Adding external DC offset to analog output

  • Thread starter Stephen Bennett
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Stephen Bennett

Greetings List: I just encountered a problem in the field where I have a -10Vdc to 10Vdc analog output from a robot controller card. The D/A signal from the robot is eight bits, -10 being zero and +10V being this highest count of 255(2^n -1). During normal operation this signal is held at -10 V dc. I would like to know how I can easily give this signal an external DC offset so that the output range of the card could be bumped up to 0 to 10V. This would allow it to meet with specs of the equipment receiving the signal. I do not want to damage this equipment and or effect the signal in any way. Would a simple +10V dc power supply with some type of over current protection and voltage regulation be the best alternative? Perhaps a high pass / low pass filter to get rid of any unwanted noise from the DC source? I have been reading articles on this list for a few years now and I know that some of the smartest minds are lurking out there so please feel free to let me know what you think at your convenience. Thanks In Advance: Stephen Bennett Kawasaki Robotics (USA), Inc.

Steve Myres, PE

A simple offset will not help with your application, as your input signal has a range of 20V, and your input, only 10V, so you will need range scaling as well as the offset. The best way I know of to handle this is by using a signal conditioner / isolator. The unit (about the size of a deck of cards, and mounted on DIN rail, on edge), will take the +/-10VDC signal from the robot controller and an external 24VDC power supply, and output a fully ground-isolated 0-10VDC signal that you can connect to your PLC. Action Instruments model G408-1001 should do the job, and units are also available from API, Weidmuller, Entrelec, Phoenix, etc. Units are also available for 120VAC and other input voltages, as well as other output formats, like 4-20ma, if this is helpful. There is one potential trap in converting a bipolar signal to a unipolar, that you must evaluate for your application. Bipolar signals are frequently used in motion control to indicate full reverse motion to full forward. I have seen potentially hazardous, even life threatening, behavior result with a unipolar signal used in this application. The situation was that the motor drive powered up before the PLC, and was receiving no signal (0VDC), from the PLC, and this meant "go full speed in reverse". You can see why a bipolar signal may have safety advantages if the signal may assume a zero value when not under program control. You will have to evaluate if this is a risk in your application. If it is, you may be able to get a bipolar input capability for the PLC or monitoring device. Hope this helps. Feel free to contact me off list if you have other questions. Steve Myres, PE [email protected]

Michael Griffin

At 13:29 09/03/01 -0500, Stephen Bennett wrote: <clip> >I just encountered a problem in the field where I have a -10Vdc to 10Vdc >analog output from a robot controller card. <clip> >I would like to know how I can easily give this signal an external DC >offset so that the output range of the card could be bumped up to 0 to >10V. <clip> There are lots of signal converters which will do exactly this. Phoenix, Entrelec, Weiland, Action Instruments, etc. should all have suitable examples. They have small DIN rail mount modules which convert between various analogue voltage and current standards, as well as some which are field configurable. Be sure to check the frequency response of the various modules, as many are intended for process industry applications and have a 3 - 5 Hz response. Some modules however, offer much faster response than that if you require it. ********************** Michael Griffin London, Ont. Canada [email protected] **********************

Johan Bengtsson

Good points below, I agree on checking risks and so on. If you would like a simpler solution that might work in your case to do the same job you can use this solution: use two resistors as a voltage divider, possibly + a potentiometer to cover tolerance differences, and a 10V source. +10V DC ----* | --- | | |R| | | --- | | --- | | | |<------ output | | --- | | --- | | |R| | | --- | input ------* R should be big enough to be handled by the signal source and small enough to not be affected by the output impedance at the very highest 10 times smaller than the input impedance of the recieving circuit, preferably smaller. The potentiometer could be removed if you don't need to adjust for resistor tolerance, otherwise it should be kept as small as possible but big enough to cover those tolerances. The 10V source needs to be stable, noice free and at the right voltage. You have to decide for yourself if you want this, an off the shelf converter, or if you decide to not convert at all due to safety. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: ----------------------------------------