A multiturn encoder is a rotary position sensor that measures angles from 0 to 360 degrees, as well the number of complete revolutions, known as Turn Count. Conversely, encoders that only output angle data within a single revolution, are known as single turn encoders.
Multiturn encoders are commonly used in applications where turn count exceeds one encoder turn. This includes rotary-to-linear motion systems, such as monitoring the location of a moving world in a leadscrew-driven linear stage. They are also used in geared rotary applications, such as providing feedback on the input of a strain wave or planetary gear, for example in a rotary actuator.
In most applications, the number of revolutions can be monitored incrementally through the use of a single turn absolute encoder, by tracking encoder rollover (when the sensor “rolls over” the zero point) within the servo drive or motion controller. Turn count increments as the angle position counter transitions from 360 to 0 degrees, and decrement as the counter transitions from 0 to 360 degrees. The same measurement process can be applied to incremental encoders by monitoring direction and counting Z-channel index pulses.
Absolute encoders are typically used in critically safe measurement applications, as true absolute position can be read on start-up, without the need to move to a “home” or “reference” position. This is beneficial in case a loss of power event occurs, where this homing process may not be safe to perform. Likewise, absolute encoders with multiturn capability are required to retain and deliver accurate single turn and multiturn position count between power cycles.
Additionally, absolute multiturn encoders monitor turn count internally and format the turn count information into an absolute data format. For example, absolute encoders with SSI and BiSS-C outputs, suitably format both single turn and multiturn data within a single data packet.
By convention, true absolute multiturn encoders additionally count turn count when power is not supplied to the encoder.
A geared multiturn encoder will typically use magnetic or optical sensing measurement techniques. These work on a similar principle to the minute and hour hands of a watch, where the minute hand is the primary encoder disc, and the hour hand is a secondary, or combination of, measurement disc(s) connected through some gearing. The large benefit is that the encoder is its battery-less operation and ability to increment turn count when not powered and a battery is not required. These devices however are bulky, commonly do not allow for a thru bore and are susceptible to shock and vibration.
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A battery backup multiturn encoder will employ an internal incremental turn counter to track turn count. When power to the device is removed, current will be drawn from an external battery in order to increment turn count. These devices require a low power consumption mode and dependent on acceleration and environmental conditions may offer a battery life of up to several years. While these devices are not as bulky as geared multiturn encoders, their external battery maybe, dependent on application requirements. Maintenance to replace the battery may often be required dependent on environmental and application conditions.
These devices are both gearless and battery-less and work on a similar principle to energy recovery systems in electric vehicles. When powered off, the kinetic energy of the moving encoder will generate enough power to increment turn count. Whilst environmentally robust, and compact, such technologies can be susceptible to external DC fields.
Devices such as Multiturn IncOder are absolute encoders with multiturn storage. When power is removed from the device, turn count information is stored to non-volatile memory and recovered when power is restored. These devices are compact, robust, and wear-free. These sensors however are not able to increment turn count if the encoder rotates during power off, so should be used where movement is restricted when the supply is removed, for example where a power-off brake is used.