Electric drives

From FestoWiki - english
Jump to: navigation, search

When it comes to simplicity and costs, pneumatics is unmatched by any other form of technology. It can be used worldwide without any problems and without the need for extensive specialist know-how. If the requirements for speed, acceleration and precision are within the capabilities of pneumatic technology, and if high flexibility (e.g. freely programmable positions) is not required, then this is the technology you should select. All other project requirements can generally be handled using electric drives.

Differences in performance

     Drive technology  Standard pneumatics Belt drive  Lead screw drive
Ball screw
Linear motor
Symbol Standardpneumatik.png Symbol Riemenantrieb.png Symbol Gleitgewindetrieb.png Symbol Kugelgewindetrieb.png Symbol Linearmotor.png
Load ++
(up to 100 kg)
+++ (up to 200 kg)  ++
(up to 100 kg)
+++
(up to 200 kg)
 +
(up to 30 kg)
Speed ++
(3 m/s)
+++
(5…10 m/s)
 +
(0.5 m/s)
++
(3…5 m/s)
+++
(5…10 m/s)
Acceleration +
(30 m/s²)
+++
(100 m/s²)
 +
(30 m/s²)
++
(50 m/s²)
+++
(…150 m/s²)
Precision ++
(100 µm)
 ++
(100 µm)
++
(50 µm)
+++
(20 µm)
+++
(3 µm)
Noise (with
identical v and a)
 +
Very loud
++
Loud
++
Medium
+++
Quiet
 +++
Quiet
Rigidity ++
Medium
++
Medium
+++
Very high (Reverse backlash)
+++
Very high
+++
High
Stroke +++
(up to 8.5 m)
 +++
(up to 10 m)
++
(up to 2 m)
++
(up to 2 m)
+++
(up to 10 m)
Costs (Total cost
of ownership)
+++ ++
(Approx. 2 * pneumatics)
++
(Approx. 2.5 * pneumatics)
+
(Approx. 3 * pneumatics)
+
(Approx. 4 * pneumatics)
Flexibility +
Not programmable
+++
Programmable
+++
Programmable
+++
Programmable
+++
Programmable
Power density +++
High
 ++
Medium
++
Medium
++
Medium
+
Low
Maintenance +++
Maintenance-free
+++
Maintenance-free
+
Relubricate
+
Relubricate
+
Relubricate
Service life +++
Very long
+++
Very long
 +
Limited
+++
Very long
+++
Very long

 
Pneumatics

When it comes to simplicity and costs, pneumatics is unmatched by any other form of technology. It can be used worldwide without any problems and without the need for extensive specialist know-how. If the requirements for speed, acceleration and precision are within the capabilities of pneumatic technology, and if high flexibility (e.g. freely programmable positions) is not required, then this is the technology you should select.

 

Toothed belt drives

Axes with toothed belt drives are the standard solution for freely programmable linear axis systems. The dynamic response of these axes is very high and they can be designed with very long axes. However, there are limitations on precision. But for applications that call for high flexibility at a very low cost, then this is the preferred technology.

 

Lead screw drives

Lead screw drives are an inexpensive solution for spindle axes and, in vertical operation, have the advantage of being self-locking and thus meet increased safety requirements. For continuous operation, regular lubrication must be undertaken. Lead screw drives are generally used in applications that call for higher feed forces combined with low running performance and attractive costs.

 

Ball screw drives

Linear axes with recirculating ball spindles are a good solution for high-precision applications. Their procurement costs are comparatively high, although they are very reliable and have a long service life. When these characteristics are important, then axes with recirculating ball spindles are the preferred choice.

 

Linear motors

Linear motors have a reputation as being an expensive, innovative technology. However, technical solutions with plug & work functionality are now also available. Linear motors stand out because of their very high dynamic response and high precision, but they are only suitable for relatively small loads.

 

Glossary

Absolute displacement encoders

With an absolute displacement encoder, there is always a fixed zero point and the position of the axis is measured relative to this point. This is comparable with a ruler: you know where the zero point is and you can read off, for example, 25.4 millimetres (see Incremental measuring system).

 

Absolute positioning

Absolute positioning always takes a zero point as a fixed reference point, and positioning takes place relative to this zero point, i.e. you travel from the 80 mm position to the 95 mm position ( see Incremental positioning).

AC

Abbreviation for "alternating current". This is the standard abbreviation for the sinusoidal alternating current which is available through power sockets worldwide to power electrical devices.

Brushless and brushed motors

With electric motors, the power is generated by two magnetic fields that are attracted to each other. One of these magnetic fields is able to rotate, which causes the motor to rotate when a magnetic field is built up. The greatest motor power is generated when the magnetic fields are at right angles to each other. In order to achieve this, one magnetic field must be switched accordingly. On brushed motors this is done via carbon brushes, and on brushless motors via the motor electronics. 

Closed loop

See Closed-loop control

DC

Abbreviation for "direct current". This is the abbreviation frequently used for direct current as supplied by batteries. 

Direct drives

A direct drive is a specially designed motor which can generate very high forces at low speeds. This leads to very high acceleration and to reductions in cycle times. With a direct drive, the load to be moved is mounted directly on the motor without gear units and other mechanical components. This technology is available as rotary direct drives (torque motors) and linear direct drives (linear motors).

Three-phase AC

Three-phase alternating current is a special form of alternating current which is transmitted via three conductors and the neutral conductor (the return conductor). It is used to drive particularly powerful electric motors and machines.

Real-time capability

Real-time capability specifies that the reaction of an electronic device or a fieldbus take place within a certain time. In order to state this capability clearly, the required reaction time needs to be specified. A modern controller processes all inputs and outputs within 10 to 20 ms, i.e. it has a reaction time within this timeframe.

Electronic brake

The electronic brake is produced by a motor controller, which activates a motor to act like a generator, i.e. it enables current to be generated, thus converting the mechanical energy of the rotary motion into electrical energy. This energy conversion slows down the motor, but cannot be used to block or stop it completely.

Encoder

An encoder is an optical or magnetic measuring system which emits a certain number of pulses after a certain movement. The encoders used on electric motors usually output 1,000 to 4,000 pulses per revolution.

Fieldbus

Fieldbuses are serial interface modules for communication between the different types of electronics in a system. It is standardised in terms of technical data, software and reaction time, allowing electronic devices from different manufacturers to be freely combined. Fieldbuses offered for motor controllers are CAN, Profibus and DeviceNet.

Frequency converter

A frequency converter is an electronic device which enables the speed of a three-phase motor to be adjusted. It can only be used to adjust the speed; it is not possible to approach positions. To do this, a higher-level controller and measuring system are required.

Holding brake

The brakes used in handling systems are generally holding brakes, i.e. they can hold an axis at a standstill. A holding brake will usually be able to bring a motion axis to a standstill a number of times in the event of power supply failure. 

Incremental measuring system

An incremental measuring system does not have a fixed zero point. A zero point must first be defined by homing before an absolute position can be approached - see Absolute measuring system.

Incremental or relative positioning

With incremental positioning, travel is always relative to the current position, i.e. you specify that an axis should travel another 10 mm.  Absolute positioning.

Load circuit

A load circuit provides large currents for the motors. These are fed to the power stages of the electronic controllers, which then supply the motors. Frequently, only the load circuit is switched off in the case of an emergency stop, and the control circuit remains switched on. This means that the position of an axis is always known. This is possible using the motor controllers CMMS, SFC and SEC.

Linear motor

See Direct drive. 

Motor controller

A motor controller is a frequency converter with an additional closed-loop controller built in. This enables multiple positions to be approached in a selective manner. The designations for our motor controllers are CMMS, SFC and SEC.

Emergency-off circuit

The purpose of an emergency-off circuit is to bring a machine into a safe state if a hazard is detected. This generally means that first all motions are brought to a standstill and then the power supply is switched off. The motor controllers CMMS, SFC and SEC have an input which can be used for the fastest possible braking. The power supply is switched off within the user's system.

Open loop

See Closed-loop control 

PNP and NPN logic

In Europe, PNP logic is used for wiring logic inputs and outputs, i.e. switching takes place from + via the load to -. With NPN logic, switching is from - via the load to +. This is in part due to historical reasons, but also to safety-related aspects. With NPN logic, many terminals are connected to the + conductor. If one of these terminals short circuits to the housing or -, the outputs no longer work. If this happens with PNP logic, the output transistor is destroyed, but all other inputs and outputs continue to work.

Closed-loop control

Closed-loop control means that a controlled variable is measured and an electronic controller reacts accordingly. A closed-loop controller automatically detects external influences and can react appropriately to ensure that no errors occur. We talk about closed-loop control if the controlled variable is measured directly, for example using a displacement encoder on the carriage of a linear axis. The term semi-closed loop control is used when there is an additional mechanism between the measuring system and the controlled variable, e.g. in the case of a toothed belt axis with servo motor, where the measuring system is on the motor and the toothed belt is located between this and the carriage. In the case of open-loop control, on the other hand, no feedback signal is received. 

Resolver

A resolver is an inductive measuring system. The functional principle is equivalent to that of a generator, i.e. a sinusoidal AC voltage is generated with a frequency which directly depends on the speed. This frequency can be used to determine the speed, acceleration and position.

Servo motor

"Servo" stands for closed-loop control. A servo motor has an integrated measuring system so that the motor position is always precisely known. This means that external forces are detected and compensated and very high accuracies can be achieved.

Stepper motor

A stepper motor is a special motor design which advances in individual steps. This step-by-step advancing means that operation is relatively loud, but the motor does not vibrate when at a standstill. A standard stepper motor does not have an integrated measuring system. The motor uses open-loop instead of closed-loop control, with the result that it is relatively inaccurate.

Control circuit

The power supply for electronic devices is generally provided by the control circuit. It is used to supply the logic, measuring systems, switches and displays. The control circuit should be routed separately from the load circuit.

Open-loop control 

We talk about open-loop control if the electronic system feeds a signal to the motor and does not receive a feedback signal to indicate whether a position has been reached. All stepper motors without encoders work in this way, and it offers sufficient reliability as long as the motor peak torque is not reached. A controller or PLC (programmable logic controller) is an electronic device which processes all the inputs within a defined timeframe and sets the outputs accordingly.

Torque motor See Direct drive.

This term is also used for the main drives on machines. Depending on the area of application, the term "rotary direct drives" is more appropriate.