Connecting a Force Sensor to a DAQ
This digital era is marked by a lot of technologies. We have become so familiar with the systems created from the combination of them that we have lost track of the individual breakthroughs that are the component subsystems.
In the world of manufacturing, the term “instrumentation” often refers to this combination of subsystems used to accurately measure, control, or observe another system. This article discusses such an instrument that connects a force sensor and a data acquisition (DAQ) device. It describes the various subsystems of the sensor-DAQ setup using some of Tacuna System’s devices for reference.
The Elements of the Force Sensor-to-Data Acquisition System Setup
When connecting a force sensor and DAQ system, it is useful to have a general overview of how the various subsystems are integrated. The block diagram in Figure 1 below shows the individual components and how the signal flows through them. This section describes the function of each block pictured.
The Force Sensor/Transduction Unit
A sensor detects a physical phenomenon and transforms the detected quantity into a proportion output through a process called transduction. This output might be pneumatic, mechanical, or electrical. Force transducers apply physical principles that allow them to produce electrical signals when a physical force is exerted on them. They are classified depending on the physical principle used. The following are some common types of force sensors: hydraulic, pneumatic, capacitive, and strain gauge. (See An Overview Of Load Cells for more information about these sensors.)
Hydraulic Force Sensors
The hydraulic force sensor works on the principle of force-balance. It uses a piston and a fluid contained in a sealed cylinder chamber to sense the applied force. The piston-fluid interface then transmits the force exerted on the loading platform to a pressure indicator or a pressure transducer.
Pneumatic Force Sensors
The pneumatic type also works on the same principle of force-balance but it instead of a fluid, it works with compressed air or gas.
Capacitive Force Sensors
The capacitive type senses force through the relationship between the distance of separation of two charged plates and the capacitance. One plate is made stationary while the other is movable. The force exerted on the moveable plate decreases or increases its distance from the stationary plate, producing an output capacitance proportional to the applied force.
Strain Gauge Force Sensors
The most common of the sensors mentioned in this article is strain force sensor type, also called a strain gauge load cell. This load cell uses an elastic resistive material called a strain gauge, attached to the structural member of the device, to transform the force into an electrical voltage output. When a force is applied to the load cell at a specific point, it is transformed into a strain by the gauge’s structural members. This strain is mechanically transmitted to the strain gauge, which then deforms elastically producing a change in resistance. The change in resistance is then converted into an electrical voltage by including the strain gauge in a Wheatstone bridge circuit. A strain gauge load cell has typically four wires: two power supply/excitation wires and two output signal wires.
The Signal Conditioning Unit
This is the intermediary unit between the transduction unit and the DAQ unit. It converts the output from the transducer unit to a signal compatible with the data acquisition unit. This signal conditioning process generally follows three steps: isolation, signal filtering, and signal amplification
In the case where the force sensor is powered through a regulated supply on the signal conditioner circuit, an electrical isolator is necessary. This device isolates the force sensor from a direct connection to the power supply, protecting the sensor from fault currents. Isolation can be done by a magnetic isolator such as a transformer, or an optical isolator such as an optocoupler.
The output signal from a strain gauge in particular can be very small (in the order of millivolts, mV), and potentially “noisy”. The DAQ requires a clean, detectable input signal.
The process of signal filtering removes this noise, or unwanted signals from the sensor output. This can be achieved using specially designed circuits such as active and passive filters. An alternative is to simply place a bypass capacitor between the positive terminal of the signal path and the ground terminal of the signal path. This capacitor serves as a low pass filter since it conducts high-frequency signals such as AC noise to ground.
Filtering is always necessary at both the the amplifier input and output.
The Signal Amplifier
Amplification is the last step of signal conditioning. The amplifier increases the dynamic range of the filtered sensor output to a level equal to or near the dynamic range of the DAQ.
Generally, two methods are used to amplify the sensor output signal.
1. Differential Amplifier
With this method, the output terminals from the sensor are made part of a voltage divider circuit; a differential amplifier then amplifies the voltage divider output.
2. Wheatstone Bridge
With this method, the sensing device (typically a strain gauge) is incorporated into an arm of a Wheatstone bridge. The output terminals of the bridge connect to an amplifier. The most commonly used amplifier in the industry is the Instrumentation amplifier. Its advantages over the differential amplifier include its high and stable gain value, very high input impedance, very low output impedance, and an extremely high common-mode rejection ratio. The low output impedance in turn presents a low input impedance signal source to the DAQ so that changes in the input of the DAQ do not alter the output from the amplifier.
Signal Conditioning, Summarized
We can summarize the internal workings of the “Signal Conditioning Unit” block in Figure 1 as follows:
- The power supply to the load sensor is isolated,
- Noise is filtered from the load sensor output voltage,
- The filtered signal is amplified to a level compatible with the requirements of the DAQ.
The Data Acquisition (DAQ) Unit:
This unit receives the conditioned signal from the force sensor. It consists of the following digital signal processing units: The anti-aliasing filter, the sample-hold (S-H) circuit, the quantizer-encoder circuit, and the digital signal processor.
The Anti-Aliasing Filter
This is the filter circuit located directly at the output terminal of the amplifier. This analog filter band limits the DAQ unit input received from the amplifier circuit. The band limit is necessary to remove the input signal frequency components that are higher than half of the DAQ’s specified sampling frequency. Without the anti-aliasing filter, the DAQ sampling would occur at inappropriate intervals for the higher frequencies. This would produce a sampled signal containing various overlapping or indistinguishable frequency components. Aliasing is the term used for this overlapping effect.
The Sample and Hold Circuit
This component performs sampling: the acquisition of an analog signal at discrete time intervals resulting in a sequence of samples. The “sampling rate” is the term used for these discrete time intervals. Sampling is done based on the Nyquist Theorem which states that an input analog signal can be accurately reconstructed from its sampled version if it is sampled at a rate that is at least twice its highest frequency component.
This explains the need for an anti-aliasing filter. This filter limits the analog signal from the amplifier to a bandwidth lower or equal to half of the sampling rate.
The Sample and Hold circuit (S-H) consists of a solid-state high-frequency switch, a holding capacitor, and a buffer amplifier (i.e., a voltage follower).
In practice, the S-H circuit is very key to the sampling process because the input analog signal is time-varying. To truly sample this signal, the component must determine the instantaneous frequency value at each specific time. The circuit holds the instantaneous value steady till the next time interval. The held signal value must be sampled within that holding time period at the appropriate sampling rate in order to achieve an accurate result.
Figure 6 below shows a simple S-H circuit. This unit converts the analog signal into a discrete-time signal continuous in amplitude but discrete in the time domain.
The Quantizer and Encoder
The quantizer unit samples the S-H output in the amplitude domain. Quantization creates a range of equally spaced levels. Each level exists between only two possible limits; hence the signal is said to have been quantized.
The encoder then assigns a certain binary code to each of these levels. The encoder can use one of two possible encoding schemes: the two’s complement coding scheme or the offset binary coding scheme.
Note that the S-H circuit, the quantizer and the encoder together all form an analog-to-digital converter (ADC). This is so because the input to the S-H circuit is analog and the output from the encoder is digital.
The Digital Signal Processor (DSP)
This component performs certain processing operations on the digital signal: frequency and time domain analysis, filtering, convolution, and correlation.
The DSP is a unit that has its own subsystems. Also, it runs on a dedicated hardware architecture that is quite similar to that of a general microprocessor. The special hardware unit of the DSP includes multipliers, an accumulator unit, shifters, address generators, data memory, and program memory. The digitized signal is stored in DSP memory to be retrieved later for processes such as display or process control.
The architecture of a DSP is shown below in Figure 7.
Finally, a telemetry system can be integrated with the DAQ subsystem to transmit the digitized force sensor output accurately over a long distance to a remote location. See Advantages and Applications of Wireless Load Cells.
A Look at Hardware Products
The data acquisition hardware shown in Figure 8 below is a portable device that can aggregate input from several sensors. It has multiple input channels, and the capacity to transmit aggregated data to a central remote location through a wireless network. It offers both wired and wireless telemetry systems, has 16 input channels, has a data logging system, and has a sampling rate of up to 1000Hz.
Tacuna Systems also offers a wide range of amplifiers and conditioners that can then be used to interface a force sensor such as a strain gauge load cell to this DAQ hardware. A sample Tacuna Systems amplifier and its datasheet specifications are shown in Figures 9-10 below.
The datasheet shows the amplifier gain, the load cell type it supports, the input range it supports from a sensor, the output voltage and current. Note that the output voltage and current signal value all fall within the dynamic range of the DAQ device. By acquiring our suite of products, one has a complete package ready to be coupled together to have a force measurement system operational in a short period of time.