Hardware and Wiring Help for Constant Force Application

Hello all, this is my first time working with a PLC and dealing with hardware, so please have patience with me.

I have an electric rod-style actuator driven by a rotary servo motor and a servo drive. The actuator will exert force on a platform that can slide up and down. The goal is always to maintain constant force applied onto the platform. I connected a load cell in line with the direction of force transmission between the end of the actuator and the platform surface (i.e., the load cell is sandwiched between both surfaces). Additionally, I purchased a load cell amplifier that can condition the 2 mV/V load cell signal into 0-5 V/RS485 output. Ultimately, the goal is to achieve force control via the PID functions available within the S7-1200 PLC.

Here are the specific products I am using:

• Mitsubishi MR-J4-10A1-RJ servo drive

• Mitsubishi HG-KR13 servo motor

• Futek LSB201 load cell

• ATO Digital Load Cell Amplifier, Output 0-5V/RS485

• Norgren ELION Rod-Style Actuator

• Siemens CPU 1212C DC/DC/DC

The servo drive has an option for Torque control mode, which (I believe) is what is needed to achieve force control. From the attachment, I plan to use the following digital inputs EM2 (forced stop - decelerates the motor to a stop), SON (servo on), RES (reset any alarms), RS1 (forward rotation), and RS2 (reverse rotation). I will have ALM (alerts a malfunction) and RD (system is ready to operate) for the digital outputs. I bought all the necessary pushbuttons, selector switches, and e-stop for these digital inputs and outputs.

A 50-pin pigtail connector will interface the servo drive and PLC. I have provided my attempt at a wiring diagram. Some specific confusions/questions are:

1. Should the e-stop be wired to a DI or 24 VDC? I’ve seen two wiring diagrams with both cases.

2. My interpretation of shorting the unused analog inputs of the CPU is to jumper the inputs in X11 and connect them to 0 VDC. Is this correct?

3. I plan to power the CPU using a bulky lab-grade linear DC-regulated power supply (0-30 [VDC], 0-5 [A]). I calculated the power budget and found that I need a power output of >= 12 [W] and current output >= 0.46 [A]. Should I stick with this PSU or use a switching power supply (maybe a class 2 PSU that plugs into a wall socket)?

4. I’ve read in other discussion threads that I should not connect my load cell to the AI of the CPU because 10 bits are far too low resolution. Would you recommend I get the SB 1231 AI 1x12 bit, SB 1231 4x13 or 16 bit, or the SIWAREX WP 231? I am straying away from SIWAREX because of the high price tag, but I am open to your opinions.

Getting the hardware configured is the first step. Hopefully, I am heading in the right direction. Any feedback will be much appreciated. Thank you. 

Hi,

1. What do you mean DI or 24 VDC? All DI there works with 24VDC. Only I would suggest to use double e-stop contacts and connect it to servo and PLC inputs

2. Correct, but as I understand you would connect load cell amplifier output to one of these inputs

3. Always is better to use dedicated supply with correct voltage and bigger current capability than your devices will use (at least 1A). they are really cheap. With lab supply there someone can change output voltage just by accident and there is a possibility to damage something.

4. 10 bits is not so bad. You can set needed zero/span on the load cell amplifier analog output. Or you need more than 1000 values of force?

Regarding control type of the servo - if you try to control force of the servo directly - there is almost no point of using PLC at all. I would suggest to use one speed on very low measured force and another slower speed if force is near setpoint.

Hello, thank you for your response. I have made a lot of progress since I last posted this.

I bought an SM1234 to connect my load cell amplifier since it has a higher bit resolution. Also, I decided to stick to the linear PSU because it will keep noise to a minimum (less ripple and whatnot). Luckily, it is only me in my lab space, so I don’t have to worry about anyone bumping into it while it is operating!

In terms of using a double NC e-stop and connecting it to my servo, I believe I am doing that now.  I have a safety circuit that consists of a Phoenix Contact two-channel safety relay, two Allen Bradley force-guided relays, a 2NC e-stop, and a momentary push button to reset this circuit. The EM2 connects to one of the contacts in the safety relay. Whenever the e-stop is pressed, EM2 will activate, causing the motor to decelerate. The extra push button is to prevent the servo from accidentally restarting.

Please check out my new post. I have new questions regarding wiring! I am not too sure about the programming aspect, as that is something I still have to learn. As of now, though, I believe the PLC is necessary because the force needs to be constant as the actuator extends and retracts, so speed will be changing as needed. I will try using a PID loop with a set force (torque) point and using my load cell values to constantly update the torque.