Table 4.

Summary of AI-enabled DTs in advanced robotics.

Subfield	AI Category	Key Methods	Application-Case	Ref.
Control (Section 3.2)	Supervised and Unsupervised Learning	SVM, PCA	Object recognition of a smart gripper	[260]
	Reinforcement learning	Trial-and-error search	Weightlifting robot control	[261]
	Supervised Learning	GD	Understanding the added value of integrated models for through-life engineering services	[262]
	Computational Intelligence	Vision-based Markovian chain	Automate fan-blade reconditioning for aerospace maintenance, repair and overhaul	[263]
		QP	Supporting rescuers on disaster-response missions	[264]
Planning (Section 3.3)	Reinforcement Learning	Proximal policy optimization	Pick-and-place tasks for an industrial robotic arm	[265]
		DDPG	Control and trajectory planning of a planar 3-DOF manipulator and 3D arms of a humanoid robot	[266]
		DQN	Automate smart manufacturing systems	[267]
		Proposed LSTM-MACG	Collision avoidance for a number of UAVs in a confined airspace	[268]
	Computational Intelligence	Ant colony optimization	Path planning of industrial robots	[269]
HRI/HRC (Section 3.4)	Supervised Learning	CNN	Standing-posture recognition in HRC	[270]
		DL	Mechatronics system	[271]
		ANN	Enabling industrial robots to bypass obstacles	[272]
		LSTM	Visual question answering for HMC system	[273]
	Supervised and Unsupervised Learning	FFT-PCA-SVM	HRI welding and welder behavior analysis (identifying the professional level)	[274]
	Reinforcement Learning	DDPG	COVID-19, improve efficiency in assembling medical equipment	[275]
Predictive Maintenance	Supervised Learning	DNN	System health monitoring	[276]
			Maximizing the overall plant availability of modern manufacturing systems	[277]
Workspace Modeling	Supervised Learning	Monte Carlo method	Simulating the workspace of the mechanisms	[278]
Others	Supervised Learning	RF	Estimation of lawn grass lengths for robotic lawn mower	[279]

PCA: Principal Component Analysis; GD: Gradient Descent; QP: Quadratic Programming; MACG: MultiAgent Computational Guidance; FFT: Fast Fourier Transform; UAV: Unmanned Aerial Vehicle.