CANopen encoders for elevator engineering
By Heinrich Greiner (IVO) and Ellen-Christine Reiff (RBS Stutensee)
Precise positioning of the lift cabin is a vital precondition for optimized travel of the elevator. The formerly applied magnet switches with their rather big efforts in installation nowadays are more and more replaced by encoder systems. Encoders for monitoring position and speed allow any required curve of way and time. Whereas incremental encoders are perfect for measuring speed, absolute encoders are designed to fit positioning applications. The position values remain available even after power loss and make the reference run superfluous. New, “intelligent” solutions do not only allow comfortable installation and setup but also will contribute to a reduction in operational costs by enabling selective maintenance work. Thus, for example service intervals can be adapted to individual requirements.
The CANopen application profile
The CANopen protocol conform to European standards (EN 50325-4) is experiencing a breakthrough now also in elevator engineering as flexible serial communication system for electronic component network. The corresponding application profile is clearly defined but still allows utmost individuality. To the elevator manufacturer the new, OEM-independent standard means above all, devices with better price/performance ratio and a considerable relief in system integration. IVO (www.ivo.de), a specialist in encoders and controllers has contributed their experience in fieldbus systems to the working group „CANopen Lift“ by designing an encoder corresponding to the application profile and capable of direct connection to the bus system. This will open up all advantages of bus technology to the user, starting with comfortable setup and ending with comprehensive diagnostic options. The compact multiturn absolute encoder operates without any mechanical gears, but the number of turns is captured without any wear and tear by the patented „touchless encoder principle“. Even environmental temperatures from –20˚ C to +80˚ C have no impact on the encoder’s reliability. The encoder is shock-protected up to 2.0000 m/s2. The encoder resolution is 29-bit, 13 bit of which are for the angular information and 16 bit for the number of turns.
Absolute and incremental signals
Regulation tasks in elevator technology often imply a combination of two encoder types: During travel up and downwards the signals of an incremental encoder are being evaluated for controlling the speed of the cabin drive. The incremental encoder supplies a speed-irrelevant clock pulse that, for example, rather easily can be changed by a frequency/voltage transformer into a speed-relevant voltage. Precise travel to the individual floors however, calls for the absolute position signals of a multiturn encoder. For this and similar applications, for example swing forklift trucks in high multi-store shelves, one device and consequently the occurring costs can be saved.
An electronic multiturn encoder of the Multivo product series features two additional incremental track signals shifted by 90˚. The integrated Asic processes both absolute and incremental signals and makes them available by standardized interface to the control unit for speed and position regulation. The incremental signals for example, are for regulating travel profiles, whereas the absolute signals are for capturing the stop positions. The device satifies the highest requirements since processing speeds of more than 10 kHz are no problem. The basic encoder fits all current field bus systems by plugging on the desired bus cover. Separate connectors for the incremental signals enable easy installation. Slippage control
Besides speed control, the encoder’s incremental signals shifted by 90˚ open up further possibilities and applications, for example in monitoring slippage tolerances at belt drives or steel cables. Slippage means the difference between the path covered by the cable and the drive roll respectively motor. Slippage tolerances are needed to avoid extreme loads on reels, fan belts or steel cables but certain limit values must not be exceeded. It takes two encoders to capture the slippage tolerance, one for the cable and one for the drive line. The measured values supplied may be used for example by the elevator’s control unit for permanent slippage monitoring. Preventive maintenance service for controlling the slippage tolerances are thus becoming superfluous. Slippage monitoring does not always have to be up to the elevator’s control unit. Counters, for example the difference controller N 230, do as well. The difference controller designed for DIN rail mounting is capable of connecting two separated incremental encoder channels, one each for track A and B with direction recognition.
The controller evaluates the difference between the two measured values and gives a corresponding signal as soon as the slippage tolerances allowed are being exceeded. Input frequency of each channel is 30 kHz, the communication is realized by RS-485 interface. There are even more counter applications in elevator technology. By the counter series isiLine the company offers a series of electronic industrial counters that fit many applications thanks to their convincing price/performance ratio – also as drop-in replacement for electromechanical products. They can be applied for example for capturing start and stop intervals or operation hours. Thus, the service intervals can exactly be adapted to actual need.
The counters operate by count frequencies up to 12 kHz and dry-closure contacts, PNP/NPN outputs and 10 V to 260 V impulses serve as input trigger. The 8-digit LCD display allows comfortable readout with background lighting upon request. All counters are independent from the mains. A firmly integrated lithium battery with an expected service life of approx. 8 years (at 20 ˚C) secures display and runs value backup. Only the models with background lighting need external voltage supply.
Source: CAN Newsletter Special Lift 2005 (www.can-cia.org/newsletter)