Rotating Paddle Level Sensor

Predecessor to all level sensors, Rotating Paddle was first introduced in 1950s. It is the original level sensor, and has left its mark on product descriptions even today. The best anectodal evidence can be seen in the salient features of Vibrating Fork Level Sensor; the line "No moving parts" is included in several makes of Vibrating Forks across countries.

Like most first innovations, designers of the Rotating Paddle Level Sensor used a seemingly simple principle in a very effective manner, such that a variety of application materials could be covered, with minimal adjustments.

However, newer level sensors technologies based on Vibrating Tuning Forks, RF-Capacitance and RF-Admittance were being introduced since 1970s; these had one major advantage:

An electric motor drives the shaft connected to the rotating paddle. The twin blades of rotating paddle rotate freely when uncovered. The electric motor is mounted on a sliding-fit rotating disc, which in-turn is connected to a spring. When rotating freely, the spring is relaxed, as the application media covers the blades of the rotating paddle, the torque requirement increases, which extends spring. The spring extension is detected by position sensors, thus level of application media is detected. The power to the motor turned off. Subsequently, when the rotating paddle is uncovered, the spring ensures the motor is back to the position where motor can be powered on.

Based on the torque requirement and availability of space motors used in Rotating Paddle could be one of : DC Synchronous Motor, AC synhronous motor, Stepper motor.

Servo motors have not been considered as no known manufacturer is known to be using it. Although, a servo motor could make an interesting choice, more on that later. Based on low input power supply, long useful life of 3 years and the possibility of adjusting speed and torque of motor makes DC gear stepper motor emerge as the best option.

While most other types of level sensors have upgraded to AC-DC wide range universal power supply, Rotating Paddle Level Sensor in most cases continue to be available in separate AC powered and DC powered models.

The choice of motor determines the required power supply. For DC motors SMPS power supplies can step down a wide range of voltage to the desired 5V/12V. For 230V/110V AC powered motors, it is expensive the step up the voltage efficiently for low voltage DC input. Also generating a sine wave from DC votlage is non-trivial and therefore not used.

Sapcon's Rotating Paddle: Orbit, has a wide range AC-DC(18-265 AC/DC) power supply which is very rare in the industry.

1. Placement of motor

Generally when the rotating paddle is covered with application media, the rotational motion faces resistance. At this point, the torque applied by the motor rotates the sliding disc instead of the rotating blade.

Instead of a circular disc an alternative arrangement of a chasis can also be used to achive the same result. Here the motor can often have an off-center shaft.

2. Position Sensing devices

The power to the motor is cut-off based on the signal from the position sensing device, the position of the motor itself or the peripheral beacon attached to the motor is sensed. The position sensing device plays an important role in determining useful life of the rotating paddle level sensor.

Sensitivity adjustment may be required to sense low density application media. At a lower torque setting is useful for low density media. This is typically done by adjusting the spring.

Low cost solution often use microswitches as sensing as well as output devices. Sapcon instead uses standard 10A rated DPDT relays, hence making the choice of power supply more flexible.

Safety: Microcontoller controlled relay outputs makes the Failsafe operation of Rotating Paddle Level sensor independent of mechanical failures.

Rotating Paddle is in-famous for the afore-mentioned mechanical wear and tear. An empirical analysis of site conditions has revealed the more factors, some of which can be avoided by proper installation and maintainance.

1. Wear-and-tear due to motion

Electric motor, ball bearings, oil-seals and PTFE bushes are primarily affected by rotational motion. As time progresses the frictional force is bound degrade the performance, however for most applications, the cost of Rotating Paddle Sensors would be recovered towards the end the of the useful life of the level sensor.

2. Ingress of dust

Dust ingress can be avoided by correctly installing and maintaining the cable glands. Dust can increase the friction between moving parts in the instrument. This may lead to fast wear-and-tear of the instrument.

3. Buildup of application media

For hygroscopic media or media that has a tendancy to stick on the level sensor; Rotating Paddle Level Sensor is not recommended in such application media.

Incorrect horizontal mounting arrangement

For horizontally mounted low level sensor, the load on the probe may result in misalignment of the shaft if baffle is not installed above the low level sensor. Misalignment of shaft may lead to an excess pressure on the motor, reducing the useful life of the instrument.



Why Rotating Paddle Level Sensor is the preferred choice for many buyers

Apart from emperical performance on the plant shop-floor, there are other factors that instill confidence in the typical buyer of a rotating paddle level sensor. The following factors are important to list as an acknowledgment of these factors may help a buyer reconsider or re-inforce Rotating Paddle Level sensor as the first choice.

• Visually verifiable mechanism: Understanding the principle by seeing the sensor in operation can instill more confidence, as the mechanism of rotating paddle level seems obvious even to modestly trained operators on the shop floor.
• Pre-installed; Replacement buys: Most rotating paddle sensors are replacement buys, the rotating paddle sensors have been around long enough to have been replaced a few times. In such cases going for a familiar option seems a less risky option.
• Immutable Purchase Specs: The specifications may be frozen by a third party consultant, all that the buyer has a choice is to select the make.
• Attractive low cost options: As seen above, manufacturers can make a choice to use low cost components and reduce their raw material cost. A combination AC Synchronous motor, single power supply option(only AC) and micro-switch ouputs can make a compelling buy. However the lack of features accompanied with low cost substitutes for bearings, motor, oil-seals, PTFE bushes the useful life of the paddle level sensor may be affected.
• Testing New Application Media: Unlike RF-Capacitance and RF-Admittance only physical parameters like specific gravity, particle size of the solid need to be known. A trial over a short duration of a day can be fairly conclusive in determining the success of the paddle level sensor in a particular application media.

How Sapcon is addressing limitations of Rotating Paddle Level Sensor

The natural generational changes have brought in important to corrections on previous designs.

Sapcon's Design Process

• Benchmarking against international products
• Proof of concept for individual modules
• Iterative Design:Only the best one makes it to production
• Extensive in-house testing
• Process qualification design for production