The K148 is a calibration unit designed for calibrating strain-gauge bridge measuring amplifiers. It serves as a replacement for a strain-gauge transducer, providing defined electrical signals that simulate the effects of mechanical quantities acting on a transducer. This allows for precise correlation between mechanical deformation and the electrical display at the end of the measurement chain.

Function Description

The K148 calibration unit is engineered to connect to both single-channel and multi-channel measuring amplifiers, supporting up to eight channels in a full-bridge circuit. It utilizes a six-wire circuitry for connection, which includes sense lines for the excitation voltage. This design ensures that the length of the measuring cables does not affect measurement accuracy.

For single-channel amplifiers, the K148 connects directly. In multi-channel setups, the channels are switched serially internally. The unit has a strain gauge equivalent resistance of 350 ohms.

The K148 can be operated in two primary modes:

Manual Control: The device features a front panel with a keyboard for stand-alone operation, allowing users to manually select channels, polarity, range steps, and calibration steps.
Remote Control: The unit can be controlled via a USB interface or a serial (RS232) interface, connecting to a computer. This enables automated calibration processes and integration into larger measurement systems. Simultaneous operation of both interfaces is not possible.

The device provides five range steps up to ±100 mV/V, each further subdivided into ten calibration steps. This granular control allows for detailed checking of an amplifier's linearity.

Important Technical Specifications

Strain Gauge Equivalent Resistance: 350 ohms.
Frequency Range:
- 225 Hz to 600 Hz: Accuracy class 0.0025%.
- DC and CF > 600...5000 Hz: Accuracy class 0.025%.
Supply Voltage: 10 V ... 15 V (DC).
Power Consumption (USB port connected): 140 mA.
External Power Supply (optional): Input 100-250 VAC, Output min. 200 mA.
Interfaces: USB port for power supply/computer communication, RS232 serial interface.
Connectors:
- 15-pin SUB-D connector for single-channel amplifiers (6-wire connection).
- 25-pin SUB-D connectors for multi-channel amplifiers (channels 1-4 and 5-8, 6-wire connection).
- 4-pin mini DIN connector for external 12 V DC power supply.

Usage Features

Channel Selection: Arrow keys on the front panel allow selection of individual channels (1-8). If no LEDs are lit, the device is in single-channel mode.
Polarity Selection: A dedicated key on the front panel allows switching between positive (+) and negative (-) polarity of the output signal.
Range and Calibration Step Selection:
- Arrow keys are used to select range steps between 2 mV/V and 100 mV/V.
- Other arrow keys are used to select calibration steps between 0% and 100% within the chosen range.
- An "Auto-Repeat" function allows continuous cycling through percentage steps by pressing and holding the corresponding key.
Status Indicators:
- LED POWER (green): Indicates power supply presence.
- LED READY: Can be controlled by software in computer mode.
- LED REMOTE: Lights up when the K148 is operated via USB or serial interface.
- LED 1 (yellow): Indicates voltage supply OK.
- LED 2 (green): Indicates active USB link to the PC.
Audible Feedback: Each key press generates a signal tone. A short tone indicates a correct input, while a long tone signifies an incorrect input (e.g., range > 100 mV/V) or remote mode. The volume of this signal tone can be adjusted using special key combinations.
Calibration Procedure:
1. Connect the K148 to a power source (USB power supply, PC, or external 12 V power supply).
2. Connect the K148 to the measuring amplifier using the appropriate 6-wire or 25-pin connection cables.
3. Select the desired polarity for de-tuning.
4. Set the range step to the required range.
5. Set the calibration value to 0% and check the amplifier's zero point.
6. Use the arrow keys to set the required calibration signals according to the value table.
7. Set the amplifier according to its operating instructions, ensuring calibration in the intended measuring range to avoid selection errors.
Remote Control Commands: The K148 responds to specific commands sent via USB or serial interface. Commands must be in upper case, can include spaces, and parameter values are decimal numbers separated by commas. Commands are terminated with LF or CRLF. Incorrect commands or parameters return "?CRLF", while executed commands return single-digit numbers with CRLF.

Maintenance Features

Maintenance-Free: The K148 is designed to be maintenance-free.
Cleaning:
- Disconnect all connections before cleaning.
- Clean the housing with a soft, slightly damp (not wet) cloth.
- Avoid using solvents, as they can damage the labeling.
- Ensure no liquid enters the module or connections during cleaning.
Product Liability: Liability for device functionality passes to the operator if the device is not used according to the operating manual, used outside its described field of application, or if unauthorized changes are made by the operator.
Conversions and Modifications: Modifications to the device's design or safety engineering are prohibited without express agreement from HBM. Repairs or soldering work on motherboards are also prohibited. Only original HBM parts should be used when exchanging modules.
Qualified Personnel: Installation, commissioning, maintenance, and repair work must be carried out by qualified personnel who have read and understood the operating manual and safety instructions. Maintenance and repair work on an open device with power on should only be performed by trained personnel aware of the associated dangers.

HBM K148 - Page 56

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