How to use autopilot in UE5?

[cxr001]

I don't find a method to set roll pitch yaw command in c++, is that I need to write interface in FCS? How to use the ap xml code in c++?

[seanmcleod]

Typically any autopilot implementation will export a number of properties, e.g.

jsbsim/aircraft/c172x/c172ap.xml

Lines 27 to 36 in 5ac73c2

	<property> ap/attitude_hold </property>
	<property> ap/altitude_hold </property>
	<property> ap/heading_hold </property>
	<property> ap/altitude_setpoint </property>
	<property> ap/heading_setpoint </property>
	<property> ap/aileron_cmd </property>
	<property> ap/elevator_cmd </property>
	<property> ap/airspeed_setpoint </property>
	<property> ap/airspeed_hold </property>
	<property> ap/throttle-cmd-norm </property>

So you would then use the FGPropertyManager to write to for example the relevant setpoint, and to enable specific autopilot modes etc. For example here is some example C++ code using the PropertyManager, in this case to read the value of a property, in your case you would be writing to the autopilot specific properties.

FGPropertyManager* pm = fdm->GetPropertyManager();
FGPropertyNode* node = pm->GetNode("/my/property/name");

while (fdm->Run()) {
  x = node->getDoubleValue();
}

[cxr001]

So I can use Tie function in FGFCS::bind, for example, write
PropertyManager->Tie("ap/altitude_setpoint", this, &FGFCS::GetAlCmd, &FGFCS::SetAlCmd); ?
But where is the attitude command here? I cannot find the pitch yaw command interface in c172ap, and it looks the flight controller doesn't use the airspeed setpoint to compute anything here. Is that I should change this autoflight by myself?

[seanmcleod]

No, you would use the PropertyManager to write a value to ap/altitude_setpoint, e.g.

FGPropertyManager* pm = fdm->GetPropertyManager();
FGPropertyNode* node = pm->GetNode("ap/altitude_setpoint");

// Set the altitude set-point to 2000ft
node->setDoubleValue(2000);

[cxr001]

I can only find fdm from the FCS.cpp file, I don't know where is the fdm in the UE project

[cxr001]

I change the function copy to jsbsim as

void UJSBSimMovementComponent::CopyToJSBSim()
{
// Basic flight controls
FGPropertyManager* pm = Exec->GetPropertyManager().get();
FGPropertyNode* node = pm->GetNode("ap/altitude_setpoint");
// Set the altitude set-point to 2000ft
node->setDoubleValue(2000);
// FCS->SetDaCmd(Commands.Aileron);
// FCS->SetRollTrimCmd(Commands.RollTrim);
// FCS->SetDeCmd(Commands.Elevator);
// FCS->SetPitchTrimCmd(Commands.PitchTrim);
// FCS->SetDrCmd(-Commands.Rudder); // Rudder
// FCS->SetDsCmd(Commands.Rudder); // Steering
// FCS->SetYawTrimCmd(-Commands.YawTrim);
// FCS->SetDfCmd(Commands.Flap);
// FCS->SetDsbCmd(Commands.SpeedBrake);
// FCS->SetDspCmd(Commands.Spoiler);

// Gears and Brake controls
FCS->SetLBrake(FMath::Max(Commands.LeftBrake, Commands.ParkingBrake));
FCS->SetRBrake(FMath::Max(Commands.RightBrake, Commands.ParkingBrake));
FCS->SetCBrake(FMath::Max(Commands.CenterBrake, Commands.ParkingBrake));
FCS->SetGearCmd(Commands.GearDown); 

ApplyEnginesCommands(); 

But it doesn't work. Why?

[seanmcleod]

But it doesn't work. Why?

What doesn't work? What are you expecting to happen, and what do you see happening?

[cxr001]

When I use

FGPropertyManager* pm =  FCS->GetPropertyManager().get(); //(or Exec->GetPropertyManager().get();)
FGPropertyNode* node_rh = pm->GetNode("ap/heading_setpoint");

the node_rh will return NULL. But I use the model of c172x, it should already include the autopilot. So I don't know how to repair my codes.

Sometimes node_al = pm->GetNode("ap/altitude_setpoint"); will return an address, but not controlling the elevator.

[gallonmate]

These functions already exist in the UE plugin and also work in Blueprints. I added them many months ago.

jsbsim/UnrealEngine/Plugins/JSBSimFlightDynamicsModel/Source/JSBSimFlightDynamicsModel/Private/JSBSimMovementComponent.cpp

Line 394 in 9f92a06

PropertyManager = Exec->GetPropertyManager();

jsbsim/UnrealEngine/Plugins/JSBSimFlightDynamicsModel/Source/JSBSimFlightDynamicsModel/Private/JSBSimMovementComponent.cpp

Lines 83 to 115 in 9f92a06

	void UJSBSimMovementComponent::PropertyManagerNode(TArray<FString>& Catalog)
	{
	auto newlist = Exec->GetPropertyCatalog();
	//convert list to array
	for (auto iterate : newlist)
	{
	Catalog.Add(iterate.c_str());
	}
	}
	//TODO, check if this is optimized, as we are using strings for convenience and we could probably cache the propertynode once we have it
	void UJSBSimMovementComponent::CommandConsole(FString Property, FString InValue, FString& OutValue)
	{
	FGPropertyNode* node = PropertyManager->GetNode(TCHAR_TO_UTF8(*Property),false);
	if (node != NULL)
	{
	//we skip setting values by using blank InValue.
	if (InValue != "")
	{
	node->setStringValue(TCHAR_TO_UTF8(*InValue));
	}
	OutValue = node->getStringValue();
	}
	////check if property has READ(1) attribute
	//auto attribute = SGPropertyNode::Attribute(1);
	//if (! node->getAttribute(attribute))
	//{
	// return;
	//}
	}

Read Me File: https://github.com/JSBSim-Team/jsbsim/tree/9f92a0603498ff09dd84e77b6e6022b866fcb50f/UnrealEngine#extended-commands-and-properties

[cxr001]

I found that the roll channel can be controlled by ap/autopilot-roll-on, and also the ap/roll-attitude-mode. However, it looks, the attitude hold and heading hold still would not have any effect.

[gallonmate]

Yes I was curious so I tested the c172x and could not find any full working combination of settings for the auto pilot. But I tested c310 and seems that works good.

I came to the same conclusion as you with the c172x, only Altitude control works. The ap/autopilot-roll-on and also the ap/roll-attitude-mode when enabled do seem to stabilize the aircraft but it defaults to a heading of 103, I tried to override some properties to change the heading but I gave up after a few tries. The Auto Pilot system is confusing. I looked at the test scripts, some of which test the auto pilot of the c172x and found the functions do not work as suggested in the scripts.
https://github.com/JSBSim-Team/jsbsim/blob/master/scripts/c1723.xml
https://github.com/JSBSim-Team/jsbsim/tree/master/scripts

In short I think c172x autopilot became broken in some update. The c310 seems to work good, but I only tested Heading and Altitude auto pilot controls.

Edit: For some reason, you also need to manually use engine starter in UE5 to start the engines on the c310. Press keys Control+Q or in Blueprint there is already Set Engines Starter in example BP_Airliner you can attach to Begin Play.

[cxr001]

As c310 is not suitable for my project, I checked the xml files again, and found that there is no "ap/aileron_cmd" property in c172r or c172x. Therefore the c172ap or other autopilot can not control the roll or heading positions of c172.

[gallonmate]

As c310 is not suitable for my project, I checked the xml files again, and found that there is no "ap/aileron_cmd" property in c172r or c172x. Therefore the c172ap or other autopilot can not control the roll or heading positions of c172.

@cxr001 good catch!
After searching I found issue already exist but xml file was never fixed #180,
Try editing the xml yourself by using this suggestion #180 (comment) by @bcoconni
I'm not sure why it was never fixed in the repo.

[seanmcleod]

Hmm, why are there two auto-pilot roll commands for roll control? Or was there some sort of renaming which was only done in one location and not the other?

jsbsim/aircraft/c172x/c172x.xml

Lines 381 to 390 in 98535e8

	<summer name="fcs/roll-trim-sum">
	<!-- <input>ap/aileron_cmd</input> -->
	<input>ap/roll-cmd-norm-output</input>
	<input>fcs/aileron-cmd-norm</input>
	<input>fcs/roll-trim-cmd-norm</input>
	<clipto>
	<min>-1</min>
	<max>1</max>
	</clipto>
	</summer>

Looking at the autopilot file and searching through it I couldn't find ap/roll-cmd-norm-output, so in addition to uncommenting ap/aileron_cmd we should remove ap/roll-cmd-norm-output.

[seanmcleod]

I don't know whether UE5 can use scripts.

Take a look at the InitializeJSBSim() and PrepareJSBSim() methods in:

https://github.com/JSBSim-Team/jsbsim/blob/master/UnrealEngine/Plugins/JSBSimFlightDynamicsModel/Source/JSBSimFlightDynamicsModel/Private/JSBSimMovementComponent.cpp

I don't see any support for scripts currently.

[seanmcleod]

This is a script file,

Hmm, that doesn't look like a script file, take a look at the example script files in the scripts directory. The snippet you've shown comes from either the FDM file or a systems file.

[cxr001]

I think the code given by @gallonmate is a script, so I cannot use it. Forgive my poor English here.
The code given by me couldn't work directly, I must read the output of the channel (the fcs/throttle-cmd-norm property), and then write the value into the dynamic by c++ or Blueprint. I don't know if this operation will cause any sampling problem. If there is any method or property to feedback the value of autopilot directly, I think it is better.

[gallonmate]

@cxr001 yes, sorry. I was confused and assumed you had a script working and just wanted to manually set throttle. There is not any default autopilot throttle.

There is an autopilot throttle example in the Global5000ap.xml
But I have no idea if it works or how to use it.

[cxr001]

Thank you, I found it, named " fcs/throttle-pos-norm". Note the throttle command value in UE would always display as 0 here.

[seanmcleod]

@cxr001 I've pushed a commit to fix the autopilot aileron command name in C172x - ae318de