Sinusoidal encoders use sine waves in place of the square waves seen on quadrature encoders. This allows intermediate encoder counts to be interpolated to over 1024 times. Resolutions of over 4 million counts per resolution are possible. These are relative feedback devices.
Encoders are the most prevalent position feedback device in motion control. Linear encoders can go to sub-micron resolutions and rotary encoders can have resolutions exceeding 100,000 counts per revolution. These are relative feedback devices.
These devices provide only incremental position updates. In order to know the motor or load’s position, incremental feedback needs to be used in conjunction with some type of absolute feedback (a limit switch, for example) to determine the initial position. Once the initial position is known, relative feedback can provide position information throughout the range of motion.
Within these two general types of feedback, there are many different feedback devices. Here are some of the devices most commonly used in motion control.
Absolute devices provide definitive position within a specified range upon power up (i.e. without a homing routine).
In modern control systems, feedback devices are used to ensure that the motor or load reaches the commanded position or velocity. Servo amplifiers and controllers use this feedback to determine how much current to deliver to the motor at any time, based on its present position and velocity versus where it needs to be. There are two main types of feedback, absolute and relative (also known as ‘incremental’).
Load considerations should include the object that is being moved, the moving parts in the machine and anything that may cause unwanted instabilities such as couplings and backlash. The total mass of the moving parts in the machine all have inertias that will be reflected onto the motor. Friction points such as from linear stages and bearings will add to the motor load. Flexible couplings will add resonances that have to be considered.
The construction of a linear motor is the same as a rotary motor but opened up and flattened out. Configuring a drive for a linear motor is identical to configuring a drive for a rotary motor. Linear motors are used in direct drive applications where the speed and accuracy requirements are more than a rotary motor and ball screw can provide.
Permanent magnet brushless servo motors have higher power density, better heat dissipation and require less maintenance than brushed motors. Brushless motors may be a little more difficult to set up due to the increased wiring so our digital line makes things easier by automating the commutation process.
Inductive loads are often used by universities and scientists to create magnetic fields for their experiments. Company drives have successfully controlled inductive loads with less than 80uH of inductance to over 1H (1,000,000uH) of inductance. There are special considerations for the energy stored in a large inductor, and our technical support department would be happy to discuss these regarding your project.
Magnetic bearings float a rotating shaft on a magnetic cushion controlled with servos. They are used when low friction is required or when the shaft speeds are too high for conventional bearings. Magnetic bearings use electromagnets to levitate the rotating shaft so nothing is physically touching it. A typical magnetic bearing system will require 4 or 5 drives - an x and y on each side of the rotating shaft and an optional thrust bearing to keep the shaft from floating in and out. The performance requirements for the drives can be extremely high due to the dynamic nature of the system.