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Why An Encoder?

June 16, 2020

Encoders provide feedback for accurate control of speed and positioning. The rotor angular position is given through two square waves 90 degrees shifted and an optional index pulse occurring once a revolution. There are multiple technologies to get these results.

Portescap offers two types of encoders.

Optical and Magnetic

The E9 is an incremental optical encoder having three output channels. This type of encoder uses a dedicated ASIC having a matrix of optoelectronic sensors which receives infrared light from an LED after its passage through a metal code wheel.  This code wheel is made up of opaque and clear segments on a disc. This type of encoder will provide very precise positioning and has no sensitivity to an external magnetic field.   

The M-sense is a magnetic encoder. This is a single-chip technology that provides three channels of feedback and has an integrated RS422 line driver. The M-sense uses a hall sensor array to sense the magnetic field. The sine/cosine signal is digitalized and converted into an ABZ incremental signal. Resolution on this is available from 1 – 1024 lines per revolution in a compact size.  

The MR2 is also a magnetic encoder. This uses a magneto-resistive sensor that provides sine/cosine magnetic field measurements. An interpolator converts these signals into 2 square wave outputs. This provides two channels of feedback, including an index pulse, in a compact design. It is insensitive to temperature and has low sensitivity to unwanted external fields. Resolution up to 512 lines are available.  

Let’s discuss some general definitions on an encoder spec sheet.

Number of Lines Available - At Portescap, we use line = pulse and this corresponds to the number of square cycles per channel. For example, our MR2 encoder can provide up to 512 lines per revolution.

Then 1 line (or pulse) = 4 counts, Quadrature. Counts correspond to the minimum angular step, and so, we have 2048 counts (or angular steps) for the MR2.

Duty Cycle – Also referred to as Period is typically 360 degrees of rotation.

Supply Voltage / Supply Current - the amount of voltage/current required for the encoder to perform properly.

Output Signals – Portescap encoders will have at least two output signals (channel A and channel B) and some have a third channel designated as the index channel. The index channel has one line per revolution and is synchronized with A/B channel, typically is referred to as the channel Z.

The Z channel Index can be used for determining a reference position.

Phase Shift - On Portescap encoders, the electrical phase shift between channels A and B is 90 degrees.

In the above diagram, channel A is leading channel B, therefore you have clockwise rotation. Channel B leading channel A would be counter-clockwise rotation.

Examples of using an encoder in an application:

Drug Delivery System – A drug delivery system requires a precise amount of medication dispensed at a specified rate. In the development process of the pump, encoder feedback can validate that the proper dose of the drug is delivered. For the patient, the encoder confirms that the correct dose is delivered.  The encoder comes in various resolutions so the design engineer can select the appropriate number of lines to ensure precision down to the minimum flow rate.

Automated Window Shades – Automated window shades can provide environmental comfort to users as well as providing the ease of adjusting the shade automatically versus manual operation. Encoder feedback provides information on the speed of the motor, the direction of the motor, as well as the position of the shade when power is removed. This information ensures the user has the correct shade position based on the input to the console.

Electronic Assembly Equipment – SMT component pick & place equipment demand high throughput for automated assembly lines. To achieve these outputs, motion control systems must move to pinpoint locations quickly and repeatably while detecting the various sizes and weights of the PCB components. The encoder resolution ensures the precision movements are completed, allowing synchronization of all axes for proper component placement on the PCB.

Robotics (Grippers) – Encoders play an important function in grippers to ensure the amount of pressure and speed of gripping is controlled, preventing damage to the component. Each component may require different speed and pressure to perform the gripping function. The encoder feedback creates an accurate motion by understanding the motor speed and position necessary for each component.

In conclusion, encoders play an important role in design engineers in developing new products. An encoder provides information on the motor speed, the direction, and the position of the rotor. Encoder parameters are used in developing the control systems that drive the end product.