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Magnetostrictive transducer requirements for the interface:
- Interface with reliable transmission and high update rate for the position data-
Identification of the transducer system
1. Features:
The Novotechnik DyMoS interface for magnetostrictive transducers is a uni-directional interface combining the data integrity of bus communication with the high transmission rate of the SSD interface. The speed at which the position values from the magnetostrictive transducer system are provided is adjusted to the transmission rate of the interface, thus achieving an overall update rate of output values to 16 kHz.
The data frame consists of 2 x 20 data bits. Formatting of the data frame is fixed in the transducer and can consist of 2 position values or one position value and the velocity value. An additional benefit of the Novotechnik interface is the time coordination of the transmitted position values with the internal sequence of the measurement and the data preparation, with a continuous cycle of data querying. This ensures error-free calculation of the velocity and acceleration. The drivers are RS422/485 conformal. The transmission speed is from 25 Kbps to 1 (2) Mbps.The traceability and documentation of the transducers used in the application represents an additional advantage of the interface. This is accomplished using a system identification which the transducer sends at each power-up. The system identification consists of the part number (model and stroke length) and a unique number. The system information can also be used for unique assignment of the transducer within the application to prevent faults and damage caused by connecting the wrong transducer to the machine.
2. Technical Description of the Interface
The communication frame consists of the 3-bit system frame, the 2 x 20-bit data frame, and the 5-bit CRC.
2.1 System Frame
The system frame consists of the funktion bit, the error bit and the system bit.
- Function Bit – is always High and indicates the start of transmission.
- Error Bit – is High-active. The function of the magnetostrictive transducer is monitored internally, both as a pure hardware function and by means of a plausibility check of the position values. Should an error occur, the error bit is set High. On the receiving side the decision can be made at the start of transmission whether the received data are valid or not.
- System Bit – The system bit is used for defined recognition of the sent data. The system bit is Low when data (position value) are being sent and High when the transducer system identification is being sent.
2.2 Data Frame:
The data frame consists of 2 independent 20-bit frames. The data are formatted from LSB to MSB within the frames.
- Position value: 20 bits. The bit format is intended for a transducer having 5 m of stroke and a resolution of 5 µm.
- Velocity: 20 bits. The velocity resolution is 2.5 mm/s.
Transducer models:
Standard = The data frame consists of 2 independent 20-bit frames. The data within the frames are formatted from LSB to MSB.
Option = 2 Hardware interfaces: Interface 1: Position 1 and Velocity 1
Interface 2: Position 2 and Velocity 2. (Fig. 1).
2.3 CRC: Cyclic Redundancy Check
The CRC-Frame consists of 5 bits (Bit 44-48). For a 5-bit CRC the generator polynomial has 6 places. The generator polynomial is defined as shown in Fig. 5:
X5+X4+X2+X0 (110101)
3. System Identification
The uni-directional Novotechnik interface also implements transmission of the device identifier. Transmission of the identification data is automatic at each power-on. The identification data are sent in the first four transmission frames. After the system identification is sent, the transducer automatically switches over to data (run) mode. The two transmission modes are differentiated using the system bit. When the identification is being sent, the system bit is High (=1), and when data are being sent the system bit is Low (=0). Unique identification of the transducer is accomplished by indicating the:
- Stroke length, 0500 for stroke length = 500 mm
- Serial No. YYCWCW1234.
4. Communication
4.1 The Novotechnik interface offers the options of a 48 cycle mode or a continuous cycle. The 48 cycle mode conforms to SSI protocol. Data are sampled with the clock of the receiver and data is output synchronous with positive edge of the clock. After the 48 data bits have been transmitted, a time tm of 10 µs follows during which the output is Low. After tm has elapsed, the output of the magnetostrictive transducer goes to High. With the clock the current data, corresponding to the last update, are sent at 16 kHz. The sequence of the internal data update and data transmission is shown in Fig. 2.
4.2 Continuous cycle mode
A simpler and more reliable solution is achieved by using continuous sampling mode (Fig. 3). The advantages of data transmission in this mode are:
- Precise timing of the data
- Easier handling of the clock generation
- The data are updated in the controller continuously at max. 16 kHz
- The controller has optimum access to the data.
In continuous sampling mode the clock is always present on the transducer system. The transducer system starts data transmission following the internal update. After the 48 bits are sent, a time tm follows, during which the output is Low. The data can be stored in the controller in a dual port RAM, so that the controller’s CPU always has access to the most current data. Even if the CPU does not access with absolutely precise time accuracy, it will still be ensured that the data can be allocated to a very accurate time sequence. This also allows the velocity to be calculated with high precision.The sequence of internal data refreshing and data transmission is shown in Fig. 3.
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