Magnetic Measurement

Magnetic measuring offers numerous advantages over other positioning and measuring methods: it allows measuring at a distance, in dirty environments with high accuracy.

A typical measuring solution consists of a measuring head with a sensor and a magnetic linear or rotary scale (pulse generator).

For measurement, the magnetic properties of a scale are evaluated. For this purpose, the scale is defined with a pattern of magnetic north and south poles to form a highly accurate scale. BOGEN can encode linear and rotary scales using a patented special process with freely Xable magnetic patterns: single-track or multi-track, incremental or absolute. With a magnetic viewer, this magnetic information can be made visible on a scale. It shows the boundaries between the north and south poles.

Magnetic measurement is contactless. The distance between the sensor and the linear or rotary scale depends on the pole length. As a rule of thumb, the distance between the scale and sensor can be half a pole pitch (length of a single pole). When using a scale with a 5 mm pole pitch, means the distance between the sensor and the scale should be less than 2.5 mm. This distance can be air, but any paramagnetic material could be used, for example, most liquids.

No visual contact is required for magnetic measurement. However, in many industrial applications, dirt, dust, or liquid droplets accumulate on the scale. However, the scale can still be read regardless of these disturbances, since only the magnetic field is relevant to the sensor.

The challenge in reading with high accuracy is to create a very precise magnetic pattern, i.e. to set the boundaries between the north and south poles exactly at the ideal position. For a scale with a 2 mm pole pitch, i.e. the length of a single pole is 2 mm each, it must be ensured that there is a pole boundary exactly every 2 mm. A measuring accuracy of 3 µm can be achieved.

The magnetic field of a scale remains unchanged over time. It can only be changed if a stronger magnetic field touches the scale in contact.

The sensing head is a sensing head device with integrated electronics to measure the magnetic information from a magnetic scale and convert it into a signal that can be used in the connected electronics. There are different types of read heads. They are characterized by the sensor used, the output signal (digital or analog), and the amount of signal processing in the read head.

The key element of a sensing head is the sensor. Different types of sensor technology are used to read the magnetic pattern: Hall sensors and magneto-resistive sensors. These sensors convert magnetic information into an electrical signal that can be used for motion control or other electronic control.

Different types of sensor technology are used for different tasks.

In the Hall principle, a current is driven through a thin conductor plate. When the conductor plate is penetrated by a magnetic flux, a voltage can be measured perpendicular to the current and the flux density vector, which is proportional to the flux density. Hall sensors are available as switching or linear sensors. Switching Hall sensors are best for sensing positions on a scale, while linear Hall cells provide an output that varies over different parts of a scale.

Magneto-resistive (MR) sensors take advantage of other material properties. The resistance of a ferromagnetic conductor depends on the field strength vector inside it. MR sensors require specific types of materials for the pulse generators and must adhere to specific geometries. Magneto-resistive elements provide information that can be further interpolated, similar to linear Hall sensors.

The signal from a sensor is not optimal. Signal conditioning optimizes the sensor's output signals to provide high-quality information for downstream electronics.

Signal processing enriches the signal and converts the analog signal into a digital signal. Analog or digital converters process the signal and make it available for many different outputs.

A standard output converts the analog signal into a format where the signal is converted into two 90-degree shifted square waves which are output on the A and B channels of the interface. The reference information is supplied as a Z channel.

Incremental measuring heads have RS485 ABZ or TTL/HTL outputs. Absolute measuring heads are equipped with SSI or BISS-C outputs. The SSI serial signal interface allows information to be provided serially. The sensor provides serialized absolute position information in a defined binary format.

Magnetic measurement solutions are available for absolute and incremental measurements.

Purely incremental systems count the number of steps between two positions, i.e. a certain number of steps between the start of the system and now, and steps can be counted in any direction. A stopwatch is an incremental system that indicates how many seconds (steps) have passed since the measurement started. For incremental systems, the design is simple.

Another type of incremental scale has a reference, with a reference position aligned with the zero point of the incremental scale. Incremental systems with a reference require a reference run to find the zero position and then count from there.

Typically, motion control systems are based on incremental technology. They control a relative motion with the input values.

An absolute measuring system is used to generate an absolute signal, such as position. Another absolute measurement system is the clock: it indicates a point in time concerning a general reference. There are many solutions for absolute measurements.

The pseudo-random code is an extension of the binary code for linear applications. Instead of using multiple tracks in parallel, the scale has a binary combination that is formed differently in each increment of the scale. Typically, the scale is generated with the same number of bits as the number of sensors in the sensing head.

Several algorithms can create such a pattern. It looks like random, which is why it got the misleading name.

While the pseudo-random code identifies each position, the principle of pseudo-random code with an additional incremental track uses this track to interpolate between steps.

This pattern has a higher resolution than the number of sensors in the pseudo-random track because the interpolation of the incremental tracks is multiplied by this factor for the total resolution.
The nonius track uses two traces with different numbers of poles, usually with a difference of one.

The nonius pattern is analyzed with two parallel sensor elements, each reading one track. The phase shift between the two tracks can be used to calculate the absolute position.
 

Rotary systems for incremental as well as absolute measurements are used for continuous or discrete positioning and motion control, such as the commutation of an electric motor. Angular measurement solutions require specialized rotary encoders and task-specific rotary scales.

Rotary scales are available in various designs for many applications:

• attachment of the scale to the flange, outside or inside diameter.
• coding of one or more tracks with any pole length from 0.5 mm
• reference point or index in the second track as a zero point
• outer diameter from 3 mm to 2.1 m
• Ferrite (also bonded to hub)
• plastoferrite (plastic filled with ferrite)
• elastomer on a hub (elastomer filled with ferrite)
• vulcanized rubber on a hub (rubber filled with ferrite)
• true angle graduation without gap or joint
• high accuracy up to +/- 25 arcseconds

Incremental as well as absolute magnetic linear scale solutions are used for continuous or discrete positioning, and motion control, such as commutation and other solutions.

Magnetic linear scales are available with one or more tracks. A reference point or index, which can be used as a zero point for control, can be written in the second track. Typical scales have pole lengths of 1, 2, or 5 mm, but there are also special sizes with 0.5 or 2.54 mm and other customized sizes.

Standard linear scales consist of magnetized elastomeric tape (or other materials) of varying widths (typically 5 to 25 mm) bonded to stainless steel. They are available with different pole lengths and in any length up to 200 m. The top of the scales can be protected with a thin stainless steel masking tape.

Accuracy is defined as the maximum deviation of a pole limit from the ideal position or the degree of agreement between the actually measured and the ideal position value and is divided into different accuracy classes from +/- 3 to +/- 100 µm.

Resolution describes the smallest measurable step for the measuring system. For an incremental measuring system, it describes the smallest difference in position. For many absolute systems, a resolution is expressed in the number of bits, i.e. how many 2x positions can be distinguished.

Precision is also known as repeatability. Precision is the maximum difference between the different measurement results when a tester applies a predefined test procedure more often to reach the same position.

Digitally controlled production technology for magnetic linear and rotary scales meets the highest requirements for accuracy, precision, and economy. Thus, pattern variations with different pole pitches and special patterns can be produced or adapted to other scale sizes and geometries.

First, the magnetic pattern is created on the scale, whereupon each pole boundary is measured and checked to ensure that the position of the pole boundaries meets the specifications. If this is not the case, a correction is calculated and in a further magnetization run all non-conforming pole boundaries are corrected. The pole boundaries are virtually shifted to the desired position. The magnetization is checked again. This process is repeated until all requirements are met. In this way, both prototype quantities and large series can be produced economically.