Application Information
APPLICATIONS:  Benefits of DC Motor Speed Sensing via Supply Current
There are several reasons to use DC motor supply current to measure motor speed, not just because sometimes it is the only way.  Yes, in some products, such as a fuel pump or electric water pump, where you cannot access the motor, the supply current is the only, and therefore the best method of measuring motor speed.  However, there are many other cases where motor current speed sensing provides significant advantages over traditional sensors.   
Allow Complete Functionality of Final Assembly
Where DC motors are embedded into a product, manufacturers need to be able to test the end functionality whether the motors have been previously tested or not.  For 100% QC testing, the complete functionality has to be tested after assembly.  What if a screw fell into the works?  It will draw current, but will it pump fuel?  If its hard to turn due to bearing end-caps being too tight, will the motor burn out? Will it fail in the field? What if the mechanisms that the motor are to drive are flawed?

CDA Systems Ltd. offers a few products to allow final test of products with embedded DC motors.  The CRSM1 and CMSS56 products offer a means of testing even the most challenging DC motor.  These products output an accurate, real-time speed readings that are not estimates. 

The CMSS56 also provides accurate measurements of RMS, average, and peak-to-peak current, along with simultaneous measurements on a secondary analog channel, allowing monitoring of another process variable such as pressure or vibration.

Simpler Test Cell
Some devices, such as a power-seat motor can be mechanically coupled for testing of the motor and assembly functions, or their speed can be measured with optical or inductive sensors.  However, these these can often be difficult to implement.  With one of CDA's motor speed sensors, the device need only be powered up through a typical current shunt or Hall-effect sensor, and the speed, RMS current, average current, and peak-to-peak current are measured.
Lower Maintenance Requirements for Test Cell
Using current sensing to determine motor speed removes the requirement for precise alignment of optical or inductive sensors.  There is no requirement for maintenance mechanics to constantly adjust and clean photo sensors to keep the test cell operating at full capacity.  There is no downtime due to physically damaged sensors.  In fact, the current shunt used as the input sensor to the CDA modules could be safely located in a control panel 20 feet away.

--> A guaranteed reduction in maintenance! <--

  Environmental Testing Advantages
Automotive manufacturers often have to wrestle with environmental test requirements.  For example, many manufacturers of engine cooling modules are required to perform "Salt Spray" testing, where the modules are constantly subjected to a spray of salt and/or salty water.  While the engine cooling modules can usually take the abuse, often the sensors used by manufacturers cannot.  Testing has to be stopped every few days to clean and re-align sensors.  Often, there are sensor failures in high-heat testing.  If the motor speed is measured via motor current, no such problems exist

--> A guaranteed reduction in maintenance! <--

(scope trace, motor speed & current on startup)

Enhanced Analysis
CDA's motor speed sensors update motor speed every time the motor commutates.  If there are 20 commutator segments in the motor, the speed is calculated 20 times per rotation.  This allows more accurate characterization of motor performance than can be done with traditional sensors. 

Further, the CMSS56 module also reports RMS, average, and peak-to-peak current, along with measurements from a secondary analog channel and digital inputs.

Some of our customers have found innovative applications for our motor speed modules, including using the tachometer output as an input strobe for motor noise "order analysis".

Motor Speed Control
CDA's motor speed sensors can be used in applications where control of motor speed is important.  In a production test environment, could be for final balance/vibration checks on assembled modules, or for performance testing.  In control systems, the applications for precise motor speed control are endless.  The fast response time, and variable output mechanisms of CDA's modules make them perfect fits in control systems that use any of the following:
  • Tachometer pulses - One pulse per rev tach output
  • Analog voltages - Precision analog output
  • Digital readout

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