Multi-turn Encoders Find Home in Industrial Applications
- Sunday, 01 February 2009
Multi-turn encoders utilize either an input shaft or the increasingly popular hollow shaft construction. When input shaft encoders are connected to another shaft, they are impossible to perfectly align. Therefore, a flexible coupling must be used to connect both shafts, or bearing overload may occur. Hollow shaft encoders allow the mating shaft to pass through them, locking to shafts with a collet-style mechanism.
A tether mechanism holds the encoder to the machine base and provides two functions. First, the encoder’s body is prevented from rotating. Second, any axial and radial misalignments between the shaft and encoder are absorbed in the tether, minimizing any overloading forces to the bearings. Hollow shaft encoders are typically self-aligning and offer a lower overall profile once installed, compared to a shaft encoder that requires a standoff flange. Reduced installation time — with no need for a coupling — and a lower profile are just some of the benefits from hollow shaft encoders.
Absolute multi-turn encoders often use optical recognition technology, which is the preferred method of sensing by many manufacturers, as it is virtually impervious to magnetic fields. These encoders are ideal candidates for use on gear motors utilizing brakes. While in the past, brakes wreaked havoc on many absolute multi-turn encoders, today’s advanced technology has made encoders more successful in these applications.
With 17 bits per turn and 12 bits of turn data, a multi-turn encoder can deliver a total data word of 29 bits. Transmitting this in parallel fashion would require 29 wires, which is a costly and time-consuming process. This is the reason why all multi-turn encoders today utilize serial transmission, which is typically carried out over two or four wires. Some popular methods are SSI, PROFIBUS, DeviceNet and CANopen. These serial types of transmission can achieve speeds up to 10MHz, providing nearly real-time position updates.
Another benefit of the serial transmission is the ability to program the encoder online, including baud rate, end of line termination resistor, addressing, or setup with dip switches inside the encoder. Things like counting direction, home (or “0”) position, instantaneous velocity, acceleration/deceleration rate, position transmission update rate, and many more can be read or programmed to the encoder. With these new advancements and many more options, multi-turn encoders to be used in industrial automation applications where they were never used before.