Mitsubishi PLC relative position:三菱PLC中相对位置控制的基本概念是什么？

Q: 三菱PLC中相对位置控制的基本概念是什么？

A: 三菱PLC中相对位置控制是指以当前位置为基准，通过指定移动量来确定目标位置，而非绝对坐标。这常用于伺服或步进电机定位，指令如DRVI或DRVA。根据三菱电机《MELSEC iQ-R定位模块用户手册》，相对位置控制适合连续增量运动，能避免绝对坐标累积误差，提高系统灵活性。

Q: 三菱PLC相对位置控制有哪些常用指令？

A: 三菱PLC相对位置控制常用指令包括DRVI（相对定位）和DVIT（相对间歇定位）。DRVI指定移动量和速度，从当前位置开始相对移动。据《MELSEC-Q系列定位模块用户手册》，DRVI支持脉冲输出，适用于简单定位。DVIT则允许在移动中暂停。这些指令需配合定位模块使用，确保精确控制。

Q: 如何设置三菱PLC相对位置控制的参数？

A: 设置三菱PLC相对位置控制参数需在工程工具中配置定位数据，如移动量、速度、加减速时间。依据三菱《GX Works2操作手册》，用户需设置脉冲单位、旋转方向等。参数通过定位模块写入，如QD77MS。相对位置控制强调移动量正负表示方向，确保与机械系统匹配，避免超程。

Q: 三菱PLC相对位置控制与绝对位置控制有何区别？

A: 相对位置控制以当前位置为起点，移动指定距离，目标随起点变化；绝对位置控制以原点为参考，直接指定坐标。根据三菱《定位模块用户手册》，相对控制适合增量运动，如传送带；绝对控制适合固定点定位。相对控制无需原点回归，但可能累积误差；绝对控制则需建立原点。

Q: 三菱PLC相对位置控制的应用场景有哪些？

A: 三菱PLC相对位置控制广泛应用于包装机械、数控机床和机器人臂等。例如，在传送带上进行增量进给，或机械手连续抓取。据三菱电机《可编程控制器应用指南》，相对控制能简化编程，提高效率。它特别适合运动距离不固定的任务，如根据传感器信号调整移动量，实现柔性生产。

Dialogue about

Common scenarios of "Mitsubishi PLC relative position"

【Engineer】 Hi everyone, today we're going to discuss the relative position control in Mitsubishi PLCs. Can anyone share their understanding first?

【Technician】 Relative position control means moving a certain distance from the current position, right? It doesn't need an absolute origin.

【Engineer】 Exactly. In Mitsubishi PLCs, we often use the DRVI instruction for relative positioning. Do you know what parameters it needs?

【Technician】 I think it needs the number of pulses and the pulse output destination. For example, DRVI K10000 K1 Y0.

【Engineer】 Correct. K10000 is the relative movement amount, K1 is the pulse output port number, and Y0 is the actual output terminal. What about the direction?

【Technician】 The sign of the pulse number determines direction. Positive moves forward, negative moves backward.

【Engineer】 Right. But many beginners confuse DRVI with DRVA. Can you explain the difference?

【Technician】 DRVA is absolute positioning, moving to a target position based on the origin. DRVI is relative, moving by a specified amount from the current position.

【Engineer】 Good. Now, what if we need to perform multiple relative moves in sequence? Any considerations?

【Technician】 We should ensure the previous pulse output is complete before starting the next. We can use the completion flag or wait for the pulse output to finish.

【Engineer】 Yes, typically we monitor the instruction's execution completion bit, like M8029 for pulse output complete. But careful: M8029 is for the entire pulse output, not just DRVI.

【Technician】 So we can use an interlock or a sequence to trigger the next DRVI after M8029 turns ON.

【Engineer】 Exactly. Also, for relative positioning, the current position register is updated automatically. Do you know which register?

【Technician】 I think it's the current position register in the positioning module, or for built-in pulse output, it might be D8340 and D8350 for channels 1 and 2?

【Engineer】 Correct for FX series. D8340 is the current position for Y0, and D8350 for Y1. These are 32-bit registers.

【Technician】 So after a DRVI, the value in D8340 changes by the pulse amount. That's useful for monitoring.

【Engineer】 Yes. But note that if you use DRVI with a negative value, the current position decreases. Also, if you use the PLSY instruction, it doesn't update the current position register.

【Technician】 That's an important distinction. So for relative positioning with position tracking, DRVI is preferred over PLSY.

【Engineer】 Exactly. Now, what about speed control? DRVI only specifies pulse number, not speed. How do we set the speed?

【Technician】 We need to set the speed separately, usually by writing to a special register like D8342 for Y0, or using the DPLSY instruction with speed parameter.

【Engineer】 Actually, for DRVI, the speed is determined by the value in the special register for the corresponding output. For FX3U, D8342 and D8343 form the speed for Y0. But it's often easier to use the DRVI instruction with a speed parameter in some PLC models? Wait, DRVI in FX series does not have a speed operand; it uses the value set in D8342. So we must set speed before executing DRVI.

【Technician】 So the typical sequence: set D8342 for speed, then execute DRVI with pulse count and output. That makes sense.

【Engineer】 Yes. And if we need to change speed on the fly, we can write to D8342 during operation, but careful about acceleration/deceleration.

【Technician】 I see. So relative position control is quite flexible but requires attention to these details.

【Engineer】 Absolutely. Any other questions about relative positioning? Maybe about using it with servo drives?

【Technician】 When using a servo, the PLC sends pulses to the servo driver, and the servo moves the motor. The relative position is still based on the PLC's pulse count, but the servo also has its own encoder. We need to ensure the electronic gear ratio is set correctly.

【Engineer】 Good point. The electronic gear ratio in the servo determines how many pulses correspond to one motor revolution or one mechanical unit. So the relative distance in PLC pulses translates to actual movement based on that ratio.

【Technician】 So if we want to move 10 mm, we calculate the required pulses based on the gear ratio and lead screw pitch, then use DRVI with that pulse number.

【Engineer】 Exactly. That's the essence of relative position control in Mitsubishi PLCs. Thanks for the discussion!