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16. CONTINUOUS ACTUATORS

16.1 ACTUATOR CONTROL

•Control systems for devices can be described using block diagrams

•Two main approaches to actuator control

1.Torque controlled by current amplifiers driving a motor

2.Speed controlled by voltage using a voltage amplifier

•Torque control maintains a constant desired torque which is desirable in assembly operations. Also maintains constant force (gripper) without drawing additional power. A disadvantage occurs when the required torque is less than the applied torque. This causes higher acceleration and velocities resulting in the non-zero velocity at the target point and inaccuracies. If the inertia is higher than expected the resisting torque would be larger than the constant applied torque causing slow motion, and loss of production time.

•Speed control is less affected by variations in inertia and the time to reach the target point is not affected. The approach is slow and smooth. Problems occur because torque is not controlled. Motor draws from the voltage amplifier the current required to overcome the disturbance. If the robot encounters a rigid obstacle the amplifiers fuse could burn-out. This makes it unsuitable for assembly tasks such as press fitting where a constant force is required, but it is good for spray painting and seam welding.

•The choice of control depends on the application and the environment

•Inputs to velocity controlled robots from the computer are,

a)A sequence of pulses - each pulse generates one BRU (Basic Resolution Unit). The number of pulses represents position and the pulse frequency is proportional to velocity

b)a binary word as samples data - used in most modern robots with both velocity-controlled and torque controlled systems.

16.1.1 Block Diagrams

•Blocks in the block diagrams will transform energy

•Summation points (circles) will compare, or add quantities

•These diagrams can be used to design physical systems, and the subcomponents

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							real world
θ	desired	+	θ	error	neural	τ applied	arm position
					system and		arm structure
					muscle
		-
	θ	actual				eyes

** This block diagram shows a system that has dynamics, actuators,

feedback sensors, error determination, and objectives

16.1.2 Linear Control Systems

• although these systems are not linear, we can approximate them with linear models and measures.

** Note: many of the concepts in this section apply to other controls sections as well

• A simple feedback control loop is shown below

θ	desired		I	τ
		Current	DC	θ	actual
	Controller			Robot
		Amplifier	servomotor	Joint

• If we expand the above system to system that has second order response we get a system like that below,