FEATURE FEATURE TITLE ASK THE EXPERT ABB
Safe torque-off (STO) function in VSDs
W
ith any industrial machinery, the
overriding and most basic requirement is that it be safe. To achieve this, the machine needs a fail-safe device, one that in the event of a failure, responds in such a way that no harm will come to people near the machine. Often, a
safety device will cut the power when there’s a problem and prevent accidental restarts. An example could be a machine that could hurt
somebody if they were to put their arm into it whilst it was operating. Normally, you would have precautions such as an emergency stop button, and sensors that react if someone comes near or opens the safety cage door. It is common for these sensors to trigger a safety contactor which cuts the power to the motor, but these have some drawbacks, if only that it’s yet another piece of equipment that needs to be installed, maintained and regularly tested. This is where safe torque-off (STO) comes in. It brings the functions of the contactor into the drive electronically. Now when something triggers the sensors, the drive itself shuts the power to the motor down instantly, allowing it to safely stop, and stay stopped until an operator instructs the drive to start the process again. STO makes use of the special property of a
variable-speed drive with an induction motor, which is that torque cannot be generated without the continuous correct active behaviour of the variable-speed drive circuit (see video at
www.abbukenergy.blogspot.co.uk). This means that STO is fail-safe, so when the
STO input is disconnected, the drive will not operate the motor. It temporarily cuts the voltage to the motor without the need to shut down the drive power supply. Safe cleaning and maintenance of processing machinery can then be carried out, while the drive remains ready to resume normal operations and can be interrogated whilst the STO is active, as the control voltages are still present. Restarts are quicker too, as the drive is ready to
provide power to the motor at a moment’s notice. So a drive with STO is safer, easier to maintain and cheaper in the long run. A number of ABB variable-speed drives offer STO. With a dual channel safety interface, this meets the dual redundancy requirements of SIL 3/PL e (Safety Integrity Level 3/Performance Level e). TUV certified, the ABB safe torque-off function
allows machine builders to interface emergency stop functions that comply with the machinery directive, all without the need for contactors or safety relays.
John Bennett, iDrives - an ABB authorised value provider T: 01925 741111; E:
energy@gb.abb.com www.abb.co.uk/energy
32 MAY 2015 | AUTOMATION
INDUSTRY FOCUS ENERGY MANAGEMENT FUELLING THE IOT
Tony Armstrong, director of product marketing, power products at Linear Technology Corporation looks at how the Internet of Things (IoT) is being fuelled by wireless charging and energy management
T
he ‘Internet of Things’ (IoT), refers to a growing trend to connect not only people
and computers, but all sorts of ‘things’ to the Internet, and therefore, each other. Traditionally, various types of sensors were connected by wires to their power sources. However, today, rather than the challenge and expense of running cables all around a facility, it is now possible to install reliable, industrial- strength wireless sensors that can operate for years on a small battery, or even harvest energy from sources such as light, vibration or temperature gradients. Furthermore, it is also possible to use a combination of a rechargeable battery and multiple ambient energy sources too. Moreover, due to intrinsic safety concerns, some rechargeable batteries cannot be charged by wires but require being charged via wireless power transfer techniques.
ENERGY HARVESTING TECHNOLOGIES State-of-the-art and off-the-shelf energy harvesting (EH) technologies, for example in vibration energy harvesting and indoor photovoltaic cells, yield power levels in the order of milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can mean that the technologies are broadly comparable to long-life primary batteries, both in terms of energy provision and the cost per energy unit provided. Moreover, systems incorporating energy harvesting will typically be capable of recharging after depletion, something that systems powered by primary batteries cannot do. Nevertheless, most implementations will use an ambient energy source as the primary power source, but supplement it with a battery that can be switched in if the ambient energy source is disrupted. This battery can be either rechargeable or not and this choice is usually driven by the end application itself. So it follows that if the end deployment allows for easy access to change a non-rechargeable battery, where maintenance personnel can readily swap it out in a cost effective manner, then this makes economic sense. However, if changing the battery is cumbersome and expensive, then the utilisation of a rechargeable battery makes more economic sense. Even if a rechargeable battery is selected,
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the question of the optimum method to charge it remains open. Some of the factors that will affect this decision are:
1. Is there a wired power source to charge
the battery. 2. Is there sufficient power available from the ambient energy sources to have sufficient power available to power the wireless sensor network (WSN), and also charge the battery. 3. Is wireless power transfer required to
charge the battery due to intrinsic safety requirements due to the hazardous nature of its deployment.
EH AND WIRELESS CHARGING SOLUTIONS Fortunately for the designer of such challenging systems, there exist a number of power ICs which have the necessary features and performance characteristics to enable such low levels of harvested power to be used in wearable technology applications. Linear Technology recently introduced two parts for this purpose, the LTC3331 and the LTC4120. The LTC3331, an energy harvesting and
battery life extender, is a complete regulating EH solution that delivers up to 50mA of continuous output current to extend battery life when harvestable energy is available. It consumes no supply current from the battery when providing regulated power to the load from harvested energy and only 950nA when powered from the battery under no-load conditions. The LTC4120 is a wireless power receiver and battery charger that also integrates technology patented from PowerbyProxi (a Linear technology partner). PowerbyProxi’s patented Dynamic Harmonization Control (DHC), technique enables high efficiency contactless charging without thermal or electrical overstress concerns in the receiver. Using this technology, up to 2W can be transmitted at a distance of up to 1.0cm.
Linear Technology (UK)
www.linear.com; E:
uksales@linear.com T: 01628 477066
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