Technical Papers

Drive System Plays it Safe

A tool breakage that is not detected immediately in the machine tool can lead to hazards for man and machine. The function load indicator integrated in the frequency converters and servo amplifiers of series SD2x comes with intelligent technology to prevent this scenario. In addition, two sensorless functions complete the safety concept of the drive system.

The drive system SD2 by SIEB & MEYER can control synchronous and asynchronous motors with or without a sensor and permits quick and easy adaptation to the individual case of application. The integrated function load indicator reliably detects load changes of the connected motor. To achieve this, only the torque-forming part of the motor current is measured, which ensures high-dynamic and accurate functionality. Furthermore, two sensorless safety functions are available. These were developed especially for rotary motors without speed sensors. For manufacturers of processing machines and machine tools the new functions make it easier to comply with the increasing demands regarding machine efficiency and safety of people.

Indication of tool wear and tool breakage

The load indicator is not based on acoustic sensors but evaluates the actual torque-forming current of the motor. This way load changes of the motor are determined very precisely. These, in turn, can be used to assess the actual status of the respective machine: If an expected current change does not occur during a machining process, tool breakage is likely. Then, the operator can stop the machine immediately.

Tool wear can also be detected thanks to the load indicator, since worn-out tools need more power. If a preset load threshold is reached, a message is generated and the corresponding tool can be replaced quickly. This is how a continuous machining quality can be guaranteed.

The load indicator integrated in the frequency converters and servo amplifiers of series SD2x can also detect the contact between tool and work piece (gap eliminator). This function is, for example, necessary for grinding processes and usually covered by expensive acoustic emission sensors. Last but not least, feed rates can be flexibly adjusted by means of the load indicator. This is particularly helpful for machining work pieces with different material densities (e.g. wood).

 

Sensorless safety functions

However, tool breakage does not pose the only possible danger to processing machines and machine tools. Tools that are still rotating or rotating at too high speeds are also a risk to the operator or the service technician. The machine manufacturer faces the challenge of implementing both demands, safety of people as well as efficiency, optimally as well as in compliance with the relevant standards. Classic safety solutions rely on motors and spindles that are equipped with safe speed sensors to avoid or detect dangerous operating states. If this is not the case, the machine manufacturer would have to accept lower efficiency of the machines or ensure the required safety integrity level by means of costly safety concepts for each individual application. To avoid this, SIEB & MEYER has extended the functional spectrum of the drive system SD2 with the sensorless functions Safe Frequency Monitor (SFM) and Safe Limited Output Frequency (SLOF). These functions were approved by TÜV Nord according to EN 61508:2010 and meet the requirements of safety integrity level SIL3. Both functions are based on the function Safe Torque Off (STO) that is integrated in all SD2 devices as standard. The outcome is a cost-effective solution for the machine manufacturer, suitable for sensorless systems. This makes using spindles and motors without speed sensors possible – a crucial point for applications in the fields of high-speed cutting or grinding, since integrating speed sensors into spindles/motors for these applications is difficult for technical or financial reasons.

 

Safe standstill monitoring

By means of the safety function SFM the operator can safely detect whether or not the speed of a sensorless spindle has reached standstill or dropped below a safe speed value after switch-off. As long as this is not the case, a protective door, for example, remains closed. SFM detects the electric rotating field of the motor. Along with the number of poles of the motor, the user can determine the exact mechanical rotating field. If a 4-pole motor is used, for example, and the electric limit frequency is set to 10 Hz, the mechanical rotating field is 300 rpm. The risk of injury due to rotating tools with low speeds is considered small because the user can visually perceive the tool rotation. That means, entering the protected area is determined 'safe'.

The SFM function is based on a frequency-dependent voltage that is induced by the motor or the remanence. This voltage can be measured at the motor terminals, regardless whether a synchronous motor or an asynchronous motor is used. The drive determines the actual rotating field frequency from that voltage and compares it with the parameterized limit value. If the rotating field frequency is below the parameterized limit frequency, the drive generates the status signal "Standstill". The drive only generates the standstill signal when the output stage is deactivated and the main voltage is switched off.

 

Maximum output frequency

The safety function SLOF is used to make sure that critical speeds are not exceeded. This can protect persons and equipment, for example, when a tool might break due to overspeed. For this purpose, the function determines the actual rotating field frequency generated by the frequency converter and compares it with the parameterized limit value. If the rotating field frequency is within the parameterized range, the drive generates the status signal "Frequency In Limits". If the limit value is exceeded, the output stage is disabled via the function STO. The system stops generating torque and thus acceleration at once. In this case, the function status is "Frequency Out Of Limits". Hence, the function SLOF allows defining a safe limit for the rotating field of a spindle. This prevents that the drive actively accelerates the connected spindle to a too high frequency. If the set maximum rotating field frequency is exceeded because of a faulty input or malfunction of the drive amplifier, the function STO disables the output stage.

 

Grinding machine
Too high speeds at grinding machines pose risks for operators and service technicians that can be avoided with the safety functions of the drive system SD2

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