• Single stage with launch detect
• Single stage with burnout detect
• Two stage – Staging and parachute with launch detect
• Two stage – Staging and parachute with burnout detect
• Dual Parachute with launch detect
• Dual Parachute with burnout detect

 

Configure Mode

In the configure mode the user can cycle through all the parameters and change their values. All values are automatically stored in the EEPROM. Table 1 describes each of the parameters and the range of values that can be set for each. Table 2 describes the values themselves. To avoid confusion a parameter name on the display is designated with a “.” (decimal point) and the value belonging to the parameter does not have the decimal point. 

Since the values can range 0 to 31, and there is only a single 7 segment display, alpha-numeric characters (0 – 9, and A – V) are used to display all values.  

To enter configure mode and change the parameters do the following:

1. Make sure power switch is set to OFF.
2. Hold down only the ARM button and set the power switch to ON.
   The “S” symbol appears on the screen indicating you are in “Set” or configure mode
3. Press the ARM button repeatedly to cycle through the parameters.
4. Press the PGM button repeatedly to cycle through the parameter values.
5. When finished changing the parameters switch the power switch to OFF.
    

Parameterer	Mnemonic	Default	Range	Display	Description
D1 Multiplier	D1Mul	4	[0 – 7]		0 = 0.01 sec / step 1 = 0.02 sec / step 2 = 0.05 sec / step 3 = 0.1 sec / step 4 = 0.2 sec / step 5 = 0.5 sec / step 6 = 1.0 sec / step 7 = 2.0 sec / step
D1 Offset	D1Off	F	[0– V]		D1 = (D1Dly + D1Off) * D1Mul
D1 Delay	D1Dly	0	[0– V]		D1 = (D1Dly + D1Off) * D1Mul
D2 Multiplier	D2Mul	4	[0– 7]		0 = 0.01 sec / step 1 = 0.02 sec / step 2 = 0.05 sec / step 3 = 0.1 sec / step 4 = 0.2 sec / step 5 = 0.5 sec / step 6 = 1.0 sec / step 7 = 2.0 sec / step
D2 Offset	D2Off	0	[0– V]		D2 = (D2Dly + D2Off) * D2Mul
D2 Delay	D2Dly	0	[0– V]		D2 = (D2Dly + D2Off) * D2Mul
M1 Position 1	M1P1	0	[0– V]		Motor 1 Position 1. (prior to launch)
M1 Position 2	M1P2	V	[0– V]		Motor 1 Position 2 (after delay D1)
M1 On Time	M1On	F (2.4 sec)	[0– V]		0.16 sec / step
M2 Position 1	M2P1	0	[0– V]		Motor 2 Position 1 (prior to launch)
M2 Position 2	M2P2	V	[0– V]		Motor 2 Position 2 (after delay D2)
M2 On Time	M2On	F (2.4 sec)	[0– V]		0.16 sec / step
Lost On Time	LOn	1	[0– V]		0.25 sec / step
Lost Off Time	LOff	4	[0– V]		0.25 sec / step
Lost Delay	LDly	5 (5min)	[0– V]		minute / step

Table 1 - Configurable Parameters

 

Value	Index	Displayed as		Value	Index	Displayed as		Value	Index	Displayed as
0	0			B	11			M	22
1	1			C	12			N	23
2	2			D	13			O	24
3	3			E	14			P	25
4	4			F	15			Q	26
5	5			G	16			R	27
6	6			H	17			S	28
7	7			I	18			T	29
8	8			J	19			U	30
9	9			K	20			V	31
A	10			L	21

Table 2 - Parameter Values

Functionality

Timing

The two main timing delays D1 and D2 are configured through 3 parameters each. The first of these parameters is the multiplier. The multiplier defines the “granularity” of the timing of the other two parameters. The multiplier can be set to increment the delays in steps as low as 0.01s or as high as 2s. This allows the timing delays to be set anywhere from 0.01seconds to 128 seconds depending on the requirement. Both D1 and D2 are configured in the same way. 

The formula below gives the delay period:

Dx = (DxDelay + DxOffset) * DxMultiplier

Where x = 1 for delay D1 and x = 2 for delay D2.

The second parameter is the Offset. The Offset allows you to set a zero point, or minimum time for the delay. The last parameter is the variable delay (DxDelay). This delay can be varied while in the normal mode and is intended to be adjusted between launches to finetune the recovery system release time or if the pressure, water volume or nozzle size change. Both the offset and delay can be set to one of 32 values.

Short delays are useful for staging rockets, and long delays are useful for opening main parachutes late into the flight.  

Timing Periods

Table 3 lists the minimum and maximum values that each specific phase can be configured to.

Table 3 - Delay Ranges

Motor Position

Each of the servomotors has two configurable positions Position 1 and Position 2. Position 1 is the position prior to launch. This would typically be the latched position of the recovery system. Position 2 is the position of the servomotor after the expiry of the appropriate period. The motors remain at their Position 2 position until power is turned off.  

RC servomotors are positioned using a specific pulse train on their control line. The FC generates this pulse train for only a short period of time determined by the M1 On Time or M2 On Time parameters. This allows battery power to be conserved when the motors are not required to move. The On Time should be adjusted in such a way that the motor has enough time to move from one position to the other. Sometimes this needs to be adjusted depending on the motor used or if there is a load on the motor and it takes longer.  

The full range of movement of each servo is divided into 32 steps. This means a servomotor that normally has a 90 degree range of movement will be able to be positioned with an accuracy of 2.8 degrees, while a servo that has a 200 degree range of movement can be positioned with an accuracy of 6.25 degrees. 

The positions are made configurable to allow the servomotors to be mounted inside the deployment system in any orientation. Sometimes clockwise operation is needed and sometime anti-clockwise is required.  

The positions can also be adjusted between flights in the Configure Mode if something becomes misaligned or stretched in the deployment system and the servomotor positions need to be updated to compensate.  

Lost Alarm

The FC can use the built in buzzer to sound an alarm after a delay to assist the user to locate the rocket if it is lost in tall grass or bushes.  

There are 3 configuration settings associated with the alarm. The first is the number of minutes it takes before the alarm is activated (Lost Delay). This is useful for two reasons. Firstly it allows the FC to conserve the battery since in most instances the rocket will be found before the alarm needs to sound. Secondly it allows the FC to remain quiet in flight when used in combination with a video camera that is also recording audio. 

The other two parameters are used to configure the sound duration during the alarm phase. Setting the Lost On Time to a short beep and the Lost Off Time to a long period allows the FC to conserve power, but for more noisy environments the alarm can be set to produce noise more frequently at the expense of power consumption. 

Setting the Lost On Time to zero allows the lost alarm functionality to be switched off altogether.  

Triggering

Most of the fun only happens after the FC is triggered in the Armed phase. Triggering can be achieved a number of different ways depending on the rocket design. Some examples include:

1. Launch detect – This is typically achieved using a G or acceleration switch. Triggering occurs as soon as the rocket leaves the launch pad. Another variation to this is a set of contacts that are closed and an insulator is removed from between them during launch. The insulator is usually attached so it stays with the launcher.
    
2. Burnout detect - This technique uses a pressure switch to detect when the pressure inside the rocket has reached atmospheric pressure, or some preset value. This is useful for staging rockets. This allows the rocket to deploy the second stage at the correct time regardless of how much pressure or water was used in the booster.
    
3. Negative G detect – This is another technique used to detect when the rocket has stopped producing thrust and the rocket starts to slow down. An inverted mercury switch can be used here where the mercury floats upwards to make contact as drag continues to slow the rocket down after burnout. Negative Gs happen shortly after burnout.

We’ll leave it up to the rocket builder to come up with their own way to trigger the FC. The FC can have a G-switch fitted directly to the PCB, but the FC also provides a connector for external triggers inputs.

Examples

Following are a number of examples demonstrating how to configure the FC for various scenarios.

Scenario 1 – Single Parachute:

You want to use a single parachute on a simple rocket. You want the parachute to deploy no earlier than 3 seconds, but you want to be able to increase that time in 0.2  second increments on subsequent flights as you change the amount of water or pressure. You also don’t want to use the lost alarm functionality. You will also be using the built in G-switch.

Figure 3 - Simple rocket with single parachute scenario

This is how you would configure the computer:

We select our Delay1 Multiplier to be 0.2 seconds (value = 4) (see table Table 1)

We set the Delay 1 Offset to be 3 seconds (value = 3 seconds / 0.2 = 15 = F)

During normal operation you can then change the “2.” parameter in 0.2 second increments meaning you can increase the time from 3 seconds to a maximum

D1 = (15 + 31) x 0.2 = 9.2 seconds.

Motor M1 is set for full range movement by setting “6.” to 0 and “7.” to V. If the servo was to move in the other direction the two values would be reversed. The servomotor will be on for 2.4 seconds. This is set by parameter “8.” (value = 2.4 / 0.16 = 15 = F)

We set the “C.” parameter to 0 to turn off the lost alarm.

	D1			D2				M1
	D1Mul	D1Off	D1Dly	D2Mul	D2Off	D2Dly		M1P1	M1P2	M1On
Param
Value	0.2sec	3 sec	0 sec					0	32	2.4sec
	M2			Lost
	M2P1	M2P2	M2On	LOn	LOff		LDly
Param
Value

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button twice and the rocket is ready to go. This makes it very simple to use once configured.

Scenario 2 – Rocket with twin parachutes:

Say you have a more advance rocket that wants to deploy a drogue parachute at apogee and then the main parachute some time later when it has had a chance to descend some way. This technique is often employed to prevent large parachutes from being opened at high speeds as well as to stop the rocket drifting too far in windy conditions.

You also want to use the lost alarm functionality as you are flying in wooded terrain.

A simulation of the rocket predicts that it will reach apogee after 7.25 seconds. Because this is the first flight you will want to have some control over the 1^st delay either side of the 7.25 seconds on subsequent launches.

Using drogue parachute descent rate calculations you decide that the main parachute should deploy 22 seconds after apogee.

This is how you would configure the computer:

Calculate D1

Here we can choose say the 0.2sec or 0.5sec step value for the D1 multiplier.  I will choose the 0.2 second multiplier (“0.” Set to 4) so that we can fine tune the deployment. I am going to allow the D1 delay to be adjustable from 6 seconds upwards. D1 Offset (“1.”) is set to 6 / 0.2 = 30 = U.

If I had chosen to use the 0.5second multiplier then the D1 offset parameter would have been set to 6 / 0.5 = 12 = C.

We now need to set the D1 delay parameter (“2.”) to bring up the total time as close to the 7.25 seconds as possible. Since we chose the 0.2s multiplier, we need to set D1 delay to 7.25s – 6s = 1.25s. Now 1.25s / 0.2 = 6.25. We pick the closest integral value to 6.25 which is 6. So this means the deploy time using the delay formula becomes:

D1 = (30 + 6) * 0.2 = 7.2seconds which is close enough to the required 7.25 seconds. This setting allows us to chance the delay in Normal Mode from 6 seconds to 12.4 seconds.

Calculate D2

D2 is less critical since the 22seconds after apogee is only a rough estimate. For this delay we may choose the 1 second or 2 second multiplier. I’ll choose the 1 second multiplier. (“3.” = 6)

Let’s say we want to set a minimum time of 14 seconds and vary the time up from there. D2 offset would be set to 14 x 1 = 14 = E. And to get to 22 seconds we set D2 delay parameter to 22 – 14 = 8. This gives us the ability to vary the D2 period from 14 seconds to 46 seconds in the Normal Mode.

Set M1 and M2

Because of the deployment system configuration we need to set the M1 servomotor to turn clockwise and the M2 servomotor to turn anticlockwise when activated. We set the positions as appropriate for the deployment system. In this example they are just set to full range. The On Time for both servos requires 1 second. See the values. See table below.

Set Lost Alarm

We choose to set the alarm for 5 minutes after main parachute deployment, and we want the sound to beep for 1 second and stay quiet for 3 seconds. Parameter “E.” sets the delay in minutes, so we set it to 5. The sound on time and off time are given in 0.25second increments so we set “C.” to 4 (1sec/0.25 = 4) and the “d.” parameter to C (3sec/0.25 = 12 = C)

	D1			D2				M1
	D1Mul	D1Off	D1Dly	D2Mul	D2Off	D2Dly		M1P1	M1P2	M1On
Param
Value	0.2sec	3sec	1.2sec	1 sec	14 sec	8 sec		0	32	.96sec
	M2			Lost
	M2P1	M2P2	M2On	LOn	LOff		Ldly
Param
Value	32	0	.96	1 sec	3 sec		1 min

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button twice and the rocket is ready to go.

Scenario 3 – 2 stage rocket with parachute:

In this scenario the FC is fitted to the booster of a 2-stage rocket and will be used for initiating the staging of the next stage and at a time later for opening a parachute on the booster. The ideal time to release an upper stage is when the booster is travelling at its fastest which happens right around burnout. It is also the worst time to deploy a parachute since it could be ripped off.

The external trigger this time is based on a pressure switch that activates when the pressure inside the rocket drops to 10psi above atmospheric pressure. This means that a short time later the rocket will be travelling the fastest as the pressure drops to 0.

This is how you would configure the FC:

Let’s assume that the time between the sensing of the pressure drop to the time that the second stage should be released is only 30ms. We need to set the D1 multiplier to 0.01s = 0. We set the D1 Offset to 3 x 0.01s = 30ms = 3. We set D1 delay to 0.

M1 and M2 positions and On Times are configured as in the previous example. And the lost alarm functionality is turned off.

In this example we set the delay to parachute opening to be 2 seconds after staging. The D2 Multiplier is set to 0.5s and the D2 offset is set to 4 giving us 2 seconds. We can then vary that in half second increments.

	D1			D2				M1
	D1Mul	D1Off	D1Dly	D2Mul	D2Off	D2Dly		M1P1	M1P2	M1On
Param
Value	0.01s	30ms	0sec	0.5s	2 sec	0 sec
	M2			Lost
	M2P1	M2P2	M2On	LOn	LOff		LDly
Param
Value

Now if the rocket will fly with the same setup on every launch, all you have to do for each launch is turn on the FC and press the ARM button twice and the rocket is ready to go.

Factory Reset

To reset the configurable parameters back to the default factory settings do the following:

1. Make sure power switch is set to OFF
2. Hold the PGM and ARM buttons down at the same time and turn the power switch to ON.
3. The ‘r’ character will be displayed.
4. Press the ARM button once more to confirm the reset.
5. After the beep, turn the power switch to OFF.

The default factory settings are set up for a typical single stage 2L water rocket.

Hardware

Circuit Diagram

The circuit diagram for version 1.5 is shown in Figure 4. Central to the design is the PIC16F628A microcontroller from Microchip. The on-board oscillator is used to reduce the external component count.

A power connector JP3 is provided that allows an altimeter such as the Z-log to be connected directly to the power supply. This connector will be used in future versions as the auxiliary device connector.

Figure 4 - Circuit Diagram