ESCON EC-S: Applications and restrictions of sensorless control

Topic:

Sensorless control of brushless motors (= EC motors) can be a quite attractive solution because there are no feedback sensors required. Finally this means less components in use and just the motor phases have to be wired. Especially if the space of cables is limited or a high flexibility or slim design of cables is required, the advantages and restrictions of a sensorless control have to be a point of discussion and judged on the base of the concrete application and its requirements.

Technical aspects:

The ESCON EC-S (part.no. 446925) is a sensorless control based on the principle of rotor position detection by the so-called back-EMF signal. This information is used for commutation (i.e. consecutive switching of the voltage applied to the motor windings) and speed control. The technical challenge (and main drawback) of this principle is that there is just a very weak back-EMF signal present at low speed motor operation (and even none at rotor's standstill). Therefore a pure back-EMF based sensorless control demands for applications running above a certain minimum speed. The ESCON EC-S offers a precise and stable speed control above the minimum speed limit even up to 120000 rpm (in case of a  1 pole pair EC motor). As an additional advantage the ESCON EC-S offers a variety of functions which can be assigned to the analog and digital inputs and outputs. The configuration and initial commissioning is easy-to-use by the "ESCON Studio" software which offers a common interface for the complete ESCON product line.

There are some applications which fit quite well for back-EMF controlled sensorless applications but there are also some restrictions which have to be taken into account.

Typical applications
controlled by the ESCON EC-S:

• Fans and blowers

• Pumps

• Scanners

• Grinding devices

• Drilling devices

• Rotational cutting devices

• Surgery tools

• Tattoo guns

Even if an application type seems to fit, there might be some specific restrictions present like mentioned below. We always recommend to check the restrictions too and even process some prototyping test before a final decision is made if a sensorless control is suitable for an application in series under probably different working conditions (resp. operation points of the motor).

Restrictions of sensorless control

Even in case of the mentioned typical application types above there might be some restrictions present depending on concrete application requirements. The following list consists of application requirements which can hardly or even not(!) be fulfilled by a sensorless control just based on back-EMF detection (like the ESCON EC-S):

• The motor has to be operated at low speeds
  => Purely back-EMF based sensorless control (like the ESCON EC-S) is NOT(!) recommended.
  
  
• The motor has to start-up from zero speed with strongly varying loads
  => Purely back-EMF based sensorless control (like the ESCON EC-S) is NOT(!) recommended, e.g. in case of ...
  
  • ... cranes or lift applications
    with an undefined load (e.g. varying from zero up to max. load) at motion start.
  • ... blowers and pumps with strongly varying counterpressure or friction at motion start.
  • ... drilling or cutting devices which can be close to a blockage at motion start.
    
    
• High inertia of the load
  Purely back-EMF based sensorless controllers (like the ESCON EC-S) are only conditionally recommended in case the inertia of the load is more than 5 or even 10 times higher than the inertia of the motor. It is highly recommended to test the suitability of the motor and sensorless control by the application's concrete load and working point (speed, torque) before the decision is made to go for a sensorless solution.
  
  
• Purely back-EMF based sensorless control is NOT(!) recommended, if the motor
  • ... has to accelerate from standstill with a defined torque and ramp profile.
  • ... has to decelerate to standstill with a defined ramp.
  • ... has to change its direction of rotation with a defined ramp passing through the zero speed area.
  • ... has to be finally operated position controlled (by an additional top level control loop).
  • ... has to hold its position at standstill even if some force is applied.

Restriction: Minimum speed

One of the most important restrictions is that a purely back-EMF based sensorless control cannot provide a defined torque and speed measurement at zero or low speed. There is a minimum speed required to get stable back-EMF signals which are the feedback required for a stable control loop again. This minimum speed can be estimated by a rule of thumb:

Minimum speed [rpm] = (Supply voltage [V] * Speed constant [rpm/V]) / 7

The speed constant is specified by the motor's data sheet.

If an application demands for a speed less than the minimum speed calculated by this simple rule of thumb formula, a more sophisticated sensorless control (than the ESCON EC-S) or a sensor-based control is required.

Not sure about your application?

If you feel unsure if a sensorless control (resp. the ESCON EC-S) fits to your application type, its concrete requirements, and selected motor, please contact your local maxon sales office or submit a request by the Support Center and let us know about your concrete application's requirements. maxon can offer customized and even more sophisticated sensorless control OEM solutions on request. We are pleased to advice you without any obligation and find the best solution together with you fitting your concrete application taking the technical and commercial challenges into account too.