What products are involved?
Rheonics SRV viscometer, SRD density-viscosity meter, DVP gas density-viscosity meter, and DVM HPHT density-viscosity meter.
What is the purpose of this article?
This article describes the procedure for integrating Rheonics Smart Module Electronics (SME) 4-20mA signal into a LOGO! Siemens.
1. Overview
Rheonics viscosity and density meters bring fluid intelligence and process control to a broad range of customer applications. Each sensor is equipped with a Smart Module Electronics unit (SME), which drives the sensor, evaluates its response, and enables communication through all major communication protocols.
This article guides the correct wiring procedures for using SME's 4-20mA channels and reading the sensor measurements on a LOGO! Siemens uses its analog inputs (0-10V).
Figure 1. LOGO! Siemens
If LOGO! AM2 expansion modules are available, it is recommended to use their 4-20mA inputs. Wiring the SME’s channels to an expansion module's embedded 4-20mA input is the preferred approach, as it offers a more robust and accurate system and a more reliable process control.
Figure 2. LOGO! Expansion Modules
2. Getting Started
- Rheonics SRV, SRD, DVP, DVM with minimum firmware version V03.20/0 or higher.
- LOGO! 24 CE, 12/24 RCE, 24CEo, 12/24 RCEo. These have 4 AI (0…10V)
- LOGO! Soft Comfort
- Windows 10, 64-bit
- 500 Ω Resistance
3. SME’s 4-20mA Channels
The Rheonics Smart Module Electronics (SME) includes three in-built output channels for 4-20mA signals on the SME’s housing, labeled CH1+, CH2+, and CH3+ (Figure 3). In this case, CH1+ will be used to get the viscosity output to the LOGO!
CH2+ and CH3+ can also be used for other measurements based on the configuration set in the RCP software and can be used depending on the application.
Figure 3. 4-20 mA channels on SME
For more information on the 4 -20 mA channels of the SME, take a look at Connecting4-20 mA outputs. It’s crucial to make sure the sensor cables are wired properly as well: Connecting sensor cable colored wires to transmitter electronics
4. LOGO! Analog input wiring
Wire the SME’s 4-20 mA output channel to one of LOGO!’s analog input terminals as shown in Figure 4. Since the LOGO! PLC uses 0-10V as input, a 500Ω resistor is required to convert the current signal into voltage. It's important to ensure the resistor is correctly installed for an accurate reading of the 4-20 mA signal.
Figure 4. LOGO! and SME wiring
Figure 4 features a 492Ω resistor, as well as connections on M and I2 for the analog input. For LOGO! 12/24 RCE, the analog inputs are I1, I2, I7 and I8. Any of them can be used.
Notice how the resistance has a margin of error and is not exactly 500Ω as expected. It is recommended to use a high-precision resistor to minimize the impact.
5. Rheonics Control Panel (RC) Settings
The 4-20mA settings panel, in the “Service” Tab, allows the user to read and set the upper and lower limits for each channel, as well as to set which parameter is assigned to each of them.
In Figure 5, viscosity is set on Channel 1 with an Upper Range Value (URV) of 3,000 cP, density on Channel 2, and temperature on Channel 3.
Figure 5. 4-20mA settings box
6. LOGO! Setup
6.1 New Device
After opening LOGO! Soft Comfort - Network View, begins by adding a new device and entering the LOGO! information (Figure 6). 
Figure 6. New Device on LOGO! Soft Comfort
Make sure to enter the appropriate IP Address and Subnet Mask for the LOGO, it should be on the same subnet as the SME (same Ethernet network). Incorrect IP configuration may prevent the device from communicating with others on the network. Other issues like IP duplicity could lead to difficult troubleshooting conflicts.
6.2 IO Configuration
After the device has been added, the next step is to navigate to "I/O settings", located on the left-hand side of the navigation bar, to enable the analog input.
Figure 7. LOGO! I/O Settings
There are three options available:
- Enable 0 AIs
- Enable 2 AIs (Input terminals I7 and I8)
- Enable 4 AIs (Input terminals I7, I8, I1 and I2)
Since the I2 input has been wired, it is necessary to enable all four analog inputs (AIs).
6.3 Reading the Value
LOGO! reads in the electric signal and, with further processing, converts it into a standardized value within the range 0 to 1000 (10-bit resolution). This new value is then used in the circuit as the input of other analog functions or blocks for further processing or logic, such as the Analog Amplifier which will be used in further steps (Figure 13). For example, a 0-10 V signal at 5 V is automatically scaled to 500, but to do it, an Analog Input Block is required first (Figure 8).
The Analog Input block can be seen and selected in the left tools bar of LOGO! Soft Comfort, under Instructions, Analog. This is all it takes to read an analog value; however, an Analog Flag is also necessary to avoid software complaints about an open connector.
Figure 8. Analog Input Block
6.4 Uploading the program to the LOGO! PLC
With the program ready and the SME configured, the final step is to upload the program to the LOGO! by clicking the icon shown in Figure 9.
Figure 9. Loading the program to LOGO!
A pop-up window will appear. If needed, enter the LOGO IP address and click OK.
Figure 10. Connection PC - LOGO!
The software will be transferred, and a progress bar will indicate the status. Finally, for easy visualization of the process, the online test feature will display the values picked up by the analog input in real-time.
Figure 11. Online Test
6.5 Signal Test Using RCP
To test the signal from the SME, the Service Tab on the RCP can be used to simulate a 10 mA value. By clicking on Test: 10 mA, the SME will output 10 mA to the LOGO!
Since the actual resistance value of the resistor in the analog input (Figure 4) is 492Ω, a voltage of around 4.92V is expected.
Figure 12. 10mA simulation on RCP
Figure 13 shows the 0-10 V analog input scaled to 491 automatically by LOGO! Analog Input Block AI4, which is mapped to I2, was used to read the signal on the physical input (see Figure 7 to read how the inputs are mapped).
Figure 13. Viscosity Signal on LOGO! Soft Comfort
To summarize, the SME outputs a current between 4 and 20 mA for viscosity values ranging from 0 to 3000 cP as configured in the RCP. By passing this current through a 500 Ω resistor, the corresponding voltage is picked up by the LOGO! in its analog input. This voltage is then scaled by the LOGO! to a range of 0 to 1000.
6.6 Scaling back the Analog Input
Depending on the application, knowing the actual viscosity value in cP may be necessary. To achieve this, the 0 - 1000 input value is multiplied by a defined gain, and an offset is added to the product, as shown in the following steps. For the calculations, the Analog Amplifier block can be used (Figure 14).
Figure 14. Analog Amplifier
The scaling needed to calculate the viscosity follows a linear equation y = mx + b. Where “ Y” is the viscosity in cP and “X” is the value between 0 and 1000 scaled by the LOGO. Take a look at Figure 15.
Figure 15. Viscosity vs Units
Two points are known:
- Point 1: At 4 mA (2V minimum signal), the reading is scaled to 200, corresponding to 0 cP. This is represented by the point (200, 0) on the graph.
- Point 2: At 20 mA (10 V maximum signal), the reading is scaled to 1000 units, corresponding to 3000 cP. This is shown by the point (1000, 3000) on the graph.
6.6.1 Gain Calculation for the Analog Amplifier
For the 4-20 mA signal, the minimum value (4 mA) corresponds to 2V (4 mA x 500Ω), which is scaled to 200 by the LOGO! leaving the remaining 800 units available to measure the entire viscosity range.
Another way to calculate it is by solving the slope from the linear equation in Figure 14.
A gain of 3.75 cP/unit means that the smallest detectable change in viscosity is 3.75 cP; this is due to LOGO!'s 10-bit resolution. While this level of resolution may be enough for some applications, if more accurate measurements are needed a lower viscosity range must be set using RCP.
6.6.2 Offset Calculation for the Analog Amplifier
The offset can be calculated using the linear equation from Figure 15.
The Analog Amplifier configuration is shown in Figure 16 with the calculated gain and offset.
Figure 16. Analog Amplifier Configuration
6.6.3 Scaled Viscosity Value in cP
Since modifications were made to the diagram, the steps from section 6.4 must be followed again to download the program to the LOGO!
The output of the analog amplifier, now configured with calculated offset and gain values, shows a scaled viscosity of 1,088 cP for a 10 mA signal (Figure 17). This value can be used for further control, define setpoint or configure lower or higher limits for alarms, etc.
Figure 17. Viscosity Value on cP
7. Analog vs. Digital - Pros and Cons
Analog communication, particularly 4-20 mA current signals, has been a longstanding standard in industrial settings. An analog signal's primary advantage is its simplicity. This simplicity means it requires very little configuration, and analog components are generally less expensive, which makes for a great option for basic measurements.
However, analog communication comes with limitations:
- Only one piece of information can be transmitted per channel, which makes it unsuitable for transferring diagnostic data or multiple measurements at the same time.
- Analog signals are also more prone to electrical interference, which can degrade signal quality in electrically noisy industrial environments.
- Over time, analog systems may need calibration to maintain accuracy, adding to maintenance efforts and potential downtime.
Many of the limitations of analog systems can be addressed by digital communication protocols like Modbus TCP and Modbus RTU which come by default on all Rheonics sensors:
- Transmits multiple data types over a single communication interface at once. This way, measurements, device status, diagnostic information, and even control commands can be exchanged between devices and control systems.
- Digital signals are inherently less susceptible to electrical noise, which makes them ideal for industrial settings with high electromagnetic interference. This is crucial for maintaining accurate and consistent data transmission and reliable process control.
- Digital networks are scalable and flexible, allowing multiple devices to communicate over a single network cable or wireless connection
The choice between analog and digital communication ultimately depends on the application, however, for a reliable viscosity control, it is heavily recommended the use of digital communication. For detailed information on all available communication options for Rheonics viscosity and density meters, visit Communication: Rheonics Support.