RPM Measurement: Sensors and Techniques

By T.K. Hareendran

32998
 

Sensor modules for unique speed sensing solutions

Today’s advanced sensor modules (and sensing techniques) provide simple and cost-efficient solutions for rotational speed measurement in many applications including automotive, industrial and medical.

Variable reluctance speed sensor

A variable reluctance (VR) sensor is composed of a wire wound around a cylindrical magnetic material, typically made of some type of ferrous material that is referred to as a pole piece. A magnet is attached behind the pole piece, creating a magnetic field through the pole piece and winding. This magnetic field projects out from the pole piece front, also known as the sensor tip. When ferrous material passes through and disrupts this magnetic field, a sine wave is generated. The frequency of the signal is directly proportional to the speed of rotation.

VR speed sensors are passive devices that do not require any external source of power to generate a signal. Note that a variant of VR sensor is available as inductive magnetic sensor. It differs from the standard VR sensor in that the magnet is not in the unit, but in the object being sensed. Inductive magnetic sensors are used when the rotating device sensed cannot be made of ferrous materials.

Fig. 2: AM4096 magnetic encoder chip from Renishaw. All of the sensor and processing electronics have been placed within the compact silicon design. The rotation of a simple north/south magnet is picked up by the AM4096’s sensor and provides absolute positional information output to an accuracy of better than 0.1 degree
Fig. 2: AM4096 magnetic encoder chip from Renishaw. All of the sensor and processing electronics have been placed within the compact silicon design. The rotation of a simple north/south magnet is picked up by the AM4096’s sensor and provides absolute positional information output to an accuracy of better than 0.1 degree

Magnetoresistive speed sensor

Rotational speed measurement using a magnetoresistive (MR) sensor is achieved by counting ferromagnetic marks, such as teeth of a passive gear wheel or the number of magnetic elements of a magnetised ring. Magnetoresistive sensors make use of the magnetoresistive effect—the property of a current-carrying magnetic material to change its resistivity in the presence of an external magnetic field. Although inductive sensors can be used for this task, magnetoresistive sensors have an advantage that the output signal level does not vary with rotation speed, as in the case of inductive sensors.

MR sensors are static and the output signal is generated by the bending of magnetic field lines according to the position of the target wheel. As bending of the magnetic field lines also occurs when the target is not moving, these sensors can measure very slow rotations, even down to zero hertz (0 Hz). However, note that MR sensors cannot directly measure rotational speed, but are sensitive to the motion of a toothed wheel made from ferrous material (passive target, a sensor fitted with a permanent magnet) or a rotating wheel having alternating magnetic poles (active target).

RPM measurement techniques

Frequency measurement and period measurement are two widely accepted methods for determining RPM. Frequency measurement is better for fast-moving devices such as motors and turbines, while period measurement is better for devices that move more slowly, such as shafts. When using frequency measurement as a method of monitoring RPM, the key factor is the number of pulses being sensed per revolution (PPR). This method works well with high-PPR sensors and works poorly for low-PPR sensors.

When using frequency measurement method, you can calculate the RPM using this equation:
RPM = (Pulse Frequency in pulses/sec)×(60 sec/min)/(Sensor pulses/revolution) = Revolutions/Minute

This equation shows the relationship between frequency and period as: Frequency = 1/Period

When using period measurement to monitor RPM, the RPM can be calculated by using the equation:
RPM = 60/Pulse period x PPR

In addition, note that frequency method is good for zero-speed detection. If you use frequency method to monitor the RPM, you can know the shaft has stopped when the frequency being sensed drops to zero. On the other hand, if you use period method, the main issue is determining how much time to allow between pulses before deciding that the shaft has stopped. Basically, this method requires a little more complex logic!

From printing to life care!

Rotational speed sensors are widely used to monitor the speeds of machine components, from printing and paper machines to aircrafts, ships and wind turbines. A tachometer can even find uses in medicine to diagnose circulatory problems such as clogged arteries. By placing a small, turbine-like device called haematachometer in an artery or vein, doctors can use a tachometer to interpret the rate of blood flow from the speed at which the turbine spins.

Nowadays, innovative rotational speed sensor technology offers a wide range of standard products for the most varied applications, with suitable measuring systems and electronics, as well as appropriate housings, designs and connections. The design trend is towards fabrication of tiny chips, i.e., chips integrated with sensor bridges, signal amplifiers, gain controls, differential analogue line drivers and digital interface circuits, for contactless RPM measurement, position control/sensing and rotating angle measurement, etc.


The author is a regular contributor to EFY

4 COMMENTS

  1. Hello , We have a requirement of RPM sensor for motor shaft , Can you please suggest the right one along with specs and pricing

  2. Hello ,
    please someone can guide me how to calculate the rpm of an equipment for example a feeding screw attached with a gear motor while motor rpm is provided??

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