Rotate bone on 1 axis to look at another object - Unity 3D

Question

I'm new to game development and I'm trying to create a basic 3D Tower Defence game as my first project. I've created a basic turret in Blender and I'm now trying to rotate 2 different bones to make the turret follow the enemy. The first bone (which I have working) rotates the turret to face the direction of the enemy. The second which I can't get right needs to rotate the gun at the top of the turret to look down at the enemy. I need it to keep facing the same direction as the rest of the turret and don't want it to tilt to either side. So it needs to move on 1 axis only.

I have tried various things, including examples from the Internet but I can't figure it out. So far I've managed to get it to look down at its base, up at the sky and spin like a wheel. Basically everything except where it's supposed to be looking.

Here is the code I have at the moment:

if (enemiesInRange.Count > 0)
    {
        //Rotate Platform //#### This section works fine
        Transform platform = transform.root.Find("Armature/BaseBone/PlatformBone");

        Quaternion targetRotation; //Final rotation (I.e. facing the enemy)

        Vector3 turretPosition = platform.position; //Store the turrets current position
        turretPosition.y = 0; //0 the Y axis to stop the turret tilting up or down

        Vector3 targetPosition = enemyManager.activeEnemies[enemiesInRange[0]].transform.position;

        targetRotation = Quaternion.LookRotation(targetPosition - turretPosition);

        float rotateSpeed = 2.0f * Time.deltaTime;
        platform.transform.rotation = Quaternion.Lerp(platform.transform.rotation, targetRotation, rotateSpeed);

        //Rotate GunBox //####TO DO: This part is not working
        Transform gunBox = transform.root.FindChild("Armature/BaseBone/PlatformBone/LowerArmBone/UpperArmBone/GunBoxBone");
        Vector3 gunBoxPosition = gunBox.position;

        float angle = Vector3.Angle(gunBoxPosition, targetPosition);

        gunBox.RotateAround(gunBox.position, Vector3.left, angle);

        Debug.Log(angle);

    }

Here's a picture of the turret to give you an idea what I'm working with. You can see in the screenshot that the bone's default X rotation is roughly -50.

Enemies walk through the trench around the turret. The PlatformBone rotates everything from the silver platform upwards.

I also don't understand why the line below makes the turret rotate instead of tilting it. I would expect to rotate the turret around the Y axis to keep it flat on the floor and face another direction. Yet I seem to have to do the opposite (If I select the PlatformBone its Y axis is facing up as expected):

turretPosition.y = 0; //0 the Y axis to stop the turret tilting up or down

Answer 1

To make an object face another object in Unity you can simply use

https://docs.unity3d.com/ScriptReference/Transform.LookAt.html

public void LookAt(Vector3 worldPosition, Vector3 worldUp = Vector3.up);

Rotates the transform so the forward vector points at worldPosition.

The interesting thing here is that if you have 2 objects (let's say A to rotate and B to look at) which have the same position.y, and you call A's transform.LookAt(B's position), A will be rotated to face B with rotation.y unchanged.

With that logic we can write a function that will rotate our object having the same rotation.y as before, it would look like:

public void LookAtXZ (Transform rotatingPart, Vector3 target ) 
{
    var origin = rotatingPart.position; // get origin point of rotation
    var targetXZ = new Vector3(target.x, origin.y, target.z); // "neutralize" target's position so that it has the same y as origin
    rotatingPart.LookAt(targetXZ);

}

Analogically we can write function that rotates only on X, Y or Z axis without difficulty.

public void LookAtY (Transform rotatingPart, Vector3 target ) 
{
    var origin = rotatingPart.position; 
    var targetY = new Vector3(origin.x, target.y, origin.z); 
    rotatingPart.LookAt(targetY);
}

Besides, transform.Find is not really a good way to get transform of an object, especially when you must do it every frame. Just create a public field in your class, attach the transform of part that needs to be rotated in Inspector and refer to it by reference in code, it would be way more performance-wise.

Your code would be like:

List<Transform> enemies;
Transform platform;
Transform gunBox;

void UpdateRotation () 
{
    foreach( var e in enemies )
    {
        var target = e.position;
        // For the sake of simplicity i'm going to take the use of 2 function above
        LookAtXZ(platform, target );
        LookAtY(gunBox, target );
    }
}

Hope this helps :)

p.s: how to make turret prefab with references to its parts

After setting up, save the object as a prefab and next time when you instantiate your turret from prefab, it will already have the references to platform and gunStation